ARM: Call syscall_trace_exit even when system call skipped

[Kees Cook]

On Thu, Mar 15, 2018 at 3:38 AM,  <T.E.Baldwin99 at members.leeds.ac.uk> wrote:
> On 15 March 2018 00:45:01 Kees Cook <keescook at chromium.org> wrote:
>
>>>> --- a/arch/arm/kernel/entry-common.S
>>>> +++ b/arch/arm/kernel/entry-common.S
>>>> @@ -288,16 +288,15 @@ __sys_trace:
>>>>       cmp     scno, #-1                       @ skip the syscall?
>>>>       bne     2b
>>>>       add     sp, sp, #S_OFF                  @ restore stack
>>>> -     b       ret_slow_syscall
>>>>
>>>> -__sys_trace_return:
>>>> -     str     r0, [sp, #S_R0 + S_OFF]!        @ save returned r0
>>>> +__sys_trace_return_nosave:
>>>> +     enable_irq_notrace
>>
>>
>> Why is __sys_trace_return_nosave the correct destination here? The
>> original handle set up for lr a few lines above is for
>> __sys_trace_return. It's not clear to me why this change is made?
>
>
> __sys_trace_return stores the current r0 value on the stack which will
> reloaded on exit to user mode. However if skipping a system call r0 is -1
> and storing it would destroy the users r0 value, unlike the case where the
> system call is made and r0 is the return value.
>
> The enabling of interrupts is redundant for this purpose, the reuse of code
> is a size optimization.

Ah right. Cool, thanks!

Reviewed-by: Kees Cook <keescook at chromium.org>
Tested-by: Kees Cook <keescook at chromium.org>

I assume Dmitry can add a Tested-by too?

This seems good to go into the ARM patch tracker...

-Kees

-- 
Kees Cook
Pixel Security