linux-kernel - [PATCH 3.11 90/94] seq_file: always update file->f_pos in seq

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Message-Id: <20131109065210.607123109@linuxfoundation.org>
Date:	Fri,  8 Nov 2013 22:52:36 -0800
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Jiaxing Wang <hello.wjx@...il.com>,
	Gu Zheng <guz.fnst@...fujitsu.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Jonghwan Choi <jhbird.choi@...il.com>
Subject: [PATCH 3.11 90/94] seq_file: always update file->f_pos in seq_lseek()

3.11-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Gu Zheng <guz.fnst@...fujitsu.com>

commit 05e16745c0c471bba313961b605b6da3b21a853d upstream.

This issue was first pointed out by Jiaxing Wang several months ago, but no
further comments:
https://lkml.org/lkml/2013/6/29/41

As we know pread() does not change f_pos, so after pread(), file->f_pos
and m->read_pos become different. And seq_lseek() does not update file->f_pos
if offset equals to m->read_pos, so after pread() and seq_lseek()(lseek to
m->read_pos), then a subsequent read may read from a wrong position, the
following program produces the problem:

    char str1[32] = { 0 };
    char str2[32] = { 0 };
    int poffset = 10;
    int count = 20;

    /*open any seq file*/
    int fd = open("/proc/modules", O_RDONLY);

    pread(fd, str1, count, poffset);
    printf("pread:%s\n", str1);

    /*seek to where m->read_pos is*/
    lseek(fd, poffset+count, SEEK_SET);

    /*supposed to read from poffset+count, but this read from position 0*/
    read(fd, str2, count);
    printf("read:%s\n", str2);

out put:
pread:
 ck_netbios_ns 12665
read:
 nf_conntrack_netbios

/proc/modules:
nf_conntrack_netbios_ns 12665 0 - Live 0xffffffffa038b000
nf_conntrack_broadcast 12589 1 nf_conntrack_netbios_ns, Live 0xffffffffa0386000

So we always update file->f_pos to offset in seq_lseek() to fix this issue.

Signed-off-by: Jiaxing Wang <hello.wjx@...il.com>
Signed-off-by: Gu Zheng <guz.fnst@...fujitsu.com>
Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
Cc: Jonghwan Choi <jhbird.choi@...il.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 fs/seq_file.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -328,6 +328,8 @@ loff_t seq_lseek(struct file *file, loff
 				m->read_pos = offset;
 				retval = file->f_pos = offset;
 			}
+		} else {
+			file->f_pos = offset;
 		}
 	}
 	file->f_version = m->version;


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