[compiler-rt][UBSan] silence_unsigned_overflow: do *NOT* ignore *fatal* unsigned overflows

[lebedev.ri]

Summary

D48660 / rL335762 added a silence_unsigned_overflow env flag for oss-fuzz needs,
that allows to silence the reports from unsigned overflows.
It makes sense, it is there because -fsanitize=integer sanitizer is not enabled on oss-fuzz,
so this allows to still use it as an interestingness signal, without getting the actual reports.

However there is a slight problem here.
All types of unsigned overflows are ignored.
Even if -fno-sanitize-recover=unsigned was used (which means the program will die after the report)
there will still be no report, the program will just silently die.

At the moment there are just two projects on oss-fuzz that care:

• libc++
• RawSpeed (me)

I suppose this could be overridden there ^, but i really don't think this is intended behavior in any case..

[lebedev.ri]

Adjust the help text, too.

[Dor1s]

Nice! LGTM!

[Dor1s]

Roman, I can land this CL for you next week, unless other reviewers have any complains :)

[lebedev.ri]

In D54771#1305646, @Dor1s wrote:

Nice! LGTM!

Thank you for the speedy review!

compiler-rt/trunk/lib/ubsan/ubsan_flags.inc

 UBSAN_FLAG(bool, halt_on_error, false,
            "Crash the program after printing the first error report")
 UBSAN_FLAG(bool, print_stacktrace, false,
            "Include full stacktrace into an error report")
 UBSAN_FLAG(const char *, suppressions, "", "Suppressions file name.")
 UBSAN_FLAG(bool, report_error_type, false,
         "Print specific error type instead of 'undefined-behavior' in summary.")
 UBSAN_FLAG(bool, silence_unsigned_overflow, false,
-  "Do not print error reports for unsigned integer overflow. "
-  "Used to provide fuzzing signal without blowing up logs.")
+        "Do not print non-fatal error reports for unsigned integer overflow. "
+        "Used to provide fuzzing signal without blowing up logs.")

compiler-rt/trunk/lib/ubsan/ubsan_handlers.cc

static void handleIntegerOverflowImpl(OverflowData *Data, ValueHandle LHS,
   SourceLocation Loc = Data->Loc.acquire();
   bool IsSigned = Data->Type.isSignedIntegerTy();
   ErrorType ET = IsSigned ? ErrorType::SignedIntegerOverflow
                           : ErrorType::UnsignedIntegerOverflow;
 
   if (ignoreReport(Loc, Opts, ET))
     return;
 
-  if (!IsSigned && flags()->silence_unsigned_overflow)
+  // If this is an unsigned overflow in non-fatal mode, potentially ignore it.
+  if (!IsSigned && !Opts.FromUnrecoverableHandler &&
+      flags()->silence_unsigned_overflow)
     return;
 
   ScopedReport R(Opts, Loc, ET);
 
   Diag(Loc, DL_Error, ET, "%0 integer overflow: "
                           "%1 %2 %3 cannot be represented in type %4")
       << (IsSigned ? "signed" : "unsigned") << Value(Data->Type, LHS)
       << Operator << RHS << Data->Type;

compiler-rt/trunk/test/ubsan/TestCases/Integer/no-recover.cpp

 // RUN: %clangxx -fsanitize=unsigned-integer-overflow %s -o %t && %run %t 2>&1 | FileCheck %s --check-prefix=RECOVER
 // RUN: %clangxx -fsanitize=unsigned-integer-overflow -fno-sanitize-recover=all -fsanitize-recover=unsigned-integer-overflow %s -o %t && %run %t 2>&1 | FileCheck %s --check-prefix=RECOVER
 // RUN: %env_ubsan_opts=silence_unsigned_overflow=1 %run %t 2>&1 | FileCheck %s --check-prefix=SILENT-RECOVER --allow-empty
-// RUN: %clangxx -fsanitize=unsigned-integer-overflow -fno-sanitize-recover=unsigned-integer-overflow %s -o %t && not %run %t 2>&1 | FileCheck %s --check-prefix=ABORT
+// RUN: %clangxx -fsanitize=unsigned-integer-overflow -fno-sanitize-recover=unsigned-integer-overflow %s -o %t
+// RUN: not %run %t 2>&1 | FileCheck %s --check-prefix=ABORT
+// RUN: %env_ubsan_opts=silence_unsigned_overflow=1 not %run %t 2>&1 | FileCheck %s --check-prefix=ABORT
 
 #include <stdint.h>
 
 int main() {
   // These promote to 'int'.
   (void)(uint8_t(0xff) + uint8_t(0xff));
   (void)(uint16_t(0xf0fff) + uint16_t(0x0fff));
   // RECOVER-NOT: runtime error