linux-kernel - Re: [PATCH v3] seq_file: reset iterator to first record for zero offset

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Date:   Tue, 29 Nov 2016 10:58:27 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Tomasz Majchrzak <tomasz.majchrzak@...el.com>
Cc:     linux-kernel@...r.kernel.org, dan.j.williams@...el.com,
        aleksey.obitotskiy@...el.com, pawel.baldysiak@...el.com,
        artur.paszkiewicz@...el.com, maksymilian.kunt@...el.com,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v3] seq_file: reset iterator to first record for zero offset

On Mon, Nov 28, 2016 at 4:11 PM, Tomasz Majchrzak
<tomasz.majchrzak@...el.com> wrote:
> 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@...el.com>

A comment above the check wouldn't hurt.  Otherwise

Acked-by: Miklos Szeredi <mszeredi@...hat.com>

> ---
>  fs/seq_file.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/fs/seq_file.c b/fs/seq_file.c
> index 368bfb9..ee21714 100644
> --- a/fs/seq_file.c
> +++ b/fs/seq_file.c
> @@ -190,6 +190,9 @@ ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
>          */
>         m->version = file->f_version;
>
> +       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)
> --
> 1.8.3.1
>