[PATCH] arm64: fix missing syscall trace exit

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Tue, Jun 02, 2015 at 06:11:48PM -0700, Josh Stone wrote:
> On 06/02/2015 06:01 PM, Josh Stone wrote:
> > If a syscall is entered without TIF_SYSCALL_TRACE set, then it goes on
> > the fast path.  It's then possible to have TIF_SYSCALL_TRACE added in
> > the middle of the syscall, but ret_fast_syscall doesn't check this flag
> > again.  This causes a ptrace syscall-exit-stop to be missed.
> > 
> > For instance, from a PTRACE_EVENT_FORK reported during do_fork, the
> > tracer might resume with PTRACE_SYSCALL, setting TIF_SYSCALL_TRACE.
> > Now the completion of the fork should have a syscall-exit-stop.
> > 
> > Russell King fixed this on arm by re-checking _TIF_SYSCALL_WORK in the
> > fast exit path.  Do the same on arm64.
> > 
> > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > Cc: Will Deacon <will.deacon@arm.com>
> > Cc: Russell King <rmk@arm.linux.org.uk>
> > Signed-off-by: Josh Stone <jistone@redhat.com>
> > ---
> >  arch/arm64/kernel/entry.S | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> > index 959fe8733560..a547a3e8a198 100644
> > --- a/arch/arm64/kernel/entry.S
> > +++ b/arch/arm64/kernel/entry.S
> > @@ -608,7 +608,9 @@ ENDPROC(cpu_switch_to)
> >   */
> >  ret_fast_syscall:
> >  	disable_irq				// disable interrupts
> > -	ldr	x1, [tsk, #TI_FLAGS]
> > +	ldr	x1, [tsk, #TI_FLAGS]		// re-check for syscall tracing
> > +	and	x2, x1, #_TIF_SYSCALL_WORK
> > +	cbnz	x2, __sys_trace_return
> >  	and	x2, x1, #_TIF_WORK_MASK
> >  	cbnz	x2, fast_work_pending
> >  	enable_step_tsk x1, x2
> 
> I do have one concern about this, also in Russell's ARM patch.  Is it
> really ok to branch to __sys_trace_return with interrupts disabled?

I'm not that happy to hear that you have concerns over the patch after
hurrying its submission into the -rc kernels.

> I didn't hit any issue from that, but my testcase only exercises this
> path once each run.  So that might have just been lucky not to hit any
> gross scenario...

It would've been good to have tested that _prior_ to me pushing the patch
into mainline and having the stable trees pick it up.  This kind of thing
can potentially de-stabilise the kernel.

I had thought you'd have tested with audit and other stuff enabled (I
don't use that stuff, and I'm clueless about how to use it.)

Surely, if you're tracing a child, and you start tracing on the exit
path of a syscall, the child should sleep - and as sleeping with IRQs
disabled is not allowed, there should've been a warning if this path
was hit.  I think this brings into question whether that path was
actually hit during testing.  I hope you tried running a kernel with
the usual suite of debugging options enabled?

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