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

Greg KH
2022-08-21T06:58:35Z

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