linux-kernel - Re: [PATCH] sysfs: correctly handle read offset on PREALLOC attrs

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Date:   Mon, 5 Sep 2016 16:25:48 +0300
From:   Konstantin Khlebnikov <koct9i@...il.com>
To:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        NeilBrown <neilb@...e.de>, Tejun Heo <tj@...nel.org>,
        Stable <stable@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] sysfs: correctly handle read offset on PREALLOC attrs

Bump

On Wed, Jun 22, 2016 at 9:42 PM, Konstantin Khlebnikov
<khlebnikov@...dex-team.ru> wrote:
> Attributes declared with __ATTR_PREALLOC use sysfs_kf_read() which returns
> zero bytes for non-zero offset. This breaks script checkarray in mdadm tool
> in debian where /bin/sh is 'dash' because its builtin 'read' reads only one
> byte at a time. Script gets 'i' instead of 'idle' when reads current action
> from /sys/block/$dev/md/sync_action and as a result does nothing.
>
> This patch adds trivial implementation of partial read: generate whole
> string and move required part into buffer head.
>
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
> Fixes: 4ef67a8c95f3 ("sysfs/kernfs: make read requests on pre-alloc files use the buffer.")
> Link: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=787950
> Cc: Stable <stable@...r.kernel.org> # v3.19+
> ---
>  fs/sysfs/file.c |    8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
> index f35523d4fa3a..b803213d1307 100644
> --- a/fs/sysfs/file.c
> +++ b/fs/sysfs/file.c
> @@ -114,9 +114,15 @@ static ssize_t sysfs_kf_read(struct kernfs_open_file *of, char *buf,
>          * If buf != of->prealloc_buf, we don't know how
>          * large it is, so cannot safely pass it to ->show
>          */
> -       if (pos || WARN_ON_ONCE(buf != of->prealloc_buf))
> +       if (WARN_ON_ONCE(buf != of->prealloc_buf))
>                 return 0;
>         len = ops->show(kobj, of->kn->priv, buf);
> +       if (pos) {
> +               if (len <= pos)
> +                       return 0;
> +               len -= pos;
> +               memmove(buf, buf + pos, len);
> +       }
>         return min(count, len);
>  }
>
>