ARM: 7579/1: arch/allow a scno of -1 to not cause a SIGILL

On tracehook-friendly platforms, a system call number of -1 falls
through without running much code or taking much action.

ARM is different. This adds a short-circuit check in the trace path to
avoid any additional work, as suggested by Russell King, to make sure
that ARM behaves the same way as other platforms.

Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Will Drewry <wad@chromium.org>
Reviewed-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 4 insertions, 1 deletions

diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
index b621871dd277..ee81dbc6fa10 100644
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S
@@ -448,7 +448,10 @@ __sys_trace:
         ldmccia r1, {r0 - r6}                   @ have to reload r0 - r6
         stmccia sp, {r4, r5}                    @ and update the stack args
         ldrcc   pc, [tbl, scno, lsl #2]         @ call sys_* routine
-        b       2b
+        cmp     scno, #-1                       @ skip the syscall?
+        bne     2b
+        add     sp, sp, #S_OFF                  @ restore stack
+        b       ret_slow_syscall
 
 __sys_trace_return:
         str     r0, [sp, #S_R0 + S_OFF]!        @ save returned r0