Add an unsigned shift base sanitizer

[jfb]

Summary

It's not undefined behavior for an unsigned left shift to overflow (i.e. to
shift bits out), but it has been the source of bugs and exploits in certain
codebases in the past. As we do in other parts of UBSan, this patch adds a
dynamic checker which acts beyond UBSan and checks other sources of errors. The
option is enabled as part of -fsanitize=integer.

The flag is named: -fsanitize=unsigned-shift-base
This matches shift-base and shift-exponent flags.

rdar://problem/46129047

[vsk]

It'd be nice to fold the new check into an existing sanitizer group to bring this to a wider audience. Do you foresee adoption issues for existing -fsanitize=integer adopters? Fwiw some recently-added implicit conversion checks were folded in without much/any pushback.

clang/test/Driver/fsanitize.c

911

Not sure I follow this one. Why is 'NORECOVER' not expecting to see "-fno-sanitize-recover=unsigned-shift-base"?

[jfb]

In D86000#2219288, @vsk wrote:

It'd be nice to fold the new check into an existing sanitizer group to bring this to a wider audience. Do you foresee adoption issues for existing -fsanitize=integer adopters? Fwiw some recently-added implicit conversion checks were folded in without much/any pushback.

integer does "not actually UB checks", right? I can certainly put it in there if you think I won't get yelled at 😄

clang/test/Driver/fsanitize.c

911

I have no idea! Other parts of this file do this:

// CHECK-implicit-conversion-NORECOVER-NOT: "-fno-sanitize-recover={{((implicit-unsigned-integer-truncation|implicit-signed-integer-truncation|implicit-integer-sign-change),?){3}"}} // ???
// CHECK-implicit-integer-arithmetic-value-change-NORECOVER-NOT: "-fno-sanitize-recover={{((implicit-signed-integer-truncation|implicit-integer-sign-change),?){2}"}} // ???
// CHECK-implicit-integer-truncation-NORECOVER-NOT: "-fno-sanitize-recover={{((implicit-unsigned-integer-truncation|implicit-signed-integer-truncation),?){2}"}} // ???

I was hoping someone who's touched this before would know.

compiler-rt/test/ubsan/TestCases/Integer/unsigned-shift.cpp

3

I don't understand this test... when I run it with lit it fails (and it seems like the bots agree), but manually it works. Am I doing it wrong?

[vsk]

clang/test/Driver/fsanitize.c

911

The CHECK-implicit-conversion-NORECOVER-NOT regex looks fishy. There's more parens in there than I'd expect: perhaps that explains why the test passes?

Just above your change, I see something that's closer to what I expect:

// RUN: %clang -fsanitize=float-divide-by-zero %s -fno-sanitize-recover=float-divide-by-zero -### 2>&1 | FileCheck %s --check-prefixes=CHECK-DIVBYZERO,CHECK-DIVBYZERO-NORECOVER
...
// CHECK-DIVBYZERO-NORECOVER-NOT: "-fsanitize-recover=float-divide-by-zero"

compiler-rt/test/ubsan/TestCases/Integer/unsigned-shift.cpp

3

Do you need to explicitly return 1 or something to get a non-zero exit code?

7

Does the test not work without the volatiles?

[vsk]

In D86000#2219322, @jfb wrote:

In D86000#2219288, @vsk wrote:

It'd be nice to fold the new check into an existing sanitizer group to bring this to a wider audience. Do you foresee adoption issues for existing -fsanitize=integer adopters? Fwiw some recently-added implicit conversion checks were folded in without much/any pushback.

integer does "not actually UB checks", right? I can certainly put it in there if you think I won't get yelled at 😄

Can't guarantee you won't get yelled at, but it does seem like the natural fit. It already includes other unsigned overflow checks.

