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

From: Matthias Kaehlcke
Date: Wed Jul 12 2017 - 18:20:47 EST

Hi Josh,

thanks for your prompt reply.

El Wed, Jul 12, 2017 at 05:12:42PM -0500 Josh Poimboeuf ha dit:

> On Wed, Jul 12, 2017 at 02:27:44PM -0700, Matthias Kaehlcke wrote:
> > Commit f05058c4d652 supposedly "forces a stack frame to be created before
> > the inline asm code if CONFIG_FRAME_POINTER is enabled by listing the
> > stack pointer as an output operand for the get_user() inline assembly
> > statement.". This doesn't work as intended, at least with gcc v4.9.2 and
> > x86-64 the generated code is exactly the same with and without the patch.
> > However clang adds an extra instruction that adjusts %rsp, which ends up
> > causing double faults all over the place.
> I don't think reverting it is the right approach, because that will
> still break frame pointers in certain cases.
> The original commit probably should have clarified:
> " ... forces a stack frame *if it doesn't already exist*."
> In *most* cases it will have no effect, as you saw, because users of
> get_user() tend to do other function calls beforehand, so they will have
> already saved the frame pointer before calling it.
> However, that isn't always the case. We found that certain configs
> change GCC's behavior such that, for certain get_user() call sites, the
> containing function doesn't saved the frame pointer before inserting
> get_user()'s inline asm.
> GCC completely ignores inline asm, so it has no idea that it has a call
> instruction in it. So in general, *any* inline asm with a call
> instruction needs this constraint, to force the frame pointer to be
> saved, if it hasn't already.

Thanks for the clarification!

> 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>