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

[Warner Losh <imp@bsdimp.com>]

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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

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