[clang][analyzer] Add checker for bad use of 'errno'.

[balazske]

Summary

Extend checker 'ErrnoModeling' with a state of 'errno' to indicate
the importance of the 'errno' value and how it should be used.
Add a new checker 'ErrnoChecker' that observes use of 'errno' and
finds possible wrong uses, based on the "errno state".
The "errno state" should be set (together with value of 'errno')
by other checkers (that perform modeling of the given function)
in the future. Currently only a test function can set this value.
The new checker has no user-observable effect yet.

[balazske]

This checker is made to add a partial support for CERT rule ERR30-C . One part of the rule is "check errno only after the function returns a value indicating failure".

To make this check possible the function (one that sets errno in some way) should be modeled by another checker that knows when a failure-indication value is returned from the function. In (but only in) that case the function sets value of errno. Return value of the function call should be constrained by the modeling checker to the failure-indicating values if the errno value is set, otherwise to some other values (a state split is needed).

The new API allows to set the errno value only together with an "errno check state". This state indicates how to handle the errno value by the ErrnoChecker. This information is available at the modeling of the errno-setting function. The CERT rule specifies classes of functions, including "functions that set errno and return an out-of-band error indicator" and "set errno and return an in-band error indicator". At the out-of-band case the errno value is not required to be checked, failure can be observed by check of the return value. At the in-band case the return value at failure is a valid return value too, here errno must be checked to observe if the function has failed. This case is modeled by the Errno_MustBeChecked errno check state. At many functions value of errno may be undefined after the function call if the function has not failed (the function is not required to not change errno), this is modeled by the Errno_MustNotBeChecked value.

[steakhal]

I'm liking it. We need to improve the diagnostics and the user-facing docs as well.

clang/include/clang/StaticAnalyzer/Checkers/Checkers.td

552

Then we should have documentation, and examples for it.

clang/lib/StaticAnalyzer/Checkers/ErrnoChecker.cpp

48–50

One should not define member variables like this.
The analysis engine might explore exploded nodes in an unpredictable order, invoking the checker's handler callbacks in an indeterminate order.
You should have registered simple value traits for this purpose, associating the necessary information with a given State.

69–72

Loc always wrap a memregion of some sort.

115–117

I think an early return could have spared an indentation level in the outer scope.

199

204–207

clang/lib/StaticAnalyzer/Checkers/ErrnoModeling.cpp

271–272

clang/lib/StaticAnalyzer/Checkers/ErrnoModeling.h

24

Did you consider enum classes?

42

I would appreciate some notes about when it returns None.

clang/test/Analysis/errno.c

73–75

We should have a note describing which function left the errno in an undefined/indeterminate state.
You can chain multiple NoteTags by using the ExplodedNode returned by the addTransition() and passing it along if needed.

170

So this is the case when errno gets indirectly invalidated by some opaque function call. I see.
However, it will test that the value associated with the errno region gets invalidated, that's fine. However, you should test if the metadata (the enum value) attached to memregion also gets invalidated.
Please make sure you have tests for that as well.
A ErrnoTesterChecker_dumpCheckState() should be perfect for this.

[NoQ]

clang/lib/StaticAnalyzer/Checkers/ErrnoChecker.cpp

48–50

In this case it's probably fine given that the variable doesn't really change throughout a single ExplodedGraph construction. But I agree that it doesn't really make sense to cache this data; getErrnoLoc() an O(1) lookup anyway.

69–72

Nope; null pointer is a Loc but doesn't correspond to any memory region.

I usually print out doxygen inheritance graphs:

• https://clang.llvm.org/doxygen/classclang_1_1ento_1_1SVal__inherit__graph.png
• https://clang.llvm.org/doxygen/classclang_1_1ento_1_1SymExpr__inherit__graph.png
• https://clang.llvm.org/doxygen/classclang_1_1ento_1_1MemRegion__inherit__graph.png

and hang them on the wall near my desk. They're really handy.

On a separate note, why not make this assertion part of getErrnoLoc()?

94

"May" is more neutral.

98

This path-insensitive note is attached to the exact same source location as the warning, right? I think this should be part of the warning instead. Something like this:

foo(); // (1) path note: Call to 'foo()' indicates failure only by setting 'errno'
bar(); // (2) warning: Value of 'errno' potentially overwritten by 'bar()' was not checked

105

Note that checkBeginFunction() is every time a nested stack frame is entered. This happens much more often than an update to the variable is needed.

[steakhal]

clang/lib/StaticAnalyzer/Checkers/ErrnoChecker.cpp

69–72

ah true

[balazske]

Address review comments.

• Removed caching of errno-related values from the checker.
• Added note tags.
• Added documentation comments.

[balazske]

clang/include/clang/StaticAnalyzer/Checkers/Checkers.td

552

Yes this is missing. But currently the errno value is not set by any non-debug checker, if there is any real example in the documentation it will not work (until the function is modeled correctly). Adding errno support for the functions (or some of them) is a separate change, we can add documentation here and commit the changes together (as close as possible).

clang/lib/StaticAnalyzer/Checkers/ErrnoChecker.cpp

69–72

This assert is not needed, the errno value has always a place that is a MemRegion according to how the ErrnoModeling works.

105

The "cached" errno values are removed, this function is not needed.

clang/lib/StaticAnalyzer/Checkers/ErrnoModeling.h

24

Originally it was enum class but I had to change it for some reason. Probably it did not have a default value.

clang/test/Analysis/errno.c

170

The check is now done indirectly by checking that no warning is produced.

[balazske]

This errno check will work if a state split is added at every standard function that may set errno at failure. (The success and failure branches have different errno check state, and probably different return value for the function.) Probably this is too many state splits. But a correct program should have anyway checks for the return value that make the same state splits.

[martong]

In D122150#3408156, @balazske wrote:

This errno check will work if a state split is added at every standard function that may set errno at failure. (The success and failure branches have different errno check state, and probably different return value for the function.) Probably this is too many state splits. But a correct program should have anyway checks for the return value that make the same state splits.

I think this is a good initiative and valuable work. You might be right about the too many state splits, however, we should measure this empirically. We should have a measurement on opensource projects to highlight peak memory usage and runtime discrepancies compared to the baseline. It might turn out that we indeed have to create a list of functions on which we are allowed to do the state split. Also, would be useful to evaluate the new reports.

[balazske]

added option to allow read-only of errno
other small change in the test checker

[balazske]

rebase, fix of failing tests

[martong]

clang/include/clang/StaticAnalyzer/Checkers/Checkers.td

554

I think this name would be much more easier to read.

clang/lib/StaticAnalyzer/Checkers/ErrnoChecker.cpp

47

Or in a statement without a condition ?

68

Please add a documentation to this.