[eugenis]

In D86000#2219322, @jfb wrote:

In D86000#2219288, @vsk wrote:

It'd be nice to fold the new check into an existing sanitizer group to bring this to a wider audience. Do you foresee adoption issues for existing -fsanitize=integer adopters? Fwiw some recently-added implicit conversion checks were folded in without much/any pushback.

integer does "not actually UB checks", right? I can certainly put it in there if you think I won't get yelled at 😄

I'd support this.
"integer" includes unsigned-integer-overflow, it's only recommended to the truly fearless developers :)

clang/test/Driver/fsanitize.c

911

I think that + the following line mean that the frontend depends on the default behavior (neither recover nor no-recover).

[lebedev.ri]

What's next, a sanitizer that input to signed right-shift is always non-negative? :)

FWIW "fixing" these "bugs" via (x & ~(~1U << (32-shamt))) << shamt is going to be fine,
i've already taught instcombine to drop such pointless masking before left-shift
in PR42563: https://godbolt.org/z/5rar14

[jfb]

As Vedant suggested, make it part of 'integer' sanitizer.

[jfb]

Re-upload with full context.

[lebedev.ri]

Release notes missing.
I think the canonical way to silence it (via masking?) should be at least mentioned.

clang/lib/CodeGen/CGExprScalar.cpp

3872–3884

Why is this so complicated? Shouldn't this just be: https://alive2.llvm.org/ce/z/scTqfX

$ /repositories/alive2/build-Clang-release/alive-tv /tmp/test.ll --smt-to=100000 --disable-undef-input

----------------------------------------
@2 = global 32 bytes, align 16

define i32 @src(i32 %arg, i32 %arg1) {
%bb:
  %i = icmp ugt i32 %arg1, 31
  %i2 = sub nsw nuw i32 31, %arg1    ; NOPE
  %i3 = lshr i32 %arg, %i2           ; NOPE
  %i4 = icmp ult i32 %i3, 2          ; NOPE
  %i5 = or i1 %i, %i4
  br i1 %i5, label %bb9, label %bb6

%bb6:
  %i7 = zext i32 %arg to i64
  %i8 = zext i32 %arg1 to i64
  %__constexpr_0 = bitcast * @2 to *
  call void @__ubsan_handle_shift_out_of_bounds(* %__constexpr_0, i64 %i7, i64 %i8)
  br label %bb9

%bb9:
  %i10 = shl i32 %arg, %arg1
  ret i32 %i10
}
=>
@2 = global 32 bytes, align 16

define i32 @tgt(i32 %arg, i32 %arg1) {
%bb:
  %i = icmp ugt i32 %arg1, 31
  %iZZ0 = shl i32 %arg, %arg1        ; HI!
  %iZZ1 = lshr i32 %iZZ0, %arg1      ; OVER HERE
  %i4 = icmp eq i32 %arg, %iZZ1      ; LOOK!
  %i5 = or i1 %i, %i4
  br i1 %i5, label %bb9, label %bb6

%bb6:
  %i7 = zext i32 %arg to i64
  %i8 = zext i32 %arg1 to i64
  %__constexpr_0 = bitcast * @2 to *
  call void @__ubsan_handle_shift_out_of_bounds(* %__constexpr_0, i64 %i7, i64 %i8)
  br label %bb9

%bb9:
  ret i32 %iZZ0
}
Transformation seems to be correct!

which will then be migrated to use @llvm.ushl.with.overflow once it's there.

[jfb]

Address comments

[jfb]

clang/lib/CodeGen/CGExprScalar.cpp

3872–3884

Sure, but that's pre-existing and I'd rather not change it in this patch.

compiler-rt/test/ubsan/TestCases/Integer/unsigned-shift.cpp

3

That should be implicit, but I added it regardless to make sure. It's happy now 🤷‍♂️

7

It seems that LLVM sees too much without volatile, yes.

[lebedev.ri]

llvm/docs/ReleaseNotes.rst

153

Surely not ~1U.

[jfb]

Fix notes.

llvm/docs/ReleaseNotes.rst

153

Indeed, fixed.

[vsk]

compiler-rt/test/ubsan/TestCases/Integer/unsigned-shift.cpp

7

The optimizer isn't running. Perhaps this is necessary because clang's constant folder kicks in?

llvm/docs/ReleaseNotes.rst

151

This could use some more explanation, maybe s/with masking/by clearing bits that would be shifted out/?

