From: Tomasz Majchrzak <tomasz.majchrzak@xxxxxxxxx> Date: Tue, 29 Nov 2016 15:18:20 +0100 If kernfs file is empty on a first read, successive read operations using the same file descriptor will return no data, even when data is available. Default kernfs 'seq_next' implementation advances iterator position even when next object is not there. Kernfs 'seq_start' for following requests will not return iterator as position is already on the second object. This defect doesn't allow to monitor badblocks sysfs files from MD raid. They are initially empty but if data appears at some stage, userspace is not able to read it. Signed-off-by: Tomasz Majchrzak <tomasz.majchrzak@xxxxxxxxx> Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx> --- fs/seq_file.c | 7 +++++++ 1 file changed, 7 insertions(+) --- a/fs/seq_file.c +++ b/fs/seq_file.c @@ -190,6 +190,13 @@ ssize_t seq_read(struct file *file, char */ m->version = file->f_version; + /* + * if request is to read from zero offset, reset iterator to first + * record as it might have been already advanced by previous requests + */ + if (*ppos == 0) + m->index = 0; + /* Don't assume *ppos is where we left it */ if (unlikely(*ppos != m->read_pos)) { while ((err = traverse(m, *ppos)) == -EAGAIN) -- 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