Re: [PATCH 3/4] x86: save user rsp in pt_regs->sp on SYSCALL64 fastpath

[Andy Lutomirski <luto@amacapital.net>]

On Mon, Mar 9, 2015 at 11:39 AM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
> PER_CPU(old_rsp) usage is simplified - now it is used only
> as temp storage, and userspace stack pointer is immediately stored
> in pt_regs->sp on syscall entry, instead of being used later,
> on syscall exit.
>
> Instead of PER_CPU(old_rsp) and task->thread.usersp, C code
> uses pt_regs->sp now.
>
> FIXUP/RESTORE_TOP_OF_STACK are simplified.
>
> Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
> CC: Linus Torvalds <torvalds@linux-foundation.org>
> CC: Steven Rostedt <rostedt@goodmis.org>
> CC: Ingo Molnar <mingo@kernel.org>
> CC: Borislav Petkov <bp@alien8.de>
> CC: "H. Peter Anvin" <hpa@zytor.com>
> CC: Andy Lutomirski <luto@amacapital.net>
> CC: Oleg Nesterov <oleg@redhat.com>
> CC: Frederic Weisbecker <fweisbec@gmail.com>
> CC: Alexei Starovoitov <ast@plumgrid.com>
> CC: Will Drewry <wad@chromium.org>
> CC: Kees Cook <keescook@chromium.org>
> CC: x86@kernel.org
> CC: linux-kernel@vger.kernel.org

Looks correct.

> @@ -253,11 +247,13 @@ GLOBAL(system_call_after_swapgs)
>          */
>         ENABLE_INTERRUPTS(CLBR_NONE)
>         ALLOC_PT_GPREGS_ON_STACK 8              /* +8: space for orig_ax */
> +       movq    %rcx,RIP(%rsp)
> +       movq    PER_CPU_VAR(old_rsp),%rcx
> +       movq    %r11,EFLAGS(%rsp)
> +       movq    %rcx,RSP(%rsp)
> +       movq_cfi rax,ORIG_RAX
>         SAVE_C_REGS_EXCEPT_RAX_RCX_R11
>         movq    $-ENOSYS,RAX(%rsp)
> -       movq_cfi rax,ORIG_RAX
> -       movq    %r11,EFLAGS(%rsp)
> -       movq    %rcx,RIP(%rsp)

Why the reordering?

--Andy