arm64: fix missing syscall trace exit

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.

Reviewed-by: Will Deacon <will.deacon@arm.com>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Josh Stone <jistone@redhat.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 6 insertions, 1 deletions

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index d369ccff67e2..0eeb1b95c65a 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -611,11 +611,16 @@ 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, ret_fast_syscall_trace
         and     x2, x1, #_TIF_WORK_MASK
         cbnz    x2, fast_work_pending
         enable_step_tsk x1, x2
         kernel_exit 0, ret = 1
+ret_fast_syscall_trace:
+        enable_irq                              // enable interrupts
+        b       __sys_trace_return
 
 /*
  * Ok, we need to do extra processing, enter the slow path.