[PATCH] arm64: entry: always restore x0 from the stack on syscall return

[will.deacon@arm.com (Will Deacon)]

On Wed, Aug 19, 2015 at 05:03:20PM +0100, Catalin Marinas wrote:
> On Wed, Aug 19, 2015 at 04:09:49PM +0100, Will Deacon wrote:
> > @@ -613,13 +609,14 @@ ENDPROC(cpu_switch_to)
> >   */
> >  ret_fast_syscall:
> >  	disable_irq				// disable interrupts
> > +	str	x0, [sp, #S_X0]			// returned x0
> >  	ldr	x1, [tsk, #TI_FLAGS]		// re-check for syscall tracing
> >  	and	x2, x1, #_TIF_SYSCALL_WORK
> >  	cbnz	x2, ret_fast_syscall_trace
> >  	and	x2, x1, #_TIF_WORK_MASK
> > -	cbnz	x2, fast_work_pending
> > +	cbnz	x2, work_pending
> >  	enable_step_tsk x1, x2
> > -	kernel_exit 0, ret = 1
> > +	kernel_exit 0
> >  ret_fast_syscall_trace:
> >  	enable_irq				// enable interrupts
> >  	b	__sys_trace_return
> 
> There is another str x0 in __sys_trace_return which I think we could
> remove.

Hmm, I don't think we can remove that. It's needed on the slowpath to
update the pt_regs with either -ENOSYS (for __ni_sys_trace) or the
syscall return value from the blr in __sys_trace.

What we can do instead is change the branch above to branch to
__sys_trace_return_skipped. Patch below.

Will

--->8

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 2a5e64ccc991..088322ff1ba0 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -619,7 +619,7 @@ ret_fast_syscall:
        kernel_exit 0
 ret_fast_syscall_trace:
        enable_irq                              // enable interrupts
-       b       __sys_trace_return
+       b       __sys_trace_return_skipped      // we already saved x0
 
 /*
  * Ok, we need to do extra processing, enter the slow path.