153

I don't think this pattern works when rhs = 0 (https://godbolt.org/z/rvEGqh).

[lebedev.ri]

llvm/docs/ReleaseNotes.rst

153

Surely not 1U either :)

[jfb]

Remove the "suppress this" in release notes.

compiler-rt/test/ubsan/TestCases/Integer/unsigned-shift.cpp

7

Probably, I haven't investigate. It's brittle is the main thing, and this forces it to not be.

llvm/docs/ReleaseNotes.rst

151

I just dropped it, I don't think release notes are the right place for this anyways.

153

Right, I don't think it's something we want to explain here.

[vsk]

Lgtm, thanks.

clang/docs/UndefinedBehaviorSanitizer.rst

-  ``-fsanitize=returns-nonnull-attribute``: Returning null pointer
      from a function which is declared to never return null.
   -  ``-fsanitize=shift``: Shift operators where the amount shifted is
      greater or equal to the promoted bit-width of the left hand side
      or less than zero, or where the left hand side is negative. For a
      signed left shift, also checks for signed overflow in C, and for
      unsigned overflow in C++. You can use ``-fsanitize=shift-base`` or
      ``-fsanitize=shift-exponent`` to check only left-hand side or
      right-hand side of shift operation, respectively.
+  -  ``-fsanitize=unsigned-shift-base``: check that an unsigned left-hand side of
+     a left shift operation doesn't overflow.
   -  ``-fsanitize=signed-integer-overflow``: Signed integer overflow, where the
      result of a signed integer computation cannot be represented in its type.
      This includes all the checks covered by ``-ftrapv``, as well as checks for
      signed division overflow (``INT_MIN/-1``), but not checks for
      lossy implicit conversions performed before the computation
      (see ``-fsanitize=implicit-conversion``). Both of these two issues are
      handled by ``-fsanitize=implicit-conversion`` group of checks.
   -  ``-fsanitize=unreachable``: If control flow reaches an unreachable

clang/include/clang/Basic/Sanitizers.def

 SANITIZER_GROUP("shift", Shift, ShiftBase | ShiftExponent)
 SANITIZER("signed-integer-overflow", SignedIntegerOverflow)
 SANITIZER("unreachable", Unreachable)
 SANITIZER("vla-bound", VLABound)
 SANITIZER("vptr", Vptr)
 
 // IntegerSanitizer
 SANITIZER("unsigned-integer-overflow", UnsignedIntegerOverflow)
+SANITIZER("unsigned-shift-base", UnsignedShiftBase)
 
 // DataFlowSanitizer
 SANITIZER("dataflow", DataFlow)
 
 // Control Flow Integrity
 SANITIZER("cfi-cast-strict", CFICastStrict)
 SANITIZER("cfi-derived-cast", CFIDerivedCast)
 SANITIZER("cfi-icall", CFIICall)

 //                ImplicitIntegerConversion)
 
 SANITIZER_GROUP("implicit-conversion", ImplicitConversion,
                 ImplicitIntegerArithmeticValueChange |
                     ImplicitUnsignedIntegerTruncation)
 
 SANITIZER_GROUP("integer", Integer,
                 ImplicitConversion | IntegerDivideByZero | Shift |
-                    SignedIntegerOverflow | UnsignedIntegerOverflow)
+                    SignedIntegerOverflow | UnsignedIntegerOverflow |
+                    UnsignedShiftBase)
 
 SANITIZER("local-bounds", LocalBounds)
 SANITIZER_GROUP("bounds", Bounds, ArrayBounds | LocalBounds)
 
 // Scudo hardened allocator
 SANITIZER("scudo", Scudo)
 
 // Magic group, containing all sanitizers. For example, "-fno-sanitize=all"
 // can be used to disable all the sanitizers.
 SANITIZER_GROUP("all", All, ~SanitizerMask())
 