[steakhal]

D126801 should simplify the reinterpret casts.

clang/include/clang/StaticAnalyzer/Checkers/Checkers.td

558

I think we have 'Hidden', but not 'Hide'.
It means that it is a modeling checker. Any checker that emits diagnostics, should not be marked 'Hidden'.

clang/lib/StaticAnalyzer/Checkers/ErrnoModeling.cpp

267–268

It looks odd that in the very next if block you have II and here you don't.

clang/lib/StaticAnalyzer/Checkers/ErrnoModeling.h

34

You don't need to prefix these with Errno_. It's already contained by a specific namespace.
https://llvm.org/docs/CodingStandards.html#name-types-functions-variables-and-enumerators-properly

Unless the enumerators are defined in their own small namespace or inside a class, enumerators should have a prefix corresponding to the enum declaration name

[balazske]

applied review comments, test improvement

[balazske]

clang/include/clang/StaticAnalyzer/Checkers/Checkers.td

558

This "Hide" applies for the option, not for the checker. It was copied from another place. I am not sure if it must be used here.

[martong]

clang/docs/analyzer/checkers.rst

2553–2555 ↗

(On Diff #435081)

This sentence is a bit hard to follow, I'd split this up.

2559–2560 ↗

(On Diff #435081)

"all times" and "the first read" seems to be in contradiction to each other. Could you please rephrase? Use a standalone sentence that explains the "(at the first read)" part.

2560 ↗

(On Diff #435081)

?

[balazske]

fixed documentation, added release note

[martong]

LGTM, it is a good start for an alpha checker.

clang/docs/ReleaseNotes.rst

525 ↗

(On Diff #435557)

[steakhal]

I believe evalAssume would be a better fit for diagnosing such issues, but I can see your point that we don't have CheckerContext there to emit reports.
That being said, the check::Location is the best alternative.

Please also check the not done comments.

clang/docs/analyzer/checkers.rst

2529 ↗

(On Diff #435557)

successful

2538–2539 ↗

(On Diff #435557)

Maybe place a crossref for this.

Also, am I right that there is a CERT rule about this issue?
We should probably mention that, while placing a crossref too.

clang/include/clang/StaticAnalyzer/Checkers/Checkers.td

550

antipatterns?

554

My problem with this name is that Unconditional seems to bind to the action, not to the place where the read happens (inside condition expressions).
I find this pretty confusing.

How about calling this AllowReadingErrnoInsideConditionExpressions. Still bad, but probably better.

558

Quote CheckerBase.td:39

/// Marks the entry hidden. Hidden entries won't be displayed in
/// -analyzer-checker-option-help.
class HiddenEnum<bit val> {
  bit Val = val;
}
def DontHide : HiddenEnum<0>;
def Hide : HiddenEnum<1>;

I believe, we should expose this option. Otherwise why would we introduce it in the first place?
The default value of this is actually DontHide in the CmdLineOption<> td class. So, you should be fine by simply omitting this keyword here.

clang/lib/StaticAnalyzer/Checkers/ErrnoChecker.cpp

11–12

and the StdLibraryFunctionsChecker per D125400.

102–103

We should also handle BinaryOperator for conditional operators (EQ, NE, LT, LE, ...). Or at least leave a FIXME about this.
Have a test demonstrating the behavior.

clang/lib/StaticAnalyzer/Checkers/ErrnoModeling.cpp

261

I believe you no longer need to cast enums to numbers and back for enums and other types.
get<ErrnoState>() should return the right type.
See D126801.

280

[balazske]

applied review comments

[balazske]

clang/docs/analyzer/checkers.rst

2538–2539 ↗

(On Diff #435557)

Probably there is not a preferred or "standard" site for man page lookup and it can be done from command line too. But I can include a POSIX link https://pubs.opengroup.org/onlinepubs/9699919799/.

clang/include/clang/StaticAnalyzer/Checkers/Checkers.td

550

I like "improper use of errno" better, it is similar to HelpText of other checkers.

clang/lib/StaticAnalyzer/Checkers/ErrnoChecker.cpp

102–103

The idea is here that this search does not stop at binary operators but goes up the AST until a condition is found.

clang/lib/StaticAnalyzer/Checkers/ErrnoModeling.cpp

261

How to handle if no value exists?

267–268

I do not get what you mean here.

280

I can not omit c_str, there is a compile error probably related to lambda return type.

clang/lib/StaticAnalyzer/Checkers/ErrnoModeling.h

34

Without Errno_ these sound too generic. Probably have only Errno prefix, not Errno_?

[balazske]

Use typed state trait.

[steakhal]

It looks great.
Let me check the test coverage and the generated docs page it everything looks great.

clang/docs/analyzer/checkers.rst

2538 ↗

(On Diff #437513)

2540 ↗

(On Diff #437513)

Please mention at least one of those functions.

clang/include/clang/StaticAnalyzer/Checkers/Checkers.td

553–554

Well, now I see why you marked this Hide previously.
This is a modeling checker, thus it won't appear in the documentation nor in the -analyzer-checker-help etc.

Interestingly, other checkers mark some options Hide and other times DontHide.
E.g. DynamicMemoryModeling.Optimistic is displayed, but the DynamicMemoryModeling.AddNoOwnershipChangeNotes is hidden.
Whatever.

clang/lib/StaticAnalyzer/Checkers/ErrnoChecker.cpp

47

I don't really understand this comment. What is a non-condition part of a statement?

146

Might be more readable to early return instead.

206

Does this refer to the callsite or to the Decl location?
I think the former, which would be a bug I think.

clang/lib/StaticAnalyzer/Checkers/ErrnoModeling.cpp

267–268

I was thinking about this:

bool isErrno(const Decl *D) {
  if (const auto *VD = dyn_cast_or_null<VarDecl>(D))
    if (const IdentifierInfo *II = VD->getIdentifier())
      return II->getName() == ErrnoVarName;
  if (const auto *FD = dyn_cast_or_null<FunctionDecl>(D))
    if (const IdentifierInfo *II = FD->getIdentifier())
      return llvm::is_contained(ErrnoLocationFuncNames, II->getName());
  return false;
}

This way both checks would follow the same pattern, symmetry.

280

Yes, you will need to specify explicitly the return type of the lambda: -> std::string.

clang/lib/StaticAnalyzer/Checkers/ErrnoModeling.h

34

But you always refer to it by it's qualified name: errno_modeling::Errno_Irrelevant.
In which case this prefix is just noise IMO. That's what I'm about.

[steakhal]

clang/lib/StaticAnalyzer/Checkers/ErrnoModeling.cpp

79

You will need this to get it compiled.

[steakhal]

Awesome!

The generated doc section looks great. The test coverage it excellent, but I would recommend adding tests for covering the following lines:

• ErrnoChecker.cpp:99
• ErrnoModeling.cpp:259
• ErrnoModeling.cpp:264

There are two other uncovered cases, but those are mainly defensive checks, so I don't mind them.

clang/docs/analyzer/checkers.rst

2565 ↗

(On Diff #437513)

by

2586 ↗

(On Diff #437513)

I think it should be surrounded by double backticks. It looks ugly this way:

[balazske]

Removed Errno_ prefix from ErrnoCheckState, documentation fixes, other small fixes.

[steakhal]

clang/lib/StaticAnalyzer/Checkers/ErrnoChecker.cpp

48

You prefixed the previous two keywords with the \c marker. Why is it missing from the switch?

101

Oh, so the test added for this actually uncovered a bug?

[balazske]

clang/include/clang/StaticAnalyzer/Checkers/Checkers.td

553–554

It looks good now, ErrnoChecker is not a modeling checker and the option should be accessible for users.

clang/lib/StaticAnalyzer/Checkers/ErrnoChecker.cpp

48

Is it really missing? (I do not check Doxygen documentation, probably the new documentation appears in the online pages after the commit.)

