FAILED: patch "[PATCH] mm: shrinker: fix NULL pointer dereference in debugfs" failed to apply to 6.1-stable tree

FAILED: patch "[PATCH] mm: shrinker: fix NULL pointer dereference in debugfs" failed to apply to 6.1-stable tree

[gregkh]

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x e30453c61e185e914fde83c650e268067b140218
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026071340-handed-risk-625c@gregkh' --subject-prefix 'PATCH 6.1.y' 'HEAD^..'

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From e30453c61e185e914fde83c650e268067b140218 Mon Sep 17 00:00:00 2001
From: Qi Zheng <qi.zheng@linux.dev>
Date: Wed, 17 Jun 2026 17:00:52 +0800
Subject: [PATCH] mm: shrinker: fix NULL pointer dereference in debugfs

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: bbf535fd6f06 ("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>

diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c
index cda4e86428c8..cafb56630132 100644
--- a/mm/shrinker_debug.c
+++ b/mm/shrinker_debug.c
@@ -183,10 +183,12 @@ 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;
 }
 

[PATCH 6.1.y 1/2] mm: shrinkers: fix debugfs file permissions

[Sasha Levin]

From: John Keeping <john@metanate.com>

[ Upstream commit 2124f79de6a909630d1a62b01ecc32db9f967181 ]

The permissions for the files here are swapped as "count" is read-only and
"scan" is write-only.  While this doesn't really matter as these
permissions don't stop the files being opened for reading/writing as
appropriate, they are shown by "ls -l" and are confusing.

Link: https://lkml.kernel.org/r/20230418101906.3131303-1-john@metanate.com
Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Stable-dep-of: e30453c61e18 ("mm: shrinker: fix NULL pointer dereference in debugfs")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 mm/shrinker_debug.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c
index 39c3491e28a3ef..6133c8d00f116b 100644
--- a/mm/shrinker_debug.c
+++ b/mm/shrinker_debug.c
@@ -198,9 +198,9 @@ int shrinker_debugfs_add(struct shrinker *shrinker)
 	}
 	shrinker->debugfs_entry = entry;
 
-	debugfs_create_file("count", 0220, entry, shrinker,
+	debugfs_create_file("count", 0440, entry, shrinker,
 			    &shrinker_debugfs_count_fops);
-	debugfs_create_file("scan", 0440, entry, shrinker,
+	debugfs_create_file("scan", 0220, entry, shrinker,
 			    &shrinker_debugfs_scan_fops);
 	return 0;
 }
-- 
2.53.0

[PATCH 6.1.y 2/2] mm: shrinker: fix NULL pointer dereference in debugfs

[Sasha Levin]

From: Qi Zheng <zhengqi.arch@bytedance.com>

[ Upstream commit e30453c61e185e914fde83c650e268067b140218 ]

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: bbf535fd6f06 ("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: Sasha Levin <sashal@kernel.org>
---
 mm/shrinker_debug.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c
index 6133c8d00f116b..9f3fabe1bf30cd 100644
--- a/mm/shrinker_debug.c
+++ b/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;
 }
 
-- 
2.53.0