The offset must be multiplied by 4 to be sure to access the correct
32bit word in the stack scratch space.
For instance, a store at scratch memory cell #1 was generating the
following:
st r4, [sp, #1]
While the correct code for this is:
st r4, [sp, #4]
To reproduce the bug (assuming your system has a NIC with the mac
address 52:54:00:12:34:56):
echo 0 > /proc/sys/net/core/bpf_jit_enable
tcpdump -ni eth0 "ether[1] + ether[2] - ether[3] * ether[4] - ether[5] \
== -0x3AA" # this will capture packets as expected
echo 1 > /proc/sys/net/core/bpf_jit_enable
tcpdump -ni eth0 "ether[1] + ether[2] - ether[3] * ether[4] - ether[5] \
== -0x3AA" # this will not.
This bug was present since the original inclusion of bpf_jit for ARM
(ddecdfce: ARM: 7259/3: net: JIT compiler for packet filters).
Signed-off-by: Nicolas Schichan <nschic...@freebox.fr>
---
arch/arm/net/bpf_jit_32.c | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)
diff --git a/arch/arm/net/bpf_jit_32.c b/arch/arm/net/bpf_jit_32.c
index a64d349..b6f305e 100644
--- a/arch/arm/net/bpf_jit_32.c
+++ b/arch/arm/net/bpf_jit_32.c
@@ -42,7 +42,7 @@
#define r_skb_hl ARM_R8
#define SCRATCH_SP_OFFSET 0
-#define SCRATCH_OFF(k) (SCRATCH_SP_OFFSET + (k))
+#define SCRATCH_OFF(k) (SCRATCH_SP_OFFSET + 4 * (k))
#define SEEN_MEM ((1 << BPF_MEMWORDS) - 1)
#define SEEN_MEM_WORD(k) (1 << (k))
--
1.7.5.4
--
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