101

Yes: Even if it looks natural that getCond can be used the same way as at normal ConditionalOperator this is not true: It returns something other (casted or converted) than the root condition expressions.

146

There is a else part.

206

CallF is a FunctionDecl that should be the original (first) declaration of the function.

clang/include/clang/StaticAnalyzer/Checkers/Checkers.td

 def VforkChecker : Checker<"Vfork">,
   HelpText<"Check for proper usage of vfork">,
   Documentation<HasDocumentation>;
 
 } // end "unix"
 
 let ParentPackage = UnixAlpha in {
 
+def ErrnoChecker : Checker<"Errno">,
+  HelpText<"Check not recommended uses of 'errno'">,
+  Dependencies<[ErrnoModeling]>,
+  CheckerOptions<[
+    CmdLineOption<Boolean,
+                  "AllowNonConditionErrnoRead",
+                  "Allow read of undefined value from errno outside of conditions",
+                  "true",
+                  InAlpha,
+                  Hide>, // ??
+  ]>,
+  Documentation<HasDocumentation>;
+
 def ChrootChecker : Checker<"Chroot">,
   HelpText<"Check improper use of chroot">,
   Documentation<HasDocumentation>;
 
 def PthreadLockChecker : Checker<"PthreadLock">,
   HelpText<"Simple lock -> unlock checker">,
   Dependencies<[PthreadLockBase]>,
   Documentation<HasDocumentation>;

clang/lib/StaticAnalyzer/Checkers/CMakeLists.txt

add_clang_library(clangStaticAnalyzerCheckers
   DebugIteratorModeling.cpp
   DeleteWithNonVirtualDtorChecker.cpp
   DereferenceChecker.cpp
   DirectIvarAssignment.cpp
   DivZeroChecker.cpp
   DynamicTypePropagation.cpp
   DynamicTypeChecker.cpp
   EnumCastOutOfRangeChecker.cpp
+  ErrnoChecker.cpp
   ErrnoModeling.cpp
   ErrnoTesterChecker.cpp
   ExprInspectionChecker.cpp
   FixedAddressChecker.cpp
   FuchsiaHandleChecker.cpp
   GCDAntipatternChecker.cpp
   GenericTaintChecker.cpp
   GTestChecker.cpp

