Subject: [PATCH v8 2/6] arm64: ptrace: allow tracer to skip a system call

From: [email-masked] (Will Deacon)
Date: Thu, 20 Nov 2014 19:17:38 +0000

On Thu, Nov 20, 2014 at 05:13:04AM +0000, AKASHI Takahiro wrote:
> On 11/20/2014 04:06 AM, Will Deacon wrote:
> > On Wed, Nov 19, 2014 at 08:46:19AM +0000, AKASHI Takahiro wrote:
> >> Syscall(-1) will return -ENOSYS whether or not a syscallno is explicitly
> >> replaced with -1 by a tracer, and, in this sense, it is *skipped*.
> >
> > Ok, but now userspace sees -ENOSYS for a skipped system call in that case,
> > whereas it would usually see whatever the trace put in x0, right?
> 
> If you don't really like this behavior, how about this patch instead of my [2/6] patch?
> 
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 726b910..1ef57d0 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -668,8 +668,15 @@ ENDPROC(el0_svc)
>           * switches, and waiting for our parent to respond.
>           */
>   __sys_trace:
> +       cmp     w8, #-1                         // default errno for invalid
> +       b.ne    1f                              // system call
> +       mov     x0, #-ENOSYS
> +       str     x0, [sp, #S_X0]
> +1:
>          mov     x0, sp
>          bl      syscall_trace_enter
> +       cmp     w0, #-1                         // skip the syscall?
> +       b.eq    __sys_trace_return_skipped
>          adr     lr, __sys_trace_return          // return address
>          uxtw    scno, w0                        // syscall number (possibly new)
>          mov     x1, sp                          // pointer to regs
> @@ -684,6 +691,7 @@ __sys_trace:
> 
>   __sys_trace_return:
>          str     x0, [sp]                        // save returned x0
> +__sys_trace_return_skipped:
>          mov     x0, sp
>          bl      syscall_trace_exit
>          b       ret_to_user
> 
> With this change, I believe, syscall(-1) returns -ENOSYS by default whether traced
> or not, and still you can change a return value when tracing.
> (But a drawback here is that a tracer will see -ENOSYS in x0 even at syscall entry
> for syscall(-1).)

But it's exactly these drawbacks that I'm objected to. syscall(-1) shouldn't
be treated any differently to syscall(42) with respect to restarting,
exactly like x86.

Will