[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!

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;

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