[ASan] fail to detect a stack-use-after-scope

[zhendongsu]

Compiler Explorer: https://godbolt.org/z/GToMdvo6K

% clangtk -v
clang version 15.0.0 (https://github.com/llvm/llvm-project.git 40d2ef841b68f6b493ce88bd750a92105a2b567d)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /local/suz-local/opfuzz/bin
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/10
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/11
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6.5.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.5.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/8
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/11
Candidate multilib: .;@m64
Selected multilib: .;@m64
% 
% clangtk -O0 -fsanitize=address small.c
% ./a.out
% 
% cat small.c
int a, *b, **c = &b;
int main() {
  {
    int d = 0;
  L:
    *c = &d;
  }
  while (0)
    goto L;
  a = *b;
  return 0;
}

[luxufan]

Out of interest, I debugged this issue and I found the reason for this problem is clang doesn't emit llvm.lifetime intrinsic for scoped local variable d. Without llvm.lifetime intrinsics, addresssanitizer pass would not insert code that poisons the memory of d. https://reviews.llvm.org/D24693 This patch disabled emitting llvm.lifetime intrinsics for such a case.

[vitalybuka]

D24693 was done explicitly for that.
Without D24693 asan complains on the line "*c = &d;" because it didn't see lifetime.start

[vitalybuka]

Also keep in mind that for sanitizers if trade off is false-positive (invalid report) vs false-negative (missed report), we almost always prefer to avoid the former one.