[all-commits] [llvm/llvm-project] 1b71ea: [msan] Automatically print shadow for failing outl...

[Thurston Dang via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 1b71ea411a9d36705663b1724ececbdfec7cc98c
      https://github.com/llvm/llvm-project/commit/1b71ea411a9d36705663b1724ececbdfec7cc98c
  Author: Thurston Dang <thurston at google.com>
  Date:   2025-06-24 (Tue, 24 Jun 2025)

  Changed paths:
    M compiler-rt/lib/msan/msan.cpp
    M compiler-rt/lib/msan/msan_interface_internal.h
    A compiler-rt/test/msan/msan_print_shadow_on_outlined_check.cpp
    M llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
    M llvm/test/Instrumentation/MemorySanitizer/with-call-type-size.ll

  Log Message:
  -----------
  [msan] Automatically print shadow for failing outlined checks (#145107)

A commonly used aid for debugging MSan reports is `__msan_print_shadow()`, which requires manual app code annotations (typically of the variable in the UUM report or nearby). This is in contrast to ASan, which automatically prints out the shadow map when a check fails.

This patch changes MSan to print the shadow that failed an outlined check (checks are outlined per function after the `-msan-instrumentation-with-call-threshold` is exceeded) if verbosity >= 1. Note that we do not print out the shadow map of "neighboring" variables because this is technically infeasible; see "Caveat" below.

This patch can be easier to use than `__msan_print_shadow()` because this does not require manual app code annotations. Additionally, due to optimizations, `__msan_print_shadow()` calls can sometimes spuriously affect whether a variable is initialized.

As a side effect, this patch also enables outlined checks for arbitrary-sized shadows (vs. the current hardcoded handlers for {1,2,4,8}-byte shadows).

Caveat: the shadow does not necessarily correspond to an individual user variable, because MSan instrumentation may combine and/or truncate multiple shadows prior to emitting a check that the mangled shadow is zero (e.g., `convertShadowToScalar()`, `handleSSEVectorConvertIntrinsic()`, `materializeInstructionChecks()`). OTOH it is arguably a strength that this feature emit the shadow that directly matters for the MSan check, but which cannot be obtained using the MSan API.



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