clang/lib/StaticAnalyzer/Checkers/ErrnoChecker.cpp

+//=== ErrnoChecker.cpp ------------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This defines an "errno checker" that can detect some invalid use of the
+// system-defined value 'errno'. This checker works together with the
+// ErrnoModeling checker.
+//
+//===----------------------------------------------------------------------===//
+
+#include "ErrnoModeling.h"
+#include "clang/AST/ParentMapContext.h"
+#include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
+#include "clang/StaticAnalyzer/Core/Checker.h"
+#include "clang/StaticAnalyzer/Core/CheckerManager.h"
+#include "clang/StaticAnalyzer/Core/PathSensitive/CallDescription.h"
+#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
+#include "clang/StaticAnalyzer/Core/PathSensitive/ProgramState.h"
+#include "clang/StaticAnalyzer/Core/PathSensitive/SVals.h"
+#include "llvm/ADT/STLExtras.h"
+
+using namespace clang;
+using namespace ento;
+using namespace errno_modeling;
+
+namespace {
+
+class ErrnoChecker
+    : public Checker<check::Location, check::PreCall, check::RegionChanges> {
+public:
+  void checkLocation(SVal Loc, bool IsLoad, const Stmt *S,
+                     CheckerContext &) const;
+  void checkPreCall(const CallEvent &Call, CheckerContext &C) const;
+  ProgramStateRef
+  checkRegionChanges(ProgramStateRef State,
+                     const InvalidatedSymbols *Invalidated,
+                     ArrayRef<const MemRegion *> ExplicitRegions,
+                     ArrayRef<const MemRegion *> Regions,
+                     const LocationContext *LCtx, const CallEvent *Call) const;
+  void checkBranchCondition(const Stmt *Condition, CheckerContext &Ctx) const;
+
+  /// Indicates if a read (load) of 'errno' is allowed in a non-conditional
+  /// statement.
+  bool AllowNonConditionErrnoRead = true;
+
+private:
+  void generateErrnoNotCheckedBug(CheckerContext &C, ProgramStateRef State,
+                                  const MemRegion *ErrnoRegion,
+                                  const CallEvent *CallMayChangeErrno) const;
+
+  BugType BT_InvalidErrnoRead{this, "Value of 'errno' could be undefined",
+                              "Error handling"};
+  BugType BT_ErrnoNotChecked{this, "Value of 'errno' was not checked",
+                             "Error handling"};
+};
+
+} // namespace
+
+static ProgramStateRef setErrnoStateIrrelevant(ProgramStateRef State) {
+  return setErrnoState(State, Errno_Irrelevant);
+}
+
+static bool isInCondition(const Stmt *S, CheckerContext &C) {
+  ParentMapContext &ParentCtx = C.getASTContext().getParentMapContext();
+  bool CondFound = false;
+  while (S && !CondFound) {
+    const DynTypedNodeList Parents = ParentCtx.getParents(*S);
+    if (Parents.empty())
+      break;
+    const auto *ParentS = Parents[0].get<Stmt>();
+    if (!ParentS || isa<CallExpr>(ParentS))
+      break;
+    switch (ParentS->getStmtClass()) {
+    case Expr::IfStmtClass:
+      CondFound = (S == cast<IfStmt>(ParentS)->getCond());
+      break;
+    case Expr::ForStmtClass:
+      CondFound = (S == cast<ForStmt>(ParentS)->getCond());
+      break;
+    case Expr::DoStmtClass:
+      CondFound = (S == cast<DoStmt>(ParentS)->getCond());
+      break;
+    case Expr::WhileStmtClass:
+      CondFound = (S == cast<WhileStmt>(ParentS)->getCond());
+      break;
+    case Expr::SwitchStmtClass:
+      CondFound = (S == cast<SwitchStmt>(ParentS)->getCond());
+      break;
+    case Expr::ConditionalOperatorClass:
+      CondFound = (S == cast<ConditionalOperator>(ParentS)->getCond());
+      break;
+    case Expr::BinaryConditionalOperatorClass:
+      CondFound = (S == cast<BinaryConditionalOperator>(ParentS)->getCond());
+      break;
+    default:
+      break;
+    }
+    S = ParentS;
+  }
+  return CondFound;
+}
+
+void ErrnoChecker::generateErrnoNotCheckedBug(
+    CheckerContext &C, ProgramStateRef State, const MemRegion *ErrnoRegion,
+    const CallEvent *CallMayChangeErrno) const {
+  if (ExplodedNode *N = C.generateNonFatalErrorNode(State)) {
+    SmallString<100> StrBuf;
+    llvm::raw_svector_ostream OS(StrBuf);
+    if (CallMayChangeErrno) {
+      OS << "Value of 'errno' was not checked and may be overwritten by "
+            "function '";
+      const auto *CallD =
+          dyn_cast_or_null<FunctionDecl>(CallMayChangeErrno->getDecl());
+      assert(CallD && CallD->getIdentifier());
+      OS << CallD->getIdentifier()->getName() << "'";
+    } else {
+      OS << "Value of 'errno' was not checked and is overwritten here";
+    }
+    auto BR = std::make_unique<PathSensitiveBugReport>(BT_ErrnoNotChecked,
+                                                       OS.str(), N);
+    BR->markInteresting(ErrnoRegion);
+    C.emitReport(std::move(BR));
+  }
+}
+
+void ErrnoChecker::checkLocation(SVal Loc, bool IsLoad, const Stmt *S,
+                                 CheckerContext &C) const {
+  Optional<ento::Loc> ErrnoLoc = getErrnoLoc(C.getState());
+  if (!ErrnoLoc)
+    return;
+
+  auto L = Loc.getAs<ento::Loc>();
+  if (!L || *ErrnoLoc != *L)
+    return;
+
+  ProgramStateRef State = C.getState();
+  ErrnoCheckState EState = getErrnoState(State);
+
+  if (IsLoad) {
+    switch (EState) {
+    case Errno_MustNotBeChecked:
+      // Read of 'errno' when it may have undefined value.
+      if (!AllowNonConditionErrnoRead || isInCondition(S, C)) {
+        if (ExplodedNode *N = C.generateErrorNode()) {
+          auto BR = std::make_unique<PathSensitiveBugReport>(
+              BT_InvalidErrnoRead,
+              "An undefined value may be read from 'errno'", N);
+          BR->markInteresting(ErrnoLoc->getAsRegion());
+          C.emitReport(std::move(BR));
+        }
+      }
+      break;
+    case Errno_MustBeChecked:
+      // 'errno' has to be checked. A load is required for this, with no more
+      // information we can assume that it is checked somehow.
+      // After this place 'errno' is allowed to be read and written.
+      State = setErrnoStateIrrelevant(State);
+      C.addTransition(State);
+      break;
+    default:
+      break;
+    }
+  } else {
+    switch (EState) {
+    case Errno_MustBeChecked:
+      // 'errno' is overwritten without a read before but it should have been
+      // checked.
+      generateErrnoNotCheckedBug(C, setErrnoStateIrrelevant(State),
+                                 ErrnoLoc->getAsRegion(), nullptr);
+      break;
+    case Errno_MustNotBeChecked:
+      // Write to 'errno' when it is not allowed to be read.
+      // After this place 'errno' is allowed to be read and written.
+      State = setErrnoStateIrrelevant(State);
+      C.addTransition(State);
+      break;
+    default:
+      break;
+    }
+  }
+}
+
+void ErrnoChecker::checkPreCall(const CallEvent &Call,
+                                CheckerContext &C) const {
+  const auto *CallF = dyn_cast_or_null<FunctionDecl>(Call.getDecl());
+  if (!CallF)
+    return;
+
+  CallF = CallF->getCanonicalDecl();
+  // If 'errno' must be checked, it should be done as soon as possible, and
+  // before any other call to a system function (something in a system header).
+  // To avoid use of a long list of functions that may change 'errno'
+  // (which may be different with standard library versions) assume that any
+  // function can change it.
+  // A list of special functions can be used that are allowed here without
+  // generation of diagnostic. For now the only such case is 'errno' itself.
+  // Probably 'strerror'?
+  if (CallF->isExternC() && CallF->isGlobal() &&
+      C.getSourceManager().isInSystemHeader(CallF->getLocation()) &&
+      !isErrno(CallF)) {
+    if (getErrnoState(C.getState()) == Errno_MustBeChecked) {
+      Optional<ento::Loc> ErrnoLoc = getErrnoLoc(C.getState());
+      assert(ErrnoLoc && "ErrnoLoc should exist if an errno state is set.");
+      generateErrnoNotCheckedBug(C, setErrnoStateIrrelevant(C.getState()),
+                                 ErrnoLoc->getAsRegion(), &Call);
+    }
+  }
+}
+
+ProgramStateRef ErrnoChecker::checkRegionChanges(
+    ProgramStateRef State, const InvalidatedSymbols *Invalidated,
+    ArrayRef<const MemRegion *> ExplicitRegions,
+    ArrayRef<const MemRegion *> Regions, const LocationContext *LCtx,
+    const CallEvent *Call) const {
+  Optional<ento::Loc> ErrnoLoc = getErrnoLoc(State);
+  if (!ErrnoLoc)
+    return State;
+  const MemRegion *ErrnoRegion = ErrnoLoc->getAsRegion();
+
+  // If 'errno' is invalidated we can not know if it is checked or written into,
+  // allow read and write without bug reports.
+  if (llvm::is_contained(Regions, ErrnoRegion))
+    return setErrnoStateIrrelevant(State);
+
+  // Always reset errno state when the system memory space is invalidated.
+  // The ErrnoRegion is not always found in the list in this case.
+  if (llvm::is_contained(Regions, ErrnoRegion->getMemorySpace()))
+    return setErrnoStateIrrelevant(State);
+
+  return State;
+}
+
+void ento::registerErrnoChecker(CheckerManager &mgr) {
+  const AnalyzerOptions &Opts = mgr.getAnalyzerOptions();
+  auto *Checker = mgr.registerChecker<ErrnoChecker>();
+  Checker->AllowNonConditionErrnoRead =
+      Opts.getCheckerBooleanOption(Checker, "AllowNonConditionErrnoRead");
+}
+
+bool ento::shouldRegisterErrnoChecker(const CheckerManager &mgr) {
+  return true;
+}

clang/lib/StaticAnalyzer/Checkers/ErrnoModeling.h

 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
 #include "clang/StaticAnalyzer/Core/PathSensitive/ProgramState.h"
 #include "clang/StaticAnalyzer/Core/PathSensitive/SVals.h"
 
 namespace clang {
 namespace ento {
 namespace errno_modeling {
 
+enum ErrnoCheckState : unsigned {
+  /// We do not know anything about 'errno'.
+  Errno_Irrelevant = 0,
+
+  /// Value of 'errno' should be checked to find out if a previous function call
+  /// has failed.
+  Errno_MustBeChecked = 1,
+
+  /// Value of 'errno' is not allowed to be read, it can contain an unspecified
+  /// value.
+  Errno_MustNotBeChecked = 2
+};
+
 /// Returns the value of 'errno', if 'errno' was found in the AST.
 llvm::Optional<SVal> getErrnoValue(ProgramStateRef State);
 
+/// Returns the errno check state, \c Errno_Irrelevant if 'errno' was not found
+/// (this is not the only case for that value).
+ErrnoCheckState getErrnoState(ProgramStateRef State);
+
+/// Returns the location that points to the \c MemoryRegion where the 'errno'
+/// value is stored. Returns \c None if 'errno' was not found. Otherwise it
+/// always returns a valid memory region in the system global memory space.
+llvm::Optional<Loc> getErrnoLoc(ProgramStateRef State);
+
 /// Set value of 'errno' to any SVal, if possible.
+/// The errno check state is set always when the 'errno' value is set.
 ProgramStateRef setErrnoValue(ProgramStateRef State,
-                              const LocationContext *LCtx, SVal Value);
+                              const LocationContext *LCtx, SVal Value,
+                              ErrnoCheckState EState);
 
