Re: [PATCH v1 1/8] x86/entry/clearregs: Remove partial stack frame in fast system call

[Brian Gerst]

On Tue, Jan 9, 2018 at 8:03 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
>
> Remove the partial stack frame in the 64bit syscall fast path.
> In the next patch we want to clear the extra registers, which requires
> to always save all registers. So remove the partial stack frame
> in the syscall fast path and always save everything.
>
> This actually simplifies the code because the ptregs stubs
> are not needed anymore.
>
> arch/x86/entry/entry_64.S   | 57 ++++-----------------------------------------------------
> arch/x86/entry/syscall_64.c |  2 +-
>
> Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/entry/entry_64.S   | 57 ++++-----------------------------------------
>  arch/x86/entry/syscall_64.c |  2 +-
>  2 files changed, 5 insertions(+), 54 deletions(-)
>
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 58dbf7a12a05..bbdfbdd817d6 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -234,7 +234,9 @@ GLOBAL(entry_SYSCALL_64_after_hwframe)
>         pushq   %r9                             /* pt_regs->r9 */
>         pushq   %r10                            /* pt_regs->r10 */
>         pushq   %r11                            /* pt_regs->r11 */
> -       sub     $(6*8), %rsp                    /* pt_regs->bp, bx, r12-15 not saved */
> +       sub     $(6*8), %rsp
> +       SAVE_EXTRA_REGS
> +

Continue using pushes here

>         UNWIND_HINT_REGS extra=0
>
>         /*
> @@ -262,11 +264,6 @@ entry_SYSCALL_64_fastpath:
>         ja      1f                              /* return -ENOSYS (already in pt_regs->ax) */
>         movq    %r10, %rcx
>
> -       /*
> -        * This call instruction is handled specially in stub_ptregs_64.
> -        * It might end up jumping to the slow path.  If it jumps, RAX
> -        * and all argument registers are clobbered.
> -        */
>  #ifdef CONFIG_RETPOLINE
>         movq    sys_call_table(, %rax, 8), %rax
>         call    __x86_indirect_thunk_rax
> @@ -293,9 +290,7 @@ entry_SYSCALL_64_fastpath:
>         TRACE_IRQS_ON           /* user mode is traced as IRQs on */
>         movq    RIP(%rsp), %rcx
>         movq    EFLAGS(%rsp), %r11
> -       addq    $6*8, %rsp      /* skip extra regs -- they were preserved */
> -       UNWIND_HINT_EMPTY
> -       jmp     .Lpop_c_regs_except_rcx_r11_and_sysret
> +       jmp     syscall_return_via_sysret
>
>  1:
>         /*
> @@ -305,14 +300,12 @@ entry_SYSCALL_64_fastpath:
>          */
>         TRACE_IRQS_ON
>         ENABLE_INTERRUPTS(CLBR_ANY)
> -       SAVE_EXTRA_REGS
>         movq    %rsp, %rdi
>         call    syscall_return_slowpath /* returns with IRQs disabled */
>         jmp     return_from_SYSCALL_64
>
>  entry_SYSCALL64_slow_path:
>         /* IRQs are off. */
> -       SAVE_EXTRA_REGS
>         movq    %rsp, %rdi
>         call    do_syscall_64           /* returns with IRQs disabled */
>
> @@ -389,7 +382,6 @@ syscall_return_via_sysret:
>         /* rcx and r11 are already restored (see code above) */
>         UNWIND_HINT_EMPTY
>         POP_EXTRA_REGS
> -.Lpop_c_regs_except_rcx_r11_and_sysret:
>         popq    %rsi    /* skip r11 */
>         popq    %r10
>         popq    %r9
> @@ -420,47 +412,6 @@ syscall_return_via_sysret:
>         USERGS_SYSRET64
>  END(entry_SYSCALL_64)
>
> -ENTRY(stub_ptregs_64)
> -       /*
> -        * Syscalls marked as needing ptregs land here.
> -        * If we are on the fast path, we need to save the extra regs,
> -        * which we achieve by trying again on the slow path.  If we are on
> -        * the slow path, the extra regs are already saved.
> -        *
> -        * RAX stores a pointer to the C function implementing the syscall.
> -        * IRQs are on.
> -        */
> -       cmpq    $.Lentry_SYSCALL_64_after_fastpath_call, (%rsp)
> -       jne     1f
> -
> -       /*
> -        * Called from fast path -- disable IRQs again, pop return address
> -        * and jump to slow path
> -        */
> -       DISABLE_INTERRUPTS(CLBR_ANY)
> -       TRACE_IRQS_OFF
> -       popq    %rax
> -       UNWIND_HINT_REGS extra=0
> -       jmp     entry_SYSCALL64_slow_path
> -
> -1:
> -       JMP_NOSPEC %rax                         /* Called from C */
> -END(stub_ptregs_64)
> -
> -.macro ptregs_stub func
> -ENTRY(ptregs_\func)
> -       UNWIND_HINT_FUNC
> -       leaq    \func(%rip), %rax
> -       jmp     stub_ptregs_64
> -END(ptregs_\func)
> -.endm
> -
> -/* Instantiate ptregs_stub for each ptregs-using syscall */
> -#define __SYSCALL_64_QUAL_(sym)
> -#define __SYSCALL_64_QUAL_ptregs(sym) ptregs_stub sym
> -#define __SYSCALL_64(nr, sym, qual) __SYSCALL_64_QUAL_##qual(sym)
> -#include <asm/syscalls_64.h>
> -

You can't just blindly remove this.  We need to make sure that
syscalls that modify registers take the slow path exit, because they
may change the registers to be incompatible with SYSRET.

--
Brian Gerst