[PATCH 3/4] arch/arm: allow a scno of -1 to not cause a SIGILL

[Kees Cook]

From: Will Drewry <wad@xxxxxxxxxxxx>

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 lightweight check to arm_syscall()
to make sure that ARM behaves the same way.

Signed-off-by: Will Drewry <wad@xxxxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 arch/arm/kernel/traps.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
index b0179b8..f303ea6 100644
--- a/arch/arm/kernel/traps.c
+++ b/arch/arm/kernel/traps.c
@@ -540,6 +540,10 @@ asmlinkage int arm_syscall(int no, struct pt_regs *regs)
 	struct thread_info *thread = current_thread_info();
 	siginfo_t info;
 
+	/* Emulate/fallthrough. */
+	if (no == -1)
+		return regs->ARM_r0;
+
 	if ((no >> 16) != (__ARM_NR_BASE>> 16))
 		return bad_syscall(no, regs);
 
-- 
1.7.9.5

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