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

SeongJae Park
2022-08-21T17:52:22Z

Hi Greg,

On Sat, 20 Aug 2022 19:32:37 +0200 Greg KH <gregkh@linuxfoundation.org> wrote:

> 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 :)

Thank you for the kind explanation, Greg. I will revise this patch to simply
check the return value with a comment noticing it's really needed due to the
potential duplicate names.

Thanks,
SJ

>
> thanks,
>
> greg k-h
>