 #undef SANITIZER
 #undef SANITIZER_GROUP

clang/lib/CodeGen/CGExprScalar.cpp

 
 Value *ScalarExprEmitter::EmitShl(const BinOpInfo &Ops) {
   // LLVM requires the LHS and RHS to be the same type: promote or truncate the
   // RHS to the same size as the LHS.
   Value *RHS = Ops.RHS;
   if (Ops.LHS->getType() != RHS->getType())
     RHS = Builder.CreateIntCast(RHS, Ops.LHS->getType(), false, "sh_prom");
 
-  bool SanitizeBase = CGF.SanOpts.has(SanitizerKind::ShiftBase) &&
+  bool SanitizeSignedBase = CGF.SanOpts.has(SanitizerKind::ShiftBase) &&
                             Ops.Ty->hasSignedIntegerRepresentation() &&
                             !CGF.getLangOpts().isSignedOverflowDefined() &&
                             !CGF.getLangOpts().CPlusPlus20;
+  bool SanitizeUnsignedBase =
+      CGF.SanOpts.has(SanitizerKind::UnsignedShiftBase) &&
+      Ops.Ty->hasUnsignedIntegerRepresentation();
+  bool SanitizeBase = SanitizeSignedBase || SanitizeUnsignedBase;
   bool SanitizeExponent = CGF.SanOpts.has(SanitizerKind::ShiftExponent);
   // OpenCL 6.3j: shift values are effectively % word size of LHS.
   if (CGF.getLangOpts().OpenCL)
     RHS = ConstrainShiftValue(Ops.LHS, RHS, "shl.mask");
   else if ((SanitizeBase || SanitizeExponent) &&
            isa<llvm::IntegerType>(Ops.LHS->getType())) {
     CodeGenFunction::SanitizerScope SanScope(&CGF);
     SmallVector<std::pair<Value *, SanitizerMask>, 2> Checks;
if (SanitizeBase) {
       llvm::BasicBlock *Orig = Builder.GetInsertBlock();
       llvm::BasicBlock *Cont = CGF.createBasicBlock("cont");
       llvm::BasicBlock *CheckShiftBase = CGF.createBasicBlock("check");
       Builder.CreateCondBr(ValidExponent, CheckShiftBase, Cont);
       llvm::Value *PromotedWidthMinusOne =
           (RHS == Ops.RHS) ? WidthMinusOne
                            : GetWidthMinusOneValue(Ops.LHS, RHS);
       CGF.EmitBlock(CheckShiftBase);
       llvm::Value *BitsShiftedOff = Builder.CreateLShr(
           Ops.LHS, Builder.CreateSub(PromotedWidthMinusOne, RHS, "shl.zeros",
                                      /*NUW*/ true, /*NSW*/ true),
           "shl.check");
-      if (CGF.getLangOpts().CPlusPlus) {
+      if (SanitizeUnsignedBase || CGF.getLangOpts().CPlusPlus) {
         // In C99, we are not permitted to shift a 1 bit into the sign bit.
         // Under C++11's rules, shifting a 1 bit into the sign bit is
         // OK, but shifting a 1 bit out of it is not. (C89 and C++03 don't
         // define signed left shifts, so we use the C99 and C++11 rules there).
+        // Unsigned shifts can always shift into the top bit.
         llvm::Value *One = llvm::ConstantInt::get(BitsShiftedOff->getType(), 1);
         BitsShiftedOff = Builder.CreateLShr(BitsShiftedOff, One);
       }
       llvm::Value *Zero = llvm::ConstantInt::get(BitsShiftedOff->getType(), 0);
       llvm::Value *ValidBase = Builder.CreateICmpEQ(BitsShiftedOff, Zero);
       CGF.EmitBlock(Cont);
       llvm::PHINode *BaseCheck = Builder.CreatePHI(ValidBase->getType(), 2);
       BaseCheck->addIncoming(Builder.getTrue(), Orig);
       BaseCheck->addIncoming(ValidBase, CheckShiftBase);
-      Checks.push_back(std::make_pair(BaseCheck, SanitizerKind::ShiftBase));
+      Checks.push_back(std::make_pair(
+          BaseCheck, SanitizeSignedBase ? SanitizerKind::ShiftBase
+                                        : SanitizerKind::UnsignedShiftBase));
     }
 
     assert(!Checks.empty());
     EmitBinOpCheck(Checks, Ops);
   }
 
   return Builder.CreateShl(Ops.LHS, RHS, "shl");
 }

clang/lib/Driver/ToolChain.cpp

bool ToolChain::addFastMathRuntimeIfAvailable(const ArgList &Args,
 
   return false;
 }
 
 SanitizerMask ToolChain::getSupportedSanitizers() const {
   // Return sanitizers which don't require runtime support and are not
   // platform dependent.
 
