[PATCH v3] mm/damon/dbgfs: avoid duplicate context directory creation

[SeongJae Park <sj@kernel.org>]

From: Badari Pulavarty <badari.pulavarty@intel.com>

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 fails but a new context is created and added in the
internal data structure, due to absence of the directory creation
success check.  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

Return value of 'debugfs_create_dir()' is expected to be ignored in
general, but this is an exceptional case as DAMON feature is depending
on the debugfs functionality and it has the potential duplicate name
issue.  This commit therefore fixes the issue by checking the directory
creation failure and immediately return the error in the case.

Fixes: 75c1c2b53c78 ("mm/damon/dbgfs: support multiple contexts")
Cc: <stable@vger.kernel.org> # 5.15.x
Signed-off-by: Badari Pulavarty <badari.pulavarty@intel.com>
Signed-off-by: SeongJae Park <sj@kernel.org>
---
Changes from v2
(https://lore.kernel.org/damon/20220819171930.16166-1-sj@kernel.org/)
- Simply check the debugfs_create_dir() return value (Andrew Morton)
- Put a comment for justifying check of the return value (Greg KH)

Changes from v1
(https://lore.kernel.org/damon/DM6PR11MB3978994F75A4104D714437379C679@DM6PR11MB3978.namprd11.prod.outlook.com/)
- 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
third version of the patch based on Badari's final proposal and
reposting on behalf of Badari.

 mm/damon/dbgfs.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/damon/dbgfs.c b/mm/damon/dbgfs.c
index 51d67c8050dd..3b55a1b219b5 100644
--- a/mm/damon/dbgfs.c
+++ b/mm/damon/dbgfs.c
@@ -818,6 +818,9 @@ static int dbgfs_mk_context(char *name)
 		return -ENOENT;
 
 	new_dir = debugfs_create_dir(name, root);
+	/* Below check is required for a potential duplicated name case */
+	if (IS_ERR(new_dir))
+		return PTR_ERR(new_dir);
 	dbgfs_dirs[dbgfs_nr_ctxs] = new_dir;
 
 	new_ctx = dbgfs_new_ctx();
-- 
2.25.1