Re: [PATCH v5] x86: use builtins to read eflags

[Linus Torvalds <torvalds@linux-foundation.org>]

On Fri, Mar 18, 2022 at 10:59 AM Andy Lutomirski <luto@kernel.org> wrote:
>
> I generally agree.  In this particular case, though, I will keep using
> the builtin in tools/testing/selftests/x86/helpers.h unless we actually
> hit breakage.  This is because this is *user* code, it is compiled with
> the redzone enabled, and the asm code to do the right thing when the
> redzone is enabled is too hairy for me to want to deal with it.

Yeah, redzoning is a problem for "pushf".

Or rather, it's not the redzoning itself, but the fact that the
compiler might use the word under the stack for random other things,
and the pushf will then corrupt some local variable storage.

I think it would be lovely to solve that in inline asm itself some way
- by marking the stack pointer clobbered or something.

Because you have the same issue if an inline asm might need to do a
function call - think magic calling conventions etc, but also possibly
slow-path cases.

As mentioned, it's not an issue for the kernel proper due to
-mno-red-zone which we need for entirely unrelated reasons.

Side note and kind of related: we do have this in the kernel:

  register unsigned long current_stack_pointer asm(_ASM_SP);
  #define ASM_CALL_CONSTRAINT "+r" (current_stack_pointer)

which *might* also solve the redzoning issue.

In the kernel we need it not because of redzoned stack use, but
because we need the stack frame to be set up properly or objtool
complains.

             Linus