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>