[RFC PATCH 1/1] mm/slub: fix endless "No data" printing for alloc/free_traces attribute

[Gerald Schaefer]

Reading from alloc/free_traces attribute in /sys/kernel/debug/slab/ results
in an endless sequence of "No data". This is because slab_debugfs_start()
does not check for a "past end of file" condition and return NULL.

Fix it by adding such a check and return NULL.

Fixes: 64dd68497be7 ("mm: slub: move sysfs slab alloc/free interfaces to debugfs")
Cc: <stable@xxxxxxxxxxxxxxx> # v5.14+
Reported-by: Steffen Maier <maier@xxxxxxxxxxxxx>
Signed-off-by: Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx>
---
 mm/slub.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/slub.c b/mm/slub.c
index f7368bfffb7a..336609671bc2 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -6115,6 +6115,11 @@ static void *slab_debugfs_next(struct seq_file *seq, void *v, loff_t *ppos)
 
 static void *slab_debugfs_start(struct seq_file *seq, loff_t *ppos)
 {
+	struct loc_track *t = seq->private;
+
+	if (*ppos > t->count)
+		return NULL;
+
 	return ppos;
 }
 
-- 
2.25.1