[PATCH] D48664: [HWASan] Do not retag allocas before return from the function.

[Aleksey Shlyapnikov via Phabricator via llvm-commits]

alekseyshl created this revision.
alekseyshl added a reviewer: eugenis.
Herald added a subscriber: srhines.

Retagging allocas before returning from the function might help
detecting use after return bugs, but it does not work at all in real
life, when instrumented and non-instrumented code is intermixed.
Consider the following code:

F_non_instrumented() {

  T x;
  F1_instrumented(&x);
  ...

}

{

  F_instrumented();
  F_non_instrumented();

}

- F_instrumented call leaves the stack below the current sp tagged randomly for UAR detection
- F_non_instrumented allocates its own vars on that tagged stack, not generating any tags, that is the address of x has tag 0, but the shadow memory still contains tags left behind by F_instrumented on the previous step
- F1_instrumented verifies &x before using it and traps on tag mismatch, 0 vs whatever tag was set by F_instrumented


Repository:
  rL LLVM

https://reviews.llvm.org/D48664

Files:
  lib/Transforms/Instrumentation/HWAddressSanitizer.cpp
  test/Instrumentation/HWAddressSanitizer/alloca-with-calls.ll
  test/Instrumentation/HWAddressSanitizer/alloca.ll

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