-  SanitizerMask Res = (SanitizerKind::Undefined & ~SanitizerKind::Vptr &
-                       ~SanitizerKind::Function) |
-                      (SanitizerKind::CFI & ~SanitizerKind::CFIICall) |
-                      SanitizerKind::CFICastStrict |
-                      SanitizerKind::FloatDivideByZero |
-                      SanitizerKind::UnsignedIntegerOverflow |
-                      SanitizerKind::ImplicitConversion |
-                      SanitizerKind::Nullability | SanitizerKind::LocalBounds;
+  SanitizerMask Res =
+      (SanitizerKind::Undefined & ~SanitizerKind::Vptr &
+      SanitizerKind::CFICastStrict | SanitizerKind::FloatDivideByZero |
+      SanitizerKind::UnsignedShiftBase | SanitizerKind::ImplicitConversion |
   if (getTriple().getArch() == llvm::Triple::x86 ||
       getTriple().getArch() == llvm::Triple::x86_64 ||
       getTriple().getArch() == llvm::Triple::arm || getTriple().isWasm() ||
       getTriple().isAArch64())
     Res |= SanitizerKind::CFIICall;
   if (getTriple().getArch() == llvm::Triple::x86_64 || getTriple().isAArch64())
     Res |= SanitizerKind::ShadowCallStack;
   if (getTriple().isAArch64())

clang/test/CodeGen/unsigned-shift-base.c

+// RUN: %clang_cc1 -triple x86_64-apple-darwin10 -fsanitize=unsigned-shift-base,shift-exponent %s -emit-llvm -o - | FileCheck %s
+
+// CHECK-LABEL: lsh_overflow(
+unsigned lsh_overflow(unsigned a, unsigned b) {
+  // CHECK: %[[RHS_INBOUNDS:.*]] = icmp ule i32 %[[RHS:.*]], 31
+  // CHECK-NEXT: br i1 %[[RHS_INBOUNDS]], label %[[CHECK_BB:.*]], label %[[CONT_BB:.*]],
+
+  // CHECK:      [[CHECK_BB]]:
+  // CHECK-NEXT: %[[SHIFTED_OUT_WIDTH:.*]] = sub nuw nsw i32 31, %[[RHS]]
+  // CHECK-NEXT: %[[SHIFTED_OUT:.*]] = lshr i32 %[[LHS:.*]], %[[SHIFTED_OUT_WIDTH]]
+
+  // CHECK-NEXT: %[[SHIFTED_OUT_NOT_SIGN:.*]] = lshr i32 %[[SHIFTED_OUT]], 1
+
+  // CHECK-NEXT: %[[NO_OVERFLOW:.*]] = icmp eq i32 %[[SHIFTED_OUT_NOT_SIGN]], 0
+  // CHECK-NEXT: br label %[[CONT_BB]]
+
+  // CHECK:      [[CONT_BB]]:
+  // CHECK-NEXT: %[[VALID_BASE:.*]] = phi i1 [ true, {{.*}} ], [ %[[NO_OVERFLOW]], %[[CHECK_BB]] ]
+  // CHECK-NEXT: %[[VALID:.*]] = and i1 %[[RHS_INBOUNDS]], %[[VALID_BASE]]
+  // CHECK-NEXT: br i1 %[[VALID]]
+
+  // CHECK: call void @__ubsan_handle_shift_out_of_bounds
+  // CHECK-NOT: call void @__ubsan_handle_shift_out_of_bounds
+
+  // CHECK: %[[RET:.*]] = shl i32 %[[LHS]], %[[RHS]]
+  // CHECK-NEXT: ret i32 %[[RET]]
+  return a << b;
+}

clang/test/Driver/fsanitize.c

 // CHECK-UNDEFINED-WIN-SAME: "-fsanitize={{((signed-integer-overflow|integer-divide-by-zero|shift-base|shift-exponent|unreachable|return|vla-bound|alignment|null|pointer-overflow|float-cast-overflow|array-bounds|enum|bool|builtin|returns-nonnull-attribute|nonnull-attribute),?){17}"}}
 
