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

[Andrew Morton]

On Fri, 19 Aug 2022 17:19:30 +0000 SeongJae Park <sj@xxxxxxxxxx> wrote:

> From: Badari Pulavarty <badari.pulavarty@xxxxxxxxx>
> 
> 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()?