[PATCH 3.16.y-ckt 087/110] ARM: fix missing syscall trace exit

[Luis Henriques]

3.16.7-ckt13 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Russell King <rmk+kernel at arm.linux.org.uk>

commit 1b97937246d8b97c0760d16d8992c7937bdf5e6a upstream.

Josh Stone reports:

  I've discovered a case where both arm and arm64 will miss a ptrace
  syscall-exit that they should report.  If the 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.

Fix this by always checking for a syscall trace in the fast exit path.

Reported-by: Josh Stone <jistone at redhat.com>
Signed-off-by: Russell King <rmk+kernel at arm.linux.org.uk>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 arch/arm/kernel/entry-common.S | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
index 7139d4a7dea7..29b81fc959fe 100644
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S
@@ -32,7 +32,9 @@ ret_fast_syscall:
  UNWIND(.fnstart	)
  UNWIND(.cantunwind	)
 	disable_irq				@ disable interrupts
-	ldr	r1, [tsk, #TI_FLAGS]
+	ldr	r1, [tsk, #TI_FLAGS]		@ re-check for syscall tracing
+	tst	r1, #_TIF_SYSCALL_WORK
+	bne	__sys_trace_return
 	tst	r1, #_TIF_WORK_MASK
 	bne	fast_work_pending
 	asm_trace_hardirqs_on