[PATCH 6/7] x86_64, entry: Treat regs->ax the same in fastpath and slowpath syscalls

[Andy Lutomirski]

On Wed, Jul 16, 2014 at 1:08 PM, Alexei Starovoitov <ast at plumgrid.com> wrote:
> On Tue, Jul 15, 2014 at 12:32 PM, Andy Lutomirski <luto at amacapital.net> wrote:
>> For slowpath syscalls, we initialize regs->ax to -ENOSYS and stick
>> the syscall number into regs->orig_ax prior to any possible tracing
>> and syscall execution.  This is user-visible ABI used by ptrace
>> syscall emulation and seccomp.
>>
>> For fastpath syscalls, there's no good reason not to do the same
>> thing.  It's even slightly simpler than what we're currently doing.
>> It probably has no measureable performance impact.  It should have
>> no user-visible effect.
>>
>> The purpose of this patch is to prepare for seccomp-based syscall
>> emulation in the fast path.  This change is just subtle enough that
>> I'm keeping it separate.
>>
>> Signed-off-by: Andy Lutomirski <luto at amacapital.net>
>> ---
>>  arch/x86/include/asm/calling.h |  6 +++++-
>>  arch/x86/kernel/entry_64.S     | 11 +++--------
>>  2 files changed, 8 insertions(+), 9 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/calling.h b/arch/x86/include/asm/calling.h
>> index cb4c73b..76659b6 100644
>> --- a/arch/x86/include/asm/calling.h
>> +++ b/arch/x86/include/asm/calling.h
>> @@ -85,7 +85,7 @@ For 32-bit we have the following conventions - kernel is built with
>>  #define ARGOFFSET      R11
>>  #define SWFRAME                ORIG_RAX
>>
>> -       .macro SAVE_ARGS addskip=0, save_rcx=1, save_r891011=1
>> +       .macro SAVE_ARGS addskip=0, save_rcx=1, save_r891011=1, rax_enosys=0
>>         subq  $9*8+\addskip, %rsp
>>         CFI_ADJUST_CFA_OFFSET   9*8+\addskip
>>         movq_cfi rdi, 8*8
>> @@ -96,7 +96,11 @@ For 32-bit we have the following conventions - kernel is built with
>>         movq_cfi rcx, 5*8
>>         .endif
>>
>> +       .if \rax_enosys
>> +       movq $-ENOSYS, 4*8(%rsp)
>> +       .else
>>         movq_cfi rax, 4*8
>> +       .endif
>>
>>         .if \save_r891011
>>         movq_cfi r8,  3*8
>> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
>> index b25ca96..432c190 100644
>> --- a/arch/x86/kernel/entry_64.S
>> +++ b/arch/x86/kernel/entry_64.S
>> @@ -405,8 +405,8 @@ GLOBAL(system_call_after_swapgs)
>>          * and short:
>>          */
>>         ENABLE_INTERRUPTS(CLBR_NONE)
>> -       SAVE_ARGS 8,0
>> -       movq  %rax,ORIG_RAX-ARGOFFSET(%rsp)
>> +       SAVE_ARGS 8, 0, rax_enosys=1
>> +       movq_cfi rax,(ORIG_RAX-ARGOFFSET)
>
> I think changing store rax to macro is unnecessary,
> since it breaks common style of asm with the next line:

I think it's necessary to maintain CFI correctness.  rax is no longer
saved in "ax", so "orig_ax" is now the correct slot.

>>         movq  %rcx,RIP-ARGOFFSET(%rsp)

This, in contrast, is the saved rip, not the saved rcx.  rcx is lost
when syscall happens.

> Also it made the diff harder to grasp.


>
> The change from the next patch 7/7:
>
>> -       ja   int_ret_from_sys_call      /* RAX(%rsp) set to -ENOSYS above */
>> +       ja   int_ret_from_sys_call      /* RAX(%rsp) is already set */
>
> probably belongs in this 6/7 patch as well.

Agreed.  Will do for v3.

--Andy

>
> The rest look good to me. I think it's a big improvement in readability
> comparing to v1.



-- 
Andy Lutomirski
AMA Capital Management, LLC