 /// Set value of 'errno' to a concrete (signed) integer, if possible.
+/// The errno check state is set always when the 'errno' value is set.
 ProgramStateRef setErrnoValue(ProgramStateRef State, CheckerContext &C,
-                              uint64_t Value);
+                              uint64_t Value, ErrnoCheckState EState);
+
+/// Set the errno check state, do not modify the errno value.
+ProgramStateRef setErrnoState(ProgramStateRef State, ErrnoCheckState EState);
+
+/// Determine if a `Decl` node related to 'errno'.
+/// This is true if the declaration is the errno variable or a function
+/// that returns a pointer to the 'errno' value (usually the 'errno' macro is
+/// defined with this function). \p D is not required to be a canonical
+/// declaration.
+bool isErrno(const Decl *D);
+
+/// Create a NoteTag that displays the message if the 'errno' memory region is
+/// marked as interesting, and resets the interestingness.
+const NoteTag *getErrnoNoteTag(CheckerContext &C, const std::string &Message);
 
 } // namespace errno_modeling
 } // namespace ento
 } // namespace clang
 
 #endif // LLVM_CLANG_LIB_STATICANALYZER_CHECKERS_ERRNOMODELING_H

clang/lib/StaticAnalyzer/Checkers/ErrnoModeling.cpp

 REGISTER_TRAIT_WITH_PROGRAMSTATE(ErrnoRegion, const void *)
 
 /// An internal function accessing the errno region.
 /// Returns null if there isn't any associated memory region.
 static const MemRegion *getErrnoRegion(ProgramStateRef State) {
   return reinterpret_cast<const MemRegion *>(State->get<ErrnoRegion>());
 }
 
+REGISTER_TRAIT_WITH_PROGRAMSTATE(ErrnoState, unsigned)
+
 /// Search for a variable called "errno" in the AST.
 /// Return nullptr if not found.
 static const VarDecl *getErrnoVar(ASTContext &ACtx) {
   IdentifierInfo &II = ACtx.Idents.get(ErrnoVarName);
   auto LookupRes = ACtx.getTranslationUnitDecl()->lookup(&II);
   auto Found = llvm::find_if(LookupRes, [&ACtx](const Decl *D) {
     if (auto *VD = dyn_cast<VarDecl>(D))
       return ACtx.getSourceManager().isInSystemHeader(VD->getLocation()) &&
if (const auto *ErrnoVar = dyn_cast_or_null<VarDecl>(ErrnoDecl)) {
     // There is an external 'errno' variable.
     // Use its memory region.
     // The memory region for an 'errno'-like variable is allocated in system
     // space by MemRegionManager.
     const MemRegion *ErrnoR =
         State->getRegion(ErrnoVar, C.getLocationContext());
     assert(ErrnoR && "Memory region should exist for the 'errno' variable.");
     State = State->set<ErrnoRegion>(ErrnoR);
-    State = errno_modeling::setErrnoValue(State, C, 0);
+    State = errno_modeling::setErrnoValue(State, C, 0,
+                                          errno_modeling::Errno_Irrelevant);
     C.addTransition(State);
   } else if (ErrnoDecl) {
     assert(isa<FunctionDecl>(ErrnoDecl) && "Invalid errno location function.");
     // There is a function that returns the location of 'errno'.
     // We must create a memory region for it in system space.
     // Currently a symbolic region is used with an artifical symbol.
     // FIXME: It is better to have a custom (new) kind of MemRegion for such
     // cases.
const SymbolConjured *Sym = SVB.conjureSymbol(
         ACtx.getLValueReferenceType(ACtx.IntTy), C.blockCount(), &ErrnoDecl);
 
     // The symbolic region is untyped, create a typed sub-region in it.
     // The ElementRegion is used to make the errno region a typed region.
     const MemRegion *ErrnoR = RMgr.getElementRegion(
         ACtx.IntTy, SVB.makeZeroArrayIndex(),
         RMgr.getSymbolicRegion(Sym, GlobalSystemSpace), C.getASTContext());
     State = State->set<ErrnoRegion>(ErrnoR);
-    State = errno_modeling::setErrnoValue(State, C, 0);
+    State = errno_modeling::setErrnoValue(State, C, 0,
+                                          errno_modeling::Errno_Irrelevant);
     C.addTransition(State);
   }
 }
 
 bool ErrnoModeling::evalCall(const CallEvent &Call, CheckerContext &C) const {
   // Return location of "errno" at a call to an "errno address returning"
   // function.
   if (ErrnoLocationCalls.contains(Call)) {
Optional<SVal> getErrnoValue(ProgramStateRef State) {
   const MemRegion *ErrnoR = getErrnoRegion(State);
   if (!ErrnoR)
     return {};
   QualType IntTy = State->getAnalysisManager().getASTContext().IntTy;
   return State->getSVal(ErrnoR, IntTy);
 }
 
 ProgramStateRef setErrnoValue(ProgramStateRef State,
-                              const LocationContext *LCtx, SVal Value) {
+                              const LocationContext *LCtx, SVal Value,
+                              ErrnoCheckState EState) {
   const MemRegion *ErrnoR = getErrnoRegion(State);
   if (!ErrnoR)
     return State;
-  return State->bindLoc(loc::MemRegionVal{ErrnoR}, Value, LCtx);
+  // First set the errno value, the old state is still available at 'checkBind'
+  // or 'checkLocation' for errno value.
+  State = State->bindLoc(loc::MemRegionVal{ErrnoR}, Value, LCtx);
+  return State->set<ErrnoState>(EState);
 }
 
 ProgramStateRef setErrnoValue(ProgramStateRef State, CheckerContext &C,
-                              uint64_t Value) {
+                              uint64_t Value, ErrnoCheckState EState) {
   const MemRegion *ErrnoR = getErrnoRegion(State);
   if (!ErrnoR)
     return State;
-  return State->bindLoc(
+  State = State->bindLoc(
       loc::MemRegionVal{ErrnoR},
       C.getSValBuilder().makeIntVal(Value, C.getASTContext().IntTy),
       C.getLocationContext());
+  return State->set<ErrnoState>(EState);
+}
+
+Optional<Loc> getErrnoLoc(ProgramStateRef State) {
+  const MemRegion *ErrnoR = getErrnoRegion(State);
+  if (!ErrnoR)
+    return {};
+  return loc::MemRegionVal{ErrnoR};
+}
+
+ProgramStateRef setErrnoState(ProgramStateRef State, ErrnoCheckState EState) {
+  return State->set<ErrnoState>(EState);
+}
+
+ErrnoCheckState getErrnoState(ProgramStateRef State) {
+  if (unsigned EState = State->get<ErrnoState>())
+    return static_cast<ErrnoCheckState>(EState);
+  return Errno_Irrelevant;
+}
+
+bool isErrno(const Decl *D) {
+  if (const auto *VD = dyn_cast_or_null<VarDecl>(D))
+    return VD->getIdentifier() &&
+           VD->getIdentifier()->getName() == ErrnoVarName;
+  if (const auto *FD = dyn_cast_or_null<FunctionDecl>(D))
+    if (IdentifierInfo *II = FD->getIdentifier())
+      return llvm::find(ErrnoLocationFuncNames, II->getName()) !=
+             std::end(ErrnoLocationFuncNames);
+  return false;
+}
+
+const NoteTag *getErrnoNoteTag(CheckerContext &C, const std::string &Message) {
+  return C.getNoteTag([Message](PathSensitiveBugReport &BR) {
+    const MemRegion *ErrnoR = getErrnoRegion(BR.getErrorNode()->getState());
+    if (ErrnoR && BR.isInteresting(ErrnoR)) {
+      BR.markNotInteresting(ErrnoR);
+      return Message.c_str();
+    }
+    return "";
+  });
 }
 
