CFI: asan.module_ctor is missing a type hash

[samitolvanen]

From Mark Rutland:

Looks like CFI && KASAN is borked on both arm64 and x86; the implicitly-generated asan.module_ctor functions aren't given a type hash by the compiler, so when those get invoked by do_ctors() the calls fail

cc @kees @nickdesaulniers @nathanchance @lvwr

[samitolvanen]

The asan.module_ctor function is created in the asan LLVM pass using createSanitizerCtor. Since this happens outside of Clang, it's a bit awkward to generate a type hash for the function at this point.

The reason we didn't have this problem earlier was that do_ctors is an __init function and CFI was disabled for it. I'm leaning towards simply marking the function __nocfi instead of hardcoding a type hash for these functions or hacking something together in LLVM to compute a hash. Thoughts?

[samitolvanen]

Perhaps setting the types in LLVM is reasonable after all, as we'd have to deal with module constructors as well otherwise. Here's an alternative solution: https://reviews.llvm.org/D138945

[samitolvanen]

The remaining issue with CFI+KASAN is that asan.module_ctor still won't have a type hash in object files that were not compiled with -fsanitize=kcfi. With x86/arm64 defconfigs, it looks like only the constructor in kernel/cfi.o trips KASAN, but I don't see any reason we couldn't now enable CFI also for kernel/cfi.c.

[samitolvanen]

https://lore.kernel.org/lkml/20221222225747.3538676-1-samitolvanen@google.com/