 // RUN: %clang -target i386-pc-win32 -fsanitize-coverage=bb %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-COVERAGE-WIN32
 // CHECK-COVERAGE-WIN32: "--dependent-lib={{[^"]*}}ubsan_standalone-i386.lib"
 // RUN: %clang -target x86_64-pc-win32 -fsanitize-coverage=bb %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-COVERAGE-WIN64
 // CHECK-COVERAGE-WIN64: "--dependent-lib={{[^"]*}}ubsan_standalone-x86_64.lib"
 
 // RUN: %clang -target %itanium_abi_triple -fsanitize=integer %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-INTEGER -implicit-check-not="-fsanitize-address-use-after-scope"
-// CHECK-INTEGER: "-fsanitize={{((signed-integer-overflow|unsigned-integer-overflow|integer-divide-by-zero|shift-base|shift-exponent|implicit-unsigned-integer-truncation|implicit-signed-integer-truncation|implicit-integer-sign-change),?){8}"}}
+// CHECK-INTEGER: "-fsanitize={{((signed-integer-overflow|unsigned-integer-overflow|integer-divide-by-zero|shift-base|shift-exponent|implicit-unsigned-integer-truncation|implicit-signed-integer-truncation|implicit-integer-sign-change|unsigned-shift-base),?){9}"}}
 
 // RUN: %clang -fsanitize=implicit-conversion %s -### 2>&1 | FileCheck %s --check-prefixes=CHECK-implicit-conversion,CHECK-implicit-conversion-RECOVER
 // RUN: %clang -fsanitize=implicit-conversion -fsanitize-recover=implicit-conversion %s -### 2>&1 | FileCheck %s --check-prefixes=CHECK-implicit-conversion,CHECK-implicit-conversion-RECOVER
 // RUN: %clang -fsanitize=implicit-conversion -fno-sanitize-recover=implicit-conversion %s -### 2>&1 | FileCheck %s --check-prefixes=CHECK-implicit-conversion,CHECK-implicit-conversion-NORECOVER
 // RUN: %clang -fsanitize=implicit-conversion -fsanitize-trap=implicit-conversion %s -### 2>&1 | FileCheck %s --check-prefixes=CHECK-implicit-conversion,CHECK-implicit-conversion-TRAP
 // CHECK-implicit-conversion: "-fsanitize={{((implicit-unsigned-integer-truncation|implicit-signed-integer-truncation|implicit-integer-sign-change),?){3}"}}
 // CHECK-implicit-conversion-RECOVER: "-fsanitize-recover={{((implicit-unsigned-integer-truncation|implicit-signed-integer-truncation|implicit-integer-sign-change),?){3}"}}
 // CHECK-implicit-conversion-RECOVER-NOT: "-fno-sanitize-recover={{((implicit-unsigned-integer-truncation|implicit-signed-integer-truncation|implicit-integer-sign-change),?){3}"}}

 // CHECK-DIVBYZERO-RECOVER: "-fsanitize-recover=float-divide-by-zero"
 // CHECK-DIVBYZERO-NORECOVER-NOT: "-fsanitize-recover=float-divide-by-zero"
 // CHECK-DIVBYZERO-TRAP: "-fsanitize-trap=float-divide-by-zero"
 
 // RUN: %clang -fsanitize=float-divide-by-zero -fsanitize-minimal-runtime %s -### 2>&1 | FileCheck %s --check-prefixes=CHECK-DIVBYZERO,CHECK-DIVBYZERO-MINIMAL
 // CHECK-DIVBYZERO-MINIMAL: "-fsanitize-minimal-runtime"
 
