[libc-commits] [libc] [libc] fortify jmp buffer for x86-64 (PR #112769)

[Nick Desaulniers via libc-commits]

https://github.com/nickdesaulniers commented:

oh, boy, this is getting harder and harder to follow.  This code has maximally factored any repetition via macro expansion.  While I'm a big fan of DRY (don't repeat yourself), I don't think we want that for inline asm.

I think rather than having this pattern of:

```
#ifdef __i386
...lots of #defines
#else
...lots of #defines
#endif

#ifdef LIBC_COPT_SETJMP_FORTIFICATION
...
#else
...lots of #defines
#endif

asm (
  MACRO_EXPANSION
  MACRO_EXPANSION
  MACRO_EXPANSION
  ...
```

it would be easier to maintain/read/understand if:
1. we move i386 implementations to their own file.  I split out i386 sources recently; I should have done that for jmp_buf.
2. write checksum in C and call it from inline asm, so that the checksum routine is the same between architectures.
3. perhaps have separate .cpp files selected via cmake depending on LIBC_COPT_SETJMP_FORTIFICATION.  Then we can have longjump.cpp and longjump_fortified.cpp, setjump.cpp and setjump_fortified.cpp.

With inline asm, you really want to keep it straightforward what assembler you'll get.  As is, I feel like I would need to preprocess setjmp.cpp and longjump.cpp just to fully review the sequence of instructions.

---
Please file an issue to track jmp_buf fortification for all architectures, then add a link to it in the PR description so that we can track implementing this for all other architectures.

https://github.com/llvm/llvm-project/pull/112769