[cfe-dev] Prefix Mapping for Paths used by Sanitizers

[Leonard Chan via cfe-dev]

Hi all,

I noticed that some asan-instrumented binaries varied in size depending on
the length of my build path despite using `-ffile-prefix-map`. I discovered
that asan bakes in the absolute path of whatever I'm compiling through the
module ID (
https://github.com/llvm/llvm-project/blob/80397d2d12b042586cb3bafdeb12ef8d982b8875/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp#L2254)
which is currently unaffected by `-ffile-prefix-map`. This means binaries
shipped in asan-instrumented toolchains, like libunwind, may vary depending
on where it's built.

>From what I see, there doesn't seem to be an existing way of indicating to
asan that I'd like to map prefixes, so I have a couple of implementation
ideas that I wanted to see if people had opinions on:

1. Have a separate flag orthogonal to `-fmacro-prefix-map` and
`-fdebug-prefix-map` that controls which paths sanitizers (or at least
asan) use. This can be named something like `-fsanitizer-prefix-map` and
would also be turned on when `-ffile-prefix-map` is used.

2. Same as 1, but we wouldn't be adding a new flag
(`-fsanitizer-prefix-map`) and the sanitizers would just use whatever value
is passed to `-ffile-prefix-map`.

3/4. Same as 1/2, but instead of catching this at sanitizer creation, we
catch this on llvm::Module creation, so anything that calls
`Module::getModuleIdentifier()` will always return a path whose prefix was
replaced according to whatever appropriate mapping.

Thanks,
Leonard
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