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

[AKASHI Takahiro]

On 11/21/2014 04:17 AM, Will Deacon wrote:
> 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.

Can you elaborate a bit more as to "restarting?"
We can't make any assumption about the number of arguments taken by *invalid* syscall(-1)
and so changing a value in x0 (or any other registers) doesn't make any difference.
()

-Takahiro AKASHI

> Will
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