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

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Message-ID: <20200420184445.GK11244@42.do-not-panic.com>
Date:   Mon, 20 Apr 2020 18:44:45 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Bart Van Assche <bvanassche@....org>, axboe@...nel.dk,
        viro@...iv.linux.org.uk, 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 Mon, Apr 20, 2020 at 01:40:38PM +0200, Greg KH wrote:
> On Sun, Apr 19, 2020 at 04:17:46PM -0700, Bart Van Assche wrote:
> > On 4/19/20 4:05 PM, Luis Chamberlain wrote:
> > > 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.
> > 
> > This is what I found in include/linux/debugfs.h in case debugfs is disabled:
> > 
> > static inline struct dentry *debugfs_create_file(const char *name,
> > 	umode_t mode, struct dentry *parent, void *data,
> > 	const struct file_operations *fops)
> > {
> > 	return ERR_PTR(-ENODEV);
> > }
> > 
> > I have not found any code path that can cause debugfs_create_file() to
> > return NULL. Did I perhaps overlook something? If not, it's not clear to me
> > why the above patch adds checks that check whether debugfs_create_file()
> > returns NULL?
> 
> Short answer, yes, it can return NULL.  Correct answer is, you don't
> care, don't check the value and don't do anything about it.  It's
> debugging code, userspace doesn't care, so just keep moving on.

Thing is this code *exposes* knobs to userspace for an API that *does*
exepect those files to exist. That is, blktrace *relies* on these
debugfs files to exist. So the kconfig which enables blktrace
CONFIG_BLK_DEV_IO_TRACE selects DEBUG_FS.

So typically we don't care if these files were created or not on regular
drivers, but in this case this code is only compiled when debugfs is
enabled and CONFIG_BLK_DEV_IO_TRACE, and the userspace interaction with
debugfs *expects* these files.

So what do you recommend?

  Luis