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    Subject[PATCH 3.10 70/74] seq_file: always update file->f_pos in seq_lseek()
    Date
    3.10-stable review patch.  If anyone has any objections, please let me know.

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

    From: Gu Zheng <guz.fnst@cn.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@gmail.com>
    Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
    Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
    Cc: Jonghwan Choi <jhbird.choi@gmail.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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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