linux-kernel - [PATCH] seq_file:update file->f_pos when lseek() to m->read

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Date:	Sat, 29 Jun 2013 17:11:35 +0800
From:	Jiaxing Wang <hello.wjx@...il.com>
To:	viro@...iv.linux.org.uk
Cc:	Jiaxing Wang <hello.wjx@...il.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: [PATCH] seq_file:update file->f_pos when lseek() to m->read_pos

After pread(), file->f_pos and m->read_pos get different,
and lseek() to m->read_pos did not update file->f_pos, then
a subsequent read may read from a wrong position, the following
program shows 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);

Signed-off-by: Jiaxing Wang <hello.wjx@...il.com>
---
 fs/seq_file.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/seq_file.c b/fs/seq_file.c
index 774c1eb..4b22b26 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -328,7 +328,8 @@ loff_t seq_lseek(struct file *file, loff_t offset, int whence)
 				m->read_pos = offset;
 				retval = file->f_pos = offset;
 			}
-		}
+		} else
+			file->f_pos = offset;
 	}
 	file->f_version = m->version;
 	mutex_unlock(&m->lock);
-- 
1.8.1.2

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