[libc] initialize __stack_chk_guard during startup using AT_RANDOM

[nickdesaulniers]

Forked from #121121. For linux, we should be able to use the value from AT_RANDOM to better initialize the value used for __stack_chk_guard. cc @RossComputerGuy @SchrodingerZhu

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

Forked from #121121. For linux, we should be able to use the value from `AT_RANDOM` to better initialize the value used for `__stack_chk_guard`. cc @RossComputerGuy @SchrodingerZhu

[SchrodingerZhu]

For Linux, it is already initialized via sys_getrandom. This is true at least for stack canary in TLS.

I thought the generic targets are different from Linux?

[nickdesaulniers]

If you grep for __stack_chk_guard in the code base, the ONLY definition is in the newly added src/compiler/generic/__stack_chk_fail.cpp.

Yet if you build llvm-libc on linux, and run:

$ llvm-nm libc/lib/libc.a | grep __stack_chk_guard
0000000000000000 D __stack_chk_guard

so maybe we use that convention of linux vs non-linx==generic elsewhere via some cmake magic, which isn't wired up for src/compiler/??

[SchrodingerZhu]

Actually I think it is good to provide such symbols on Linux. Even for x86, I remember it is configurable via driver cmdopts to use either global or tls stack canary. Therefore, such symbol should still be provided.

[nickdesaulniers]

So if it's thread local, should it be declared as such? How would you support global or thread local based on how the user has compiled their code?

[SchrodingerZhu]

So if it's thread local, should it be declared as such?

No, the global one and the TLS one should be separated -- they are two different checking conventions. On x86, the TLS canary is always FS plus a fixed offset.

How would you support global or thread local based on how the user has compiled their code?

Just providing both definitions. Then programs can either spill the global value onto stack or the TLS value.

mov __stack_chk_guard, (AFTER_RA) %sp 
;; or
mov %fs:48, (AFTER_RA) %sp