linux-kernel - [PATCH] ARM: net: bpf_jit: fix emit

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Message-Id: <1360776125-14542-1-git-send-email-nschichan@freebox.fr>
Date:	Wed, 13 Feb 2013 18:22:00 +0100
From:	Nicolas Schichan <nschichan@...ebox.fr>
To:	Mircea Gherzan <mgherzan@...il.com>,
	Russell King <linux@....linux.org.uk>
Cc:	Nicolas Schichan <nschichan@...ebox.fr>,
	"David S. Miller" <davem@...emloft.net>,
	Daniel Borkmann <daniel.borkmann@....ee.ethz.ch>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: [PATCH] ARM: net: bpf_jit: fix emit_swap16() for non ARMv6+.

The original code was generating an lsl instructions using the value
of ARM_R8 (skb_headlen, possibly uninitialized if no skb_headlen
access was required) as a shift amount.
---
 arch/arm/net/bpf_jit_32.c |   15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/arch/arm/net/bpf_jit_32.c b/arch/arm/net/bpf_jit_32.c
index a34f1e2..6828ef6 100644
--- a/arch/arm/net/bpf_jit_32.c
+++ b/arch/arm/net/bpf_jit_32.c
@@ -341,10 +341,17 @@ static void emit_load_be16(u8 cond, u8 r_res, u8 r_addr, struct jit_ctx *ctx)
 
 static inline void emit_swap16(u8 r_dst, u8 r_src, struct jit_ctx *ctx)
 {
-	emit(ARM_LSL_R(ARM_R1, r_src, 8), ctx);
-	emit(ARM_ORR_S(r_dst, ARM_R1, r_src, SRTYPE_LSL, 8), ctx);
-	emit(ARM_LSL_I(r_dst, r_dst, 8), ctx);
-	emit(ARM_LSL_R(r_dst, r_dst, 8), ctx);
+	/* r_dst = (r_src << 8) | (r_src >> 8) */
+	emit(ARM_LSL_I(ARM_R1, r_src, 8), ctx);
+	emit(ARM_ORR_S(r_dst, ARM_R1, r_src, SRTYPE_LSR, 8), ctx);
+
+	/*
+	 * we need to mask out the bits set in r_dst[23:16] due to
+	 * the first shift instruction.
+	 *
+	 * note that 0x8ff is the encoded immediate 0x00ff0000.
+	 */
+	emit(ARM_BIC_I(r_dst, r_dst, 0x8ff), ctx);
 }
 
 #else  /* ARMv6+ */
-- 
1.7.10.4

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