On 10/25/2013 06:15 PM, Gu Zheng wrote: > 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@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; > Thanks Gu, I should have post this patch again. And Viro, please take a few minutes to review this, thanks. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
