linux-kernel - [RFC PATCH 2/2] bpf: Initialise mod[] in bpf_trace

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Date:   Mon, 7 Aug 2017 23:25:14 +0100
From:   James Hogan <james.hogan@...tec.com>
To:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
CC:     <linux-kernel@...r.kernel.org>,
        James Hogan <james.hogan@...tec.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, <netdev@...r.kernel.org>
Subject: [RFC PATCH 2/2] bpf: Initialise mod[] in bpf_trace_printk

In bpf_trace_printk(), the elements in mod[] are left uninitialised, but
they are then incremented to track the width of the formats. Zero
initialise the array just in case the memory contains non-zero values on
entry.

Fixes: 9c959c863f82 ("tracing: Allow BPF programs to call bpf_trace_printk()")
Signed-off-by: James Hogan <james.hogan@...tec.com>
Cc: Alexei Starovoitov <ast@...nel.org>
Cc: Daniel Borkmann <daniel@...earbox.net>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: netdev@...r.kernel.org
---
When I checked (on MIPS32), the elements tended to have the value zero
anyway (does BPF zero the stack or something clever?), so this is a
purely theoretical fix.
---
 kernel/trace/bpf_trace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 32dcbe1b48f2..86a52857d941 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -129,7 +129,7 @@ BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
 	   u64, arg2, u64, arg3)
 {
 	bool str_seen = false;
-	int mod[3] = {};
+	int mod[3] = { 0, 0, 0 };
 	int fmt_cnt = 0;
 	u64 unsafe_addr;
 	char buf[64];
-- 
2.13.2