Re: [PATCH v9 2/6] arm64: ptrace: allow tracer to skip a system call

[AKASHI Takahiro]

On 11/26/2014 10:02 PM, Will Deacon wrote:
> On Wed, Nov 26, 2014 at 04:49:47AM +0000, AKASHI Takahiro wrote:
> > If tracer modifies a syscall number to -1, this traced system call should
> > be skipped with a return value specified in x0.
> > This patch implements this semantics.
> >
> > Please note:
> > * syscall entry tracing and syscall exit tracing (ftrace tracepoint and
> >    audit) are always executed, if enabled, even when skipping a system call
> >    (that is, -1).
> >    In this way, we can avoid a potential bug where audit_syscall_entry()
> >    might be called without audit_syscall_exit() at the previous system call
> >    being called, that would cause OOPs in audit_syscall_entry().
> >
> > Signed-off-by: AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx>
> > ---
> >   arch/arm64/kernel/entry.S |   10 +++++++++-
> >   1 file changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> > index 726b910..946ec52 100644
> > --- a/arch/arm64/kernel/entry.S
> > +++ b/arch/arm64/kernel/entry.S
> > @@ -161,6 +161,7 @@
> >    */
> >   sc_nr	.req	x25		// number of system calls
> >   scno	.req	x26		// syscall number
> > +scno_w	.req	w26		// syscall number (lower 32 bits)
> >   stbl	.req	x27		// syscall table pointer
> >   tsk	.req	x28		// current thread_info
> >
> > @@ -668,8 +669,14 @@ ENDPROC(el0_svc)
> >   	 * switches, and waiting for our parent to respond.
> >   	 */
> >   __sys_trace:
> > -	mov	x0, sp
> > +	cmp     scno_w, #-1			// set default errno for
>
> I hate that we have to use scno_w, but the only alternative I can think of
> is using w8 directly, which isn't any better and doesn't work for compat.
> Ho-hum, I guess we'll stick with what you have.

The possible approaches might be:
* use 32-bit registers for scno & sc_nr and use "sxtw scno, w8," or
* use an extra reg like
    __sys_trace:
        mov x0, #0xffffffff
        cmp scno, x0
        b.ne 1f

> > +	b.ne	1f				// user-issued syscall(-1)
> > +	mov	x0, #-ENOSYS
> > +	str	x0, [sp]
>
> Can you use #S_X0 here for clarity, please?

Okey.

> > +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
>
> and update this guy too.

Sure.

-Takahiro AKASHI

> Will
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