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

Andrew Morton
2022-08-19T21:08:18Z

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()?