linux-kernel - [PATCH] bpf: fix a false positive kmemcheck warning

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Date:	Fri, 5 Sep 2014 12:01:59 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Daniel Borkmann <dborkman@...hat.com>
cc:	Alexei Starovoitov <ast@...mgrid.com>,
	Pablo Neira Ayuso <pablo@...filter.org>,
	"David S. Miller" <davem@...emloft.net>,
	linux-kernel@...r.kernel.org
Subject: [PATCH] bpf: fix a false positive kmemcheck warning

This patch fixes false positive kmemcheck warning in bpf.

When we try to write the variable len, the compiler generates a code that
reads the 32-bit word, modifies the bits belonging to "len" and writes the
32-bit word back. The reading of the word results in kmemcheck warning due
to reading uninitialized memory. This patch fixes it by avoiding using bit
fields when kmemcheck is enabled.

Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>

---
 include/linux/filter.h |    5 +++++
 1 file changed, 5 insertions(+)

Index: linux-2.6/include/linux/filter.h
===================================================================
--- linux-2.6.orig/include/linux/filter.h	2014-09-04 23:04:26.000000000 +0200
+++ linux-2.6/include/linux/filter.h	2014-09-04 23:43:05.000000000 +0200
@@ -325,8 +325,13 @@ struct sock;
 struct seccomp_data;
 
 struct bpf_prog {
+#ifdef CONFIG_KMEMCHECK
+	bool			jited;
+	u32			len;
+#else
 	u32			jited:1,	/* Is our filter JIT'ed? */
 				len:31;		/* Number of filter blocks */
+#endif
 	struct sock_fprog_kern	*orig_prog;	/* Original BPF program */
 	unsigned int		(*bpf_func)(const struct sk_buff *skb,
 					    const struct bpf_insn *filter);
--
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