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

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Message-Id: <1352408373-25047-4-git-send-email-keescook@chromium.org>
Date:	Thu,  8 Nov 2012 12:59:32 -0800
From:	Kees Cook <keescook@...omium.org>
To:	linux-kernel@...r.kernel.org
Cc:	Russell King <linux@....linux.org.uk>,
	Will Deacon <will.deacon@....com>,
	Geremy Condra <gcondra@...gle.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Kees Cook <keescook@...omium.org>,
	Will Drewry <wad@...omium.org>
Subject: [PATCH 3/4] arch/arm: 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@...omium.org>
---
 arch/arm/kernel/entry-common.S |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
index 8355d4b..0bef977 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
-- 
1.7.9.5

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