 } // namespace errno_modeling
 } // namespace ento
 } // namespace clang
 
 void ento::registerErrnoModeling(CheckerManager &mgr) {
   mgr.registerChecker<ErrnoModeling>();
 }
 
 bool ento::shouldRegisterErrnoModeling(const CheckerManager &mgr) {
   return true;
 }

clang/lib/StaticAnalyzer/Checkers/ErrnoTesterChecker.cpp

 #include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
 #include "clang/StaticAnalyzer/Core/Checker.h"
 #include "clang/StaticAnalyzer/Core/CheckerManager.h"
 #include "clang/StaticAnalyzer/Core/PathSensitive/CallDescription.h"
 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
 
 using namespace clang;
 using namespace ento;
+using namespace errno_modeling;
 
 namespace {
 
 class ErrnoTesterChecker : public Checker<eval::Call> {
 public:
   bool evalCall(const CallEvent &Call, CheckerContext &C) const;
 
 private:
+  /// Evaluate function \code void ErrnoTesterChecker_setErrno(int) \endcode.
+  /// Set value of \c errno to the argument.
   static void evalSetErrno(CheckerContext &C, const CallEvent &Call);
+  /// Evaluate function \code int ErrnoTesterChecker_getErrno() \endcode.
+  /// Return the value of \c errno.
   static void evalGetErrno(CheckerContext &C, const CallEvent &Call);
+  /// Evaluate function \code int ErrnoTesterChecker_setErrnoIfError() \endcode.
+  /// Simulate a standard library function tha returns 0 on success and 1 on
+  /// failure. On the success case \c errno is not allowed to be used (may be
+  /// undefined). On the failure case \c errno is set to a fixed value 11 and
+  /// is not needed to be checked.
   static void evalSetErrnoIfError(CheckerContext &C, const CallEvent &Call);
+  /// Evaluate function \code int ErrnoTesterChecker_setErrnoIfErrorRange()
+  /// \endcode. Same as \c ErrnoTesterChecker_setErrnoIfError but \c errno is
+  /// set to a range (to be nonzero) at the failure case.
   static void evalSetErrnoIfErrorRange(CheckerContext &C,
                                        const CallEvent &Call);
+  /// Evaluate function \code int ErrnoTesterChecker_setErrnoCheckState()
+  /// \endcode. This function simulates the following:
+  /// - Return 0 and leave \c errno with undefined value.
+  ///   This is the case of a successful standard function call.
+  ///   For example if \c ftell returns not -1.
+  /// - Return 1 and sets \c errno to a specific error code (1).
+  ///   This is the case of a failed standard function call.
+  ///   The function indicates the failure by a special return value
+  ///   that is returned only at failure.
+  ///   \c errno can be checked but it is not required.
+  ///   For example if \c ftell returns -1.
+  /// - Return 2 and may set errno to a value (actually it does not set it).
+  ///   This is the case of a standard function call where the failure can only
+  ///   be checked by reading from \c errno. The value of \c errno is changed by
+  ///   the function only at failure, the user should set \c errno to 0 before
+  ///   the call (\c ErrnoChecker does not check for this rule).
+  ///   \c strtol is an example of this case, if it returns \c LONG_MIN (or
+  ///   \c LONG_MAX). This case applies only if \c LONG_MIN or \c LONG_MAX is
+  ///   returned, otherwise the first case in this list applies.
+  static void evalSetErrnoCheckState(CheckerContext &C, const CallEvent &Call);
 
   using EvalFn = std::function<void(CheckerContext &, const CallEvent &)>;
   const CallDescriptionMap<EvalFn> TestCalls{
       {{"ErrnoTesterChecker_setErrno", 1}, &ErrnoTesterChecker::evalSetErrno},
       {{"ErrnoTesterChecker_getErrno", 0}, &ErrnoTesterChecker::evalGetErrno},
       {{"ErrnoTesterChecker_setErrnoIfError", 0},
        &ErrnoTesterChecker::evalSetErrnoIfError},
       {{"ErrnoTesterChecker_setErrnoIfErrorRange", 0},
-       &ErrnoTesterChecker::evalSetErrnoIfErrorRange}};
+       &ErrnoTesterChecker::evalSetErrnoIfErrorRange},
+      {{"ErrnoTesterChecker_setErrnoCheckState", 0},
+       &ErrnoTesterChecker::evalSetErrnoCheckState}};
 };
 
 } // namespace
 
 void ErrnoTesterChecker::evalSetErrno(CheckerContext &C,
                                       const CallEvent &Call) {
-  C.addTransition(errno_modeling::setErrnoValue(
-      C.getState(), C.getLocationContext(), Call.getArgSVal(0)));
+  C.addTransition(setErrnoValue(C.getState(), C.getLocationContext(),
+                                Call.getArgSVal(0), Errno_Irrelevant));
 }
 
 void ErrnoTesterChecker::evalGetErrno(CheckerContext &C,
                                       const CallEvent &Call) {
   ProgramStateRef State = C.getState();
 
-  Optional<SVal> ErrnoVal = errno_modeling::getErrnoValue(State);
+  Optional<SVal> ErrnoVal = getErrnoValue(State);
   assert(ErrnoVal && "Errno value should be available.");
   State =
       State->BindExpr(Call.getOriginExpr(), C.getLocationContext(), *ErrnoVal);
 
   C.addTransition(State);
 }
 
 void ErrnoTesterChecker::evalSetErrnoIfError(CheckerContext &C,
                                              const CallEvent &Call) {
   ProgramStateRef State = C.getState();
   SValBuilder &SVB = C.getSValBuilder();
 
   ProgramStateRef StateSuccess = State->BindExpr(
       Call.getOriginExpr(), C.getLocationContext(), SVB.makeIntVal(0, true));
+  StateSuccess = setErrnoState(StateSuccess, Errno_MustNotBeChecked);
 
   ProgramStateRef StateFailure = State->BindExpr(
       Call.getOriginExpr(), C.getLocationContext(), SVB.makeIntVal(1, true));
-  StateFailure = errno_modeling::setErrnoValue(StateFailure, C, 11);
+  StateFailure = setErrnoValue(StateFailure, C, 11, Errno_Irrelevant);
 
   C.addTransition(StateSuccess);
   C.addTransition(StateFailure);
 }
 
 void ErrnoTesterChecker::evalSetErrnoIfErrorRange(CheckerContext &C,
                                                   const CallEvent &Call) {
   ProgramStateRef State = C.getState();
   SValBuilder &SVB = C.getSValBuilder();
 
