• Gu Zheng's avatar
    seq_file: always update file->f_pos in seq_lseek() · dad483b7
    Gu Zheng authored
    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: 's avatarJiaxing Wang <hello.wjx@gmail.com>
    Signed-off-by: 's avatarGu Zheng <guz.fnst@cn.fujitsu.com>
    Signed-off-by: 's avatarAl Viro <viro@zeniv.linux.org.uk>
    Cc: Jonghwan Choi <jhbird.choi@gmail.com>
    Signed-off-by: 's avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    dad483b7
seq_file.c 20.1 KB