Re: [PATCH] bsd-user/x86_64/target_arch_thread.h: Align stack

[Warner Losh]

On Wed, Jul 31, 2024 at 5:03 PM Warner Losh <imp@bsdimp.com> wrote:

Hmmm... All platforms likely need this....

Oh, they all have it except x86, both 32 and 64-bit.... but i386 is already properly aligned (at least in the

FreeBSD kernel), so maybe that's not needed.

On Wed, Jul 31, 2024 at 4:43 PM Richard Henderson <richard.henderson@linaro.org> wrote:

On 8/1/24 00:44, Ilya Leoshkevich wrote:
> bsd-user qemu-x86_64 almost immediately dies with:
>
>      qemu: 0x4002201a68: unhandled CPU exception 0xd - aborting
>
> on FreeBSD 14.1-RELEASE. This is an instruction that requires
> alignment:
>
>      (gdb) x/i 0x4002201a68
>         0x4002201a68:        movaps %xmm0,-0x40(%rbp)
>
> and the argument is not aligned:
>
>      (gdb) p/x env->regs[5]
>      $1 = 0x822443b58
>
> A quick experiment shows that the userspace entry point expects
> misaligned rsp:
>
>      (gdb) starti
>      (gdb) p/x $rsp
>      $1 = 0x7fffffffeaa8
>
> Emulate this behavior in bsd-user.
>
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
>   bsd-user/x86_64/target_arch_thread.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/bsd-user/x86_64/target_arch_thread.h b/bsd-user/x86_64/target_arch_thread.h
> index 52c28906d6d..25233443c14 100644
> --- a/bsd-user/x86_64/target_arch_thread.h
> +++ b/bsd-user/x86_64/target_arch_thread.h
> @@ -31,7 +31,7 @@ static inline void target_thread_init(struct target_pt_regs *regs,
>       struct image_info *infop)
>   {
>       regs->rax = 0;
> -    regs->rsp = infop->start_stack;
> +    regs->rsp = (infop->start_stack & ~0xfUL) - 8;

The formula in sys/amd64/amd64/exec_machdep.c, exec_setregs is

   ((stack - 8) & ~0xful) + 8

With that,
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

Agreed.

Reviewed-by: Warner Losh <imp@bsdimp.com> 

There's a *lot* that's missing on for amd64 emulation relative to the other platforms.

Forgot to mention that I've queued this change with Richard's suggested modification.

Warner