   ProgramStateRef StateSuccess = State->BindExpr(
       Call.getOriginExpr(), C.getLocationContext(), SVB.makeIntVal(0, true));
+  StateSuccess = setErrnoState(StateSuccess, Errno_MustNotBeChecked);
 
   ProgramStateRef StateFailure = State->BindExpr(
       Call.getOriginExpr(), C.getLocationContext(), SVB.makeIntVal(1, true));
   DefinedOrUnknownSVal ErrnoVal = SVB.conjureSymbolVal(
       nullptr, Call.getOriginExpr(), C.getLocationContext(), C.blockCount());
   StateFailure = StateFailure->assume(ErrnoVal, true);
   assert(StateFailure && "Failed to assume on an initial value.");
-  StateFailure = errno_modeling::setErrnoValue(
-      StateFailure, C.getLocationContext(), ErrnoVal);
+  StateFailure = setErrnoValue(StateFailure, C.getLocationContext(), ErrnoVal,
+                               Errno_Irrelevant);
 
   C.addTransition(StateSuccess);
   C.addTransition(StateFailure);
 }
 
+void ErrnoTesterChecker::evalSetErrnoCheckState(CheckerContext &C,
+                                                const CallEvent &Call) {
+  ProgramStateRef State = C.getState();
+  SValBuilder &SVB = C.getSValBuilder();
+
+  ProgramStateRef StateSuccess = State->BindExpr(
+      Call.getOriginExpr(), C.getLocationContext(), SVB.makeIntVal(0, true));
+  StateSuccess = setErrnoState(StateSuccess, Errno_MustNotBeChecked);
+
+  ProgramStateRef StateFailure1 = State->BindExpr(
+      Call.getOriginExpr(), C.getLocationContext(), SVB.makeIntVal(1, true));
+  StateFailure1 = setErrnoValue(StateFailure1, C, 1, Errno_Irrelevant);
+
+  ProgramStateRef StateFailure2 = State->BindExpr(
+      Call.getOriginExpr(), C.getLocationContext(), SVB.makeIntVal(2, true));
+  StateFailure2 = setErrnoValue(StateFailure2, C, 2, Errno_MustBeChecked);
+
+  C.addTransition(StateSuccess,
+                  getErrnoNoteTag(C, "Assuming that this function succeeds but "
+                                     "sets 'errno' to an unspecified value."));
+  C.addTransition(StateFailure1);
+  C.addTransition(
+      StateFailure2,
+      getErrnoNoteTag(C, "Assuming that this function returns 2. 'errno' "
+                         "should be checked to test for failure."));
+}
+
 bool ErrnoTesterChecker::evalCall(const CallEvent &Call,
                                   CheckerContext &C) const {
   const EvalFn *Fn = TestCalls.lookup(Call);
   if (Fn) {
     (*Fn)(C, Call);
     return C.isDifferent();
   }
   return false;

clang/test/Analysis/errno-notes.c

+// RUN: %clang_analyze_cc1 -verify -analyzer-output text %s \
+// RUN:   -analyzer-checker=core \
+// RUN:   -analyzer-checker=apiModeling.Errno \
+// RUN:   -analyzer-checker=debug.ExprInspection \
+// RUN:   -analyzer-checker=debug.ErrnoTest \
+// RUN:   -analyzer-checker=alpha.unix.Errno
+
+#include "Inputs/errno_var.h"
+#include "Inputs/system-header-simulator.h"
+
+int ErrnoTesterChecker_setErrnoCheckState();
+
+void something();
+
+void testErrnoCheckUndefRead() {
+  int X = ErrnoTesterChecker_setErrnoCheckState();
+  something();
+  X = ErrnoTesterChecker_setErrnoCheckState(); // expected-note{{Assuming that this function succeeds but sets 'errno' to an unspecified value}}
+  if (X == 0) {                                // expected-note{{'X' is equal to 0}}
+                                               // expected-note@-1{{Taking true branch}}
+    if (errno) {
+    } // expected-warning{{An undefined value may be read from 'errno'}}
+      // expected-note@-1{{An undefined value may be read from 'errno'}}
+  }
+}
+
+void testErrnoCheckOverwrite() {
+  int X = ErrnoTesterChecker_setErrnoCheckState();
+  something();
+  X = ErrnoTesterChecker_setErrnoCheckState(); // expected-note{{Assuming that this function returns 2. 'errno' should be checked to test for failure}}
+  if (X == 2) {                                // expected-note{{'X' is equal to 2}}
+                                               // expected-note@-1{{Taking true branch}}
+    errno = 0;                                 // expected-warning{{Value of 'errno' was not checked and is overwritten here}}
+                                               // expected-note@-1{{Value of 'errno' was not checked and is overwritten here}}
+  }
+}
+
+void testErrnoCheckOverwriteStdCall() {
+  int X = ErrnoTesterChecker_setErrnoCheckState();
+  something();
+  X = ErrnoTesterChecker_setErrnoCheckState(); // expected-note{{Assuming that this function returns 2. 'errno' should be checked to test for failure}}
+  if (X == 2) {                                // expected-note{{'X' is equal to 2}}
+                                               // expected-note@-1{{Taking true branch}}
+    printf("");                                // expected-warning{{Value of 'errno' was not checked and may be overwritten by function 'printf'}}
+                                               // expected-note@-1{{Value of 'errno' was not checked and may be overwritten by function 'printf'}}
+  }
+}

clang/test/Analysis/errno-options.c

+// RUN: %clang_analyze_cc1 -verify %s \
+// RUN:   -analyzer-checker=core \
+// RUN:   -analyzer-checker=apiModeling.Errno \
+// RUN:   -analyzer-checker=debug.ErrnoTest \
+// RUN:   -analyzer-checker=alpha.unix.Errno \
+// RUN:   -analyzer-config alpha.unix.Errno:AllowNonConditionErrnoRead=false \
+// RUN:   -DERRNO_VAR
+
+// RUN: %clang_analyze_cc1 -verify %s \
+// RUN:   -analyzer-checker=core \
+// RUN:   -analyzer-checker=apiModeling.Errno \
+// RUN:   -analyzer-checker=debug.ErrnoTest \
+// RUN:   -analyzer-checker=alpha.unix.Errno \
+// RUN:   -analyzer-config alpha.unix.Errno:AllowNonConditionErrnoRead=false \
+// RUN:   -DERRNO_FUNC
+
+#include "Inputs/system-header-simulator.h"
+#ifdef ERRNO_VAR
+#include "Inputs/errno_var.h"
+#endif
+#ifdef ERRNO_FUNC
+#include "Inputs/errno_func.h"
+#endif
+
+int ErrnoTesterChecker_setErrnoIfError();
+
+void test_cond() {
+  ErrnoTesterChecker_setErrnoIfError();
+  int A = errno ? 1 : 2;
+  // expected-warning@-1{{An undefined value may be read from 'errno'}}
+}
+
+void test_errno_store_into_variable() {
+  ErrnoTesterChecker_setErrnoIfError();
+  int a = errno;
+  // expected-warning@-1{{An undefined value may be read from 'errno'}}
+}

clang/test/Analysis/errno.c

 // RUN: %clang_analyze_cc1 -verify %s \
 // RUN:   -analyzer-checker=core \
 // RUN:   -analyzer-checker=apiModeling.Errno \
 // RUN:   -analyzer-checker=debug.ExprInspection \
 // RUN:   -analyzer-checker=debug.ErrnoTest \
+// RUN:   -analyzer-checker=alpha.unix.Errno \
 // RUN:   -DERRNO_VAR
 
