[PATCH] mm/slub: fix endianness bug for alloc/free_traces attributes

[Gerald Schaefer]

On big-endian s390, the alloc/free_traces attributes produce endless
output, because of always 0 idx in slab_debugfs_show().

idx is de-referenced from *v, which points to a loff_t value, with

unsigned int idx = *(unsigned int *)v;

This will only give the upper 32 bits on big-endian, which remain 0.

Instead of only fixing this de-reference, during discussion it seemed
more appropriate to change the seq_ops so that they use an explicit
iterator in private loc_track struct.

This patch adds idx to loc_track, which will also fix the endianness bug.

Link: https://lore.kernel.org/r/20211117193932.4049412-1-gerald.schaefer@xxxxxxxxxxxxx
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 | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index a8626825a829..abe7db581d68 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -5081,6 +5081,7 @@ struct loc_track {
 	unsigned long max;
 	unsigned long count;
 	struct location *loc;
+	loff_t idx;
 };
 
 static struct dentry *slab_debugfs_root;
@@ -6052,11 +6053,11 @@ __initcall(slab_sysfs_init);
 #if defined(CONFIG_SLUB_DEBUG) && defined(CONFIG_DEBUG_FS)
 static int slab_debugfs_show(struct seq_file *seq, void *v)
 {
-
-	struct location *l;
-	unsigned int idx = *(unsigned int *)v;
 	struct loc_track *t = seq->private;
+	struct location *l;
+	unsigned long idx;
 
+	idx = (unsigned long) t->idx;
 	if (idx < t->count) {
 		l = &t->loc[idx];
 
@@ -6105,16 +6106,18 @@ static void *slab_debugfs_next(struct seq_file *seq, void *v, loff_t *ppos)
 {
 	struct loc_track *t = seq->private;
 
-	v = ppos;
-	++*ppos;
+	t->idx = ++(*ppos);
 	if (*ppos <= t->count)
-		return v;
+		return ppos;
 
 	return NULL;
 }
 
 static void *slab_debugfs_start(struct seq_file *seq, loff_t *ppos)
 {
+	struct loc_track *t = seq->private;
+
+	t->idx = *ppos;
 	return ppos;
 }
 
-- 
2.32.0