[PATCH] landlock: Drop __counted_by from landlock_domain.layers

[Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>]

With clang 22 and CONFIG_FORTIFY_SOURCE, the second
landlock_restrict_self() call hits a bogus fortify report in
inherit_ruleset():

  memcpy: detected buffer overflow: 16 byte read of buffer size 0
  Call Trace:
   __fortify_panic
   landlock_merge_ruleset+0xc46/0xc90
   __se_sys_landlock_restrict_self+0x2bc/0xd40

layers[] lives in an anonymous struct inside the anonymous union
that overlays work_free.  For such a flexible array, clang computes the
address of the __counted_by counter as the start of the array instead of
&num_layers.  The bound is then read from layers[0], i.e. from
the first layer's configuration, which is not a count.  The same layout
crashes clang itself in a standalone test.

The annotation was added as documentation, so drop it rather than
restructure the union.  Keep the information in the member comment.

Reported-by: syzbot+a0eecb22de4d821a9e5d@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=a0eecb22de4d821a9e5d
Fixes: bd3a19800dd1 ("landlock: Add counted_by in landlock_domain")
Signed-off-by: Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>
---
Reproduced with clang 22.1.8 and FORTIFY_SOURCE on mic/next (64b9e6eff524):
the second landlock_restrict_self() hits the report, while 16 nested
restricts run clean with this patch.  Mickaël, if you prefer to just drop
bd3a19800dd1 from next, that works as well.
 security/landlock/domain.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/security/landlock/domain.h b/security/landlock/domain.h
index 60e8eb2d6ac2..4bf1555a4349 100644
--- a/security/landlock/domain.h
+++ b/security/landlock/domain.h
@@ -246,8 +246,12 @@ struct landlock_domain {
 			 * properly handle merged rulesets without overlapping
 			 * access rights.  These layers are set once and never
 			 * changed for the lifetime of the domain.
+			 *
+			 * Not annotated with __counted_by(@num_layers): clang
+			 * miscomputes the counter address for a FAM nested in an
+			 * anonymous struct within a union, which trips FORTIFY.
 			 */
-			struct layer_config layers[] __counted_by(num_layers);
+			struct layer_config layers[];
 		};
 	};
 };
-- 
2.43.0