Re: [PATCH v2] mm/damon/dbgfs: avoid duplicate context directory creation

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, Aug 19, 2022 at 02:08:09PM -0700, Andrew Morton wrote:
> On Fri, 19 Aug 2022 17:19:30 +0000 SeongJae Park <sj@kernel.org> wrote:
> 
> > From: Badari Pulavarty <badari.pulavarty@intel.com>
> > 
> > When user tries to create a DAMON context via the DAMON debugfs
> > interface with a name of an already existing context, the context
> > directory creation silently fails but the context is added in the
> > internal data structure.  As a result, memory could leak and DAMON
> > cannot be turned on.  An example test case is as below:
> > 
> >     # cd /sys/kernel/debug/damon/
> >     # echo "off" >  monitor_on
> >     # echo paddr > target_ids
> >     # echo "abc" > mk_context
> >     # echo "abc" > mk_context
> >     # echo $$ > abc/target_ids
> >     # echo "on" > monitor_on  <<< fails
> > 
> > This commit fixes the issue by checking if the name already exist and
> > immediately returning '-EEXIST' in the case.
> > 
> > ...
> >
> > --- a/mm/damon/dbgfs.c
> > +++ b/mm/damon/dbgfs.c
> > @@ -795,7 +795,7 @@ static void dbgfs_destroy_ctx(struct damon_ctx *ctx)
> >   */
> >  static int dbgfs_mk_context(char *name)
> >  {
> > -	struct dentry *root, **new_dirs, *new_dir;
> > +	struct dentry *root, **new_dirs, *new_dir, *dir;
> >  	struct damon_ctx **new_ctxs, *new_ctx;
> >  
> >  	if (damon_nr_running_ctxs())
> > @@ -817,6 +817,12 @@ static int dbgfs_mk_context(char *name)
> >  	if (!root)
> >  		return -ENOENT;
> >  
> > +	dir = debugfs_lookup(name, root);
> > +	if (dir) {
> > +		dput(dir);
> > +		return -EEXIST;
> > +	}
> > +
> >  	new_dir = debugfs_create_dir(name, root);
> >  	dbgfs_dirs[dbgfs_nr_ctxs] = new_dir;
> 
> It would be simpler (and less racy) to check the debugfs_create_dir()
> return value for IS_ERR()?
> 

Yes, if you _HAVE_ to know if the code works properly (i.e. because your
feature totally depends on debugfs like damon does), or you have a
potential duplicate name like this, then sure, check the return value
and do something based on it.

It's odd enough that you should put a comment above the check just so I
don't go and send a patch to delete it later on :)

thanks,

greg k-h