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

[SeongJae Park]

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.

Fixes: 75c1c2b53c78 ("mm/damon/dbgfs: support multiple contexts")
Cc: <stable@xxxxxxxxxxxxxxx> # 5.15.x
Signed-off-by: Badari Pulavarty <badari.pulavarty@xxxxxxxxx>
Signed-off-by: SeongJae Park <sj@xxxxxxxxxx>
---
Changes from v1
(https://lore.kernel.org/damon/DM6PR11MB3978994F75A4104D714437379C679@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/)
- Manually check duplicate entry instead of checking
  'debugfs_create_dir()' return value
- Reword commit message and the test case

Seems Badari have some email client issue, so I (SJ) am making this
second version of the patch based on Badari's final proposal and repost
on behalf of Badari.

 mm/damon/dbgfs.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/damon/dbgfs.c b/mm/damon/dbgfs.c
index 51d67c8050dd..364b44063c2f 100644
--- 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;
 
-- 
2.25.1