mm: shrinker: fix NULL pointer dereference in debugfs

commit e30453c upstream.

shrinker_debugfs_add() creates both "count" and "scan" debugfs files
unconditionally.

That assumes every shrinker implements both count_objects() and
scan_objects(), which is not guaranteed.  For example, the xen-backend
shrinker sets count_objects() but leaves scan_objects() NULL, so writing
to its scan file calls through a NULL function pointer and panics the
kernel:

BUG: kernel NULL pointer dereference, address: 0000000000000000
RIP: 0010:0x0
Code: Unable to access opcode bytes at 0xffffffffffffffd6.
Call Trace:
 <TASK>
 shrinker_debugfs_scan_write+0x12e/0x270
 full_proxy_write+0x5f/0x90
 vfs_write+0xde/0x420
 ? filp_flush+0x75/0x90
 ? filp_close+0x1d/0x30
 ? do_dup2+0xb8/0x120
 ksys_write+0x68/0xf0
 ? filp_flush+0x75/0x90
 do_syscall_64+0xb3/0x5b0
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

The count path has the same issue in principle if a shrinker omits
count_objects().

To fix it, only create "count" and "scan" debugfs files when the
corresponding callbacks are present.

Link: https://lore.kernel.org/20260617090052.27325-1-qi.zheng@linux.dev
Fixes: bbf535f ("mm: shrinkers: add scan interface for shrinker debugfs")
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Qi Zheng

mm/shrinker_debug.c

@@ -198,10 +198,12 @@ int shrinker_debugfs_add(struct shrinker *shrinker)
 	}
 	shrinker->debugfs_entry = entry;
 
-	debugfs_create_file("count", 0440, entry, shrinker,
-			    &shrinker_debugfs_count_fops);
-	debugfs_create_file("scan", 0220, entry, shrinker,
-			    &shrinker_debugfs_scan_fops);
+	if (shrinker->count_objects)
+		debugfs_create_file("count", 0440, entry, shrinker,
+				    &shrinker_debugfs_count_fops);
+	if (shrinker->scan_objects)
+		debugfs_create_file("scan", 0220, entry, shrinker,
+				    &shrinker_debugfs_scan_fops);
 	return 0;
 }