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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Sep 05, 2016 at 05:02:43PM +0300, Konstantin Khlebnikov wrote:
> On 05.09.2016 16:46, Greg Kroah-Hartman wrote:
> > On Mon, Sep 05, 2016 at 04:25:48PM +0300, Konstantin Khlebnikov wrote:
> > > Bump
> > 
> > Huh?
> > 
> > > On Wed, Jun 22, 2016 at 9:42 PM, Konstantin Khlebnikov
> > > <khlebnikov@yandex-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@yandex-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@vger.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);
> > > >  }
> > 
> > I don't have this in any queue of mine, so I don't understand what you
> > are asking about.
> > 
> > totally confused,
> > 
> 
> Well.. One user poked me about this bug.

As I just mentioned, this is in 4.8-rc5, and will show up in a stable
release "soon".  So I don't understand what else we can do here :)

thanks,

greg k-h