Re: [PATCH] Revert "x86/uaccess: Add stack frame output operand in get_user() inline asm"

[Josh Poimboeuf]

On Wed, Jul 12, 2017 at 03:20:40PM -0700, Matthias Kaehlcke wrote:
> > This is admittedly an awkward way of achieving this goal, but it's the
> > only way I know how to do it with GCC.
> > 
> > What extra instruction does clang add?
> 
> I was looking at the get_user() call in drm_mode_setcrtc(). The code
> generated by clang without the patch is:
> 
>                         if (get_user(out_id, &set_connectors_ptr[i])) {
> ffffffff81386955:       4a 8d 04 bd 00 00 00    lea    0x0(,%r15,4),%rax
> ffffffff8138695c:       00 
> ffffffff8138695d:       49 03 06                add    (%r14),%rax
> ffffffff81386960:       e8 2b a5 f0 ff          callq  ffffffff81290e90 <__get_user_4>
> 
> And with the patch:
> 
>                         if (get_user(out_id, &set_connectors_ptr[i])) {
> ffffffff81386a56:       4a 8d 04 bd 00 00 00    lea    0x0(,%r15,4),%rax
> ffffffff81386a5d:       00 
> ffffffff81386a5e:       49 03 06                add    (%r14),%rax
> ffffffff81386a61:       48 8b 64 24 28          mov    0x28(%rsp),%rsp
> ffffffff81386a66:       e8 15 a5 f0 ff          callq
> ffffffff81290f80 <__get_user_4>

Hm, that seems odd.  Can you sure the disassembly for the whole
function?

-- 
Josh