[PATCH] seq_file: always update file->f_pos in seq_lseek()

From: Gu Zheng
Date: Fri Oct 25 2013 - 06:21:07 EST

This issue was first pointed out by Jiaxing Wang several months ago, but no
further comments:

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:
ck_netbios_ns 12665

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@xxxxxxxxx>
Signed-off-by: Gu Zheng <guz.fnst@xxxxxxxxxxxxxx>
fs/seq_file.c | 2 ++
1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/fs/seq_file.c b/fs/seq_file.c
index 3135c25..a290157 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -328,6 +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;

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/