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

[Bart Van Assche]

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@xxxxxxxxxx>
> > > ---
> > >    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?

Thanks,

Bart.