linux-kernel - Re: [PATCH v2 08/10] blktrace: add checks for created debugfs files on setup

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Date:   Sun, 19 Apr 2020 23:05:37 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Bart Van Assche <bvanassche@....org>
Cc:     axboe@...nel.dk, viro@...iv.linux.org.uk,
        gregkh@...uxfoundation.org, rostedt@...dmis.org, mingo@...hat.com,
        jack@...e.cz, ming.lei@...hat.com, nstange@...e.de,
        akpm@...ux-foundation.org, mhocko@...e.com, yukuai3@...wei.com,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 08/10] blktrace: add checks for created debugfs files
 on setup

On Sun, Apr 19, 2020 at 03:57:58PM -0700, Bart Van Assche wrote:
> On 4/19/20 12:45 PM, Luis Chamberlain wrote:
> > Even though debugfs can be disabled, enabling BLK_DEV_IO_TRACE will
> > select DEBUG_FS, and blktrace exposes an API which userspace uses
> > relying on certain files created in debugfs. If files are not created
> > blktrace will not work correctly, so we do want to ensure that a
> > blktrace setup creates these files properly, and otherwise inform
> > userspace.
> > 
> > Signed-off-by: Luis Chamberlain <mcgrof@...nel.org>
> > ---
> >   kernel/trace/blktrace.c | 8 +++++---
> >   1 file changed, 5 insertions(+), 3 deletions(-)
> > 
> > diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
> > index 9cc0153849c3..fc32a8665ce8 100644
> > --- a/kernel/trace/blktrace.c
> > +++ b/kernel/trace/blktrace.c
> > @@ -552,17 +552,19 @@ static int blk_trace_create_debugfs_files(struct blk_user_trace_setup *buts,
> >   					  struct dentry *dir,
> >   					  struct blk_trace *bt)
> >   {
> > -	int ret = -EIO;
> > -
> >   	bt->dropped_file = debugfs_create_file("dropped", 0444, dir, bt,
> >   					       &blk_dropped_fops);
> > +	if (!bt->dropped_file)
> > +		return -ENOMEM;
> >   	bt->msg_file = debugfs_create_file("msg", 0222, dir, bt, &blk_msg_fops);
> > +	if (!bt->msg_file)
> > +		return -ENOMEM;
> >   	bt->rchan = relay_open("trace", dir, buts->buf_size,
> >   				buts->buf_nr, &blk_relay_callbacks, bt);
> >   	if (!bt->rchan)
> > -		return ret;
> > +		return -EIO;
> >   	return 0;
> >   }
> 
> I should have had a look at this patch before I replied to the previous
> patch.
> 
> Do you agree that the following code can be triggered by
> debugfs_create_file() and also that debugfs_create_file() never returns
> NULL?

If debugfs is enabled, and not that we know it is in this blktrace code,
as we select it, it can return ERR_PTR(-ERROR) if an error occurs.

  Luis