Re: [PATCHv2] firmware: Correct handling of fw_state_wait_timeout() return value

["Luis R. Rodriguez" <mcgrof@kernel.org>]

On Tue, Jan 17, 2017 at 10:04:20AM -0800, Jakub Kicinski wrote:
> On Tue, Jan 17, 2017 at 9:30 AM, Luis R. Rodriguez <mcgrof@kernel.org> wrote:
> > On Tue, Jan 17, 2017 at 08:30:37AM -0800, Jakub Kicinski wrote:
> >> On Tue, Jan 17, 2017 at 8:21 AM, Luis R. Rodriguez <mcgrof@kernel.org> wrote:
> >> >>>
> >> >>>       retval = fw_state_wait_timeout(&buf->fw_st, timeout);
> >> >>> -     if (retval < 0) {
> >> >>> +     if (retval == -ETIMEDOUT || retval == -ERESTARTSYS) {
> >> >>>               mutex_lock(&fw_lock);
> >> >>>               fw_load_abort(fw_priv);
> >> >>>               mutex_unlock(&fw_lock);
> >> >>
> >> >> This is a bit messy, two other similar issues were reported before
> >> >> and upon review I suggested Patrick Bruenn's fix with a better commit
> >> >> log seems best fit. Patrick sent a patch Jan 4, 2017 but never followed up
> >> >> despite my feedback on a small change on the commit log message [0]. Can you
> >> >> try that and if that fixes it can you adjust the commit log accordingly? Please
> >> >> note the preferred solution would be:
> >> >>
> >> >> diff --git a/drivers/base/firmware_class.c b/drivers/base/firmware_class.c
> >> >> index b9ac348e8d33..c530f8b4af01 100644
> >> >> --- a/drivers/base/firmware_class.c
> >> >> +++ b/drivers/base/firmware_class.c
> >> >> @@ -542,6 +542,8 @@ static struct firmware_priv *to_firmware_priv(struct device *dev)
> >> >>
> >> >>  static void __fw_load_abort(struct firmware_buf *buf)
> >> >>  {
> >> >> +       if (!buf)
> >> >> +               return;
> >>
> >> Allow me to try to persuade you one last time :)  My patch makes the
> >> code more logical and easier to follow.  The code says:
> >> in case no wake up happened - finish the wait (otherwise the waking
> >> thread finishes it).
> >
> > Your patch is still wrong, as Patrick great commit log notes a null defer
> > can also happen on a race with a case of -1 being sent and a -ENOENT error,
> > so we'd have to adjust for when __fw_state_wait_common() returns also
> > -ENOENT.
> 
> Sorry, I don't follow.  _Not_ calling abort on -ENOENT error is
> exactly what my patch does.

Yeah I see now what you mean. Your approach avoids the buf issue as well.
Its still not addressing the real issue though, which is the chicken
sloppy use of a status on the buf, which at one point gets set to NULL.
This later practice makes it rather hard to make it correct to use
a stateful check properly.

> >> Adding a NULL-check would just paper over the
> >> issue and can cause trouble down the line.
> >
> > We typically bail on errors and use similar code to bail out, and we
> > typically do these things. Here its no different. The *real* issue
> > is the fact that we have a waiting timeout which can fail race against
> > a user imposed error out on the sysfs interface. There is one catch:
> >
> > We already lock with the big fw_lock and use this to be able to check
> > for the status of the fw, so once aborted we technically should not have
> > to abort again. A proper way to address then this would have been to check
> > for the status of the fw prior to aborting again given we also lock on the
> > big fw_lock. A problem with this though is the status is part of the buf
> > which is set to NULL after we are done aborting.
> 
> Yes, I've seen that too :\  This race seems to have been there prior
> to 4.9, though.  I guess we could fix both issues with the NULL-check
> although I would prefer if we had both patches.
> 
> FWIW I think the NULL-check could be put in the existing conditional:
> 
>          * There is a small window in which user can write to 'loading'
>          * between loading done and disappearance of 'loading'
>          */
> -       if (fw_state_is_done(&buf->fw_st))
> +       if (!buf || fw_state_is_done(&buf->fw_st))
>                 return;
> 
>         list_del_init(&buf->pending_list);
> 
> Note that the comment above seems to be mentioning the race we're
> trying to solve.

Right, I think another approach is to *enable* the state of the buf
to be used to avoid further use on the sysfs iterface instead. Fortunately
other sysfs interfaces already use fw_state_is_done() to bail out,
so all that would be needed I think would be:

diff --git a/drivers/base/firmware_class.c b/drivers/base/firmware_class.c
index b9ac348e8d33..30ccf7aea3ca 100644
--- a/drivers/base/firmware_class.c
+++ b/drivers/base/firmware_class.c
@@ -558,9 +558,6 @@ static void fw_load_abort(struct firmware_priv *fw_priv)
 	struct firmware_buf *buf = fw_priv->buf;
 
 	__fw_load_abort(buf);
-
-	/* avoid user action after loading abort */
-	fw_priv->buf = NULL;
 }
 
 static LIST_HEAD(pending_fw_head);
@@ -713,7 +710,7 @@ static ssize_t firmware_loading_store(struct device *dev,
 
 	mutex_lock(&fw_lock);
 	fw_buf = fw_priv->buf;
-	if (!fw_buf)
+	if (!fw_buf || fw_state_is_aborted(&fw_buf->fw_st))
 		goto out;
 
 	switch (loading) {