[PATCH] arm64: fix missing syscall trace exit
will.deacon at arm.com
Wed Jun 3 02:52:41 PDT 2015
On Wed, Jun 03, 2015 at 02:11:48AM +0100, 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 at arm.com>
> > Cc: Will Deacon <will.deacon at arm.com>
> > Cc: Russell King <rmk at arm.linux.org.uk>
> > Signed-off-by: Josh Stone <jistone at 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 think you're right to be concerned!
> 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...
Did you try enabling all the audit stuff? It looks like that can call
into the scheduler, so I think we should be running the tracing callbacks
with interrupts enabled (and it looks like x86 do this on the exit path).
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