 // RUN: %clang -fsanitize=undefined,float-divide-by-zero %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-DIVBYZERO-UBSAN
 // CHECK-DIVBYZERO-UBSAN: "-fsanitize={{.*}},float-divide-by-zero,{{.*}}"

compiler-rt/test/ubsan/TestCases/Integer/unsigned-shift.cpp

+// RUN: %clangxx -fsanitize=unsigned-shift-base %s -o %t1 && not %run %t1 2>&1 | FileCheck %s
+// RUN: %clangxx -fsanitize=unsigned-shift-base,shift-exponent %s -o %t1 && not %run %t1 2>&1 | FileCheck %s
+
+#define shift(val, amount) ({      \
+  volatile unsigned _v = (val);    \
+  volatile unsigned _a = (amount); \
+  unsigned res = _v << _a;         \
+  res;                             \
+})
+
+int main() {
+
+  shift(0b00000000'00000000'00000000'00000000, 31);
+  shift(0b00000000'00000000'00000000'00000001, 31);
+  shift(0b00000000'00000000'00000000'00000010, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 2 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000000'00000000'00000000'00000100, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 4 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000000'00000000'00000000'00001000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 8 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000000'00000000'00000000'00010000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 16 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000000'00000000'00000000'00100000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 32 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000000'00000000'00000000'01000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 64 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000000'00000000'00000000'10000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 128 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000000'00000000'00000001'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 256 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000000'00000000'00000010'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 512 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000000'00000000'00000100'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 1024 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000000'00000000'00001000'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 2048 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000000'00000000'00010000'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 4096 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000000'00000000'00100000'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 8192 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000000'00000000'01000000'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 16384 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000000'00000000'10000000'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 32768 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000000'00000001'00000000'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 65536 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000000'00000010'00000000'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 131072 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000000'00000100'00000000'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 262144 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000000'00001000'00000000'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 524288 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000000'00010000'00000000'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 1048576 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000000'00100000'00000000'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 2097152 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000000'01000000'00000000'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 4194304 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000000'10000000'00000000'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 8388608 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000001'00000000'00000000'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 16777216 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000010'00000000'00000000'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 33554432 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00000100'00000000'00000000'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 67108864 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00001000'00000000'00000000'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 134217728 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00010000'00000000'00000000'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 268435456 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b00100000'00000000'00000000'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 536870912 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b01000000'00000000'00000000'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 1073741824 by 31 places cannot be represented in type 'unsigned int'
+  shift(0b10000000'00000000'00000000'00000000, 31); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 2147483648 by 31 places cannot be represented in type 'unsigned int'
+
+  shift(0b10000000'00000000'00000000'00000000, 00);
+  shift(0b10000000'00000000'00000000'00000000, 01); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 2147483648 by 1 places cannot be represented in type 'unsigned int'
+
+  shift(0xffff'ffff, 0);
+  shift(0xffff'ffff, 1); // CHECK: unsigned-shift.cpp:[[@LINE]]:3: runtime error: left shift of 4294967295 by 1 places cannot be represented in type 'unsigned int'
+
+  return 1;
+}

llvm/docs/ReleaseNotes.rst

 
 * llvm-readobj and llvm-readelf behavior has changed to report an error when
   executed with no input files instead of reading an input from stdin.
   Reading from stdin can still be achieved by specifying `-` as an input file.
 
 Changes to LLDB
 ---------------------------------
 
+Changes to Sanitizers
+---------------------
+
+The integer sanitizer `-fsanitize=integer` now has a new sanitizer:
+`-fsanitize=unsigned-shift-base`. It's not undefined behavior for an unsigned
+left shift to overflow (i.e. to shift bits out), but it has been the source of
+bugs and exploits in certain codebases in the past.
+
+One can silence the check with masking, for example:
+
+  `(lhs & ~(~1U << ((sizeof(lhs)*CHAR_BIT) - rhs))) << rhs`
+
 External Open Source Projects Using LLVM 12
 ===========================================
 
 * A project...
 
 
 Additional Information
 ======================