 // RUN: %clang_analyze_cc1 -verify %s \
 // RUN:   -analyzer-checker=core \
 // RUN:   -analyzer-checker=apiModeling.Errno \
 // RUN:   -analyzer-checker=debug.ExprInspection \
 // RUN:   -analyzer-checker=debug.ErrnoTest \
+// RUN:   -analyzer-checker=alpha.unix.Errno \
 // RUN:   -DERRNO_FUNC
 
+#include "Inputs/system-header-simulator.h"
 #ifdef ERRNO_VAR
 #include "Inputs/errno_var.h"
 #endif
 #ifdef ERRNO_FUNC
 #include "Inputs/errno_func.h"
 #endif
 
 void clang_analyzer_eval(int);
 void ErrnoTesterChecker_setErrno(int);
 int ErrnoTesterChecker_getErrno();
 int ErrnoTesterChecker_setErrnoIfError();
 int ErrnoTesterChecker_setErrnoIfErrorRange();
+int ErrnoTesterChecker_setErrnoCheckState();
 
 void something();
 
 void test() {
   // Test if errno is initialized.
   clang_analyzer_eval(errno == 0); // expected-warning{{TRUE}}
 
   ErrnoTesterChecker_setErrno(1);

 }
 
 void testIfErrorRange() {
   if (ErrnoTesterChecker_setErrnoIfErrorRange()) {
     clang_analyzer_eval(errno != 0); // expected-warning{{TRUE}}
     clang_analyzer_eval(errno == 1); // expected-warning{{FALSE}} expected-warning{{TRUE}}
   }
 }
+
+void testErrnoCheck0() {
+  // If the function returns a success result code, value of 'errno'
+  // is unspecified and it is unsafe to make any decision with it.
+  // The function did not promise to not change 'errno' if no failure happens.
+  int X = ErrnoTesterChecker_setErrnoCheckState();
+  if (X == 0) {
+    if (errno) { // expected-warning{{An undefined value may be read from 'errno' [alpha.unix.Errno]}}
+    }
+    if (errno) { // no warning for second time (analysis stops at the first warning)
+    }
+  }
+  X = ErrnoTesterChecker_setErrnoCheckState();
+  if (X == 0) {
+    if (errno) { // expected-warning{{An undefined value may be read from 'errno' [alpha.unix.Errno]}}
+    }
+    errno = 0;
+  }
+  X = ErrnoTesterChecker_setErrnoCheckState();
+  if (X == 0) {
+    errno = 0;
+    if (errno) { // no warning after overwritten 'errno'
+    }
+  }
+}
+
+void testErrnoCheck1() {
+  // If the function returns error result code that is out-of-band (not a valid
+  // non-error return value) the value of 'errno' can be checked but it is not
+  // required to do so.
+  int X = ErrnoTesterChecker_setErrnoCheckState();
+  if (X == 1) {
+    if (errno) { // no warning
+    }
+  }
+  X = ErrnoTesterChecker_setErrnoCheckState();
+  if (X == 1) {
+    errno = 0; // no warning
+  }
+}
+
+void testErrnoCheck2() {
+  // If the function returns an in-band error result the value of 'errno' is
+  // required to be checked to verify if error happened.
+  // The same applies to other functions that can indicate failure only by
+  // change of 'errno'.
+  int X = ErrnoTesterChecker_setErrnoCheckState();
+  if (X == 2) {
+    if (errno) {
+    }
+    errno = 0; // no warning after 'errno' was read
+  }
+  X = ErrnoTesterChecker_setErrnoCheckState();
+  if (X == 2) {
+    errno = 0; // expected-warning{{Value of 'errno' was not checked and is overwritten here [alpha.unix.Errno]}}
+    errno = 0;
+  }
+  X = ErrnoTesterChecker_setErrnoCheckState();
+  if (X == 2) {
+    errno = 0; // expected-warning{{Value of 'errno' was not checked and is overwritten here [alpha.unix.Errno]}}
+    if (errno) {
+    }
+  }
+}
+
+void testErrnoCheckUndefinedLoad() {
+  int X = ErrnoTesterChecker_setErrnoCheckState();
+  if (X == 0) {
+    if (errno) {
+    }; // expected-warning{{An undefined value may be read from 'errno' [alpha.unix.Errno]}}
+  }
+}
+
+void testErrnoNotCheckedAtSystemCall() {
+  int X = ErrnoTesterChecker_setErrnoCheckState();
+  if (X == 2) {
+    printf("%i", 1); // expected-warning{{Value of 'errno' was not checked and may be overwritten by function 'printf' [alpha.unix.Errno]}}
+    printf("%i", 1); // no warning ('printf' does not change errno state)
+  }
+}
+
+void testErrnoCheckStateInvalidate() {
+  int X = ErrnoTesterChecker_setErrnoCheckState();
+  if (X == 0) {
+    something();
+    if (errno) { // no warning after an invalidating function call
+    }
+  }
+  X = ErrnoTesterChecker_setErrnoCheckState();
+  if (X == 0) {
+    printf("%i", 1);
+    if (errno) { // no warning after an invalidating standard function call
+    }
+  }
+}
+
+void testErrnoCheckStateInvalidate1() {
+  int X = ErrnoTesterChecker_setErrnoCheckState();
+  if (X == 2) {
+    clang_analyzer_eval(errno); // expected-warning{{TRUE}}
+    something();
+    clang_analyzer_eval(errno); // expected-warning{{UNKNOWN}}
+    errno = 0;                  // no warning after invalidation
+  }
+}
+
+void test_for_cond() {
+  ErrnoTesterChecker_setErrnoIfError();
+  for (; errno != 0;) {
+  }
+  // expected-warning@-1{{An undefined value may be read from 'errno'}}
+}
+
+void test_do_cond() {
+  ErrnoTesterChecker_setErrnoIfError();
+  do {
+  } while (errno != 0);
+  // expected-warning@-1{{An undefined value may be read from 'errno'}}
+}
+
+void test_while_cond() {
+  ErrnoTesterChecker_setErrnoIfError();
+  while (errno != 0) {
+  }
+  // expected-warning@-1{{An undefined value may be read from 'errno'}}
+}
+
+void test_switch_cond() {
+  ErrnoTesterChecker_setErrnoIfError();
+  switch (errno) {}
+  // expected-warning@-1{{An undefined value may be read from 'errno'}}
+}
+
+void test_conditional_cond() {
+  ErrnoTesterChecker_setErrnoIfError();
+  int A = errno ? 1 : 2;
+  // expected-warning@-1{{An undefined value may be read from 'errno'}}
+}
+
+void test_errno_store_into_variable() {
+  ErrnoTesterChecker_setErrnoIfError();
+  int a = errno; // AllowNonConditionErrnoRead is on by default, no warning
+}
+
+int f(int);
+
+void test_errno_in_condition_in_function_call() {
+  ErrnoTesterChecker_setErrnoIfError();
+  if (f(errno) != 0) {
+  }
+}