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

[Konstantin Khlebnikov]

On Mon, Sep 5, 2016 at 5:13 PM, Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> On Mon, Sep 05, 2016 at 03:46:28PM +0200, 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@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
>> > > 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@xxxxxxxxxxxxxxx> # 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,
>
> Wait, this is already in Linus's tree, what are you asking for here?
>
> still confused,
>

Huh. Sorry. Haven't done git pull before checking - it only in lastest v4.8-rc5