Re: gcc-10: kernel stack is corrupted and fails to boot

[Arnd Bergmann]

On Thu, May 14, 2020 at 7:22 AM Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote:
> On Wed, May 13, 2020 at 09:52:07PM -0700, Linus Torvalds wrote:
> > On Wed, May 13, 2020, 20:50 Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> The gcc docs [1,2] at least don't inspire much confidence that this will
> continue working with plain asm("") though:
>
> "Note that GCCâs optimizers can move asm statements relative to other
> code, including across jumps."
> ...
> "Note that the compiler can move even volatile asm instructions relative
> to other code, including across jump instructions."
>
> Even if we don't include an instruction in it I think it should at least
> have a memory clobber, to stop the compiler from deciding that it can be
> moved before the call so it can do the tail-call optimization.

I think LTO would still be able to notice that cpu_startup_entry() can
be annotated __attribute__((noreturn)) and optimize the callers
accordingly, which in turn would allow a tail call again after dead code
elimination.

     Arnd