[PATCH v2] acl: Annotate struct posix_acl with __counted_by()

[Thorsten Blum <thorsten.blum@linux.dev>]

Add the __counted_by compiler attribute to the flexible array member
a_entries to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.

Use struct_size() to calculate the number of bytes to allocate for new
and cloned acls and remove the local size variables.

Change the posix_acl_alloc() function parameter count from int to
unsigned int to match posix_acl's a_count data type. Add identifier
names to the function definition to silence two checkpatch warnings.

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Cc: Nathan Chancellor <nathan@kernel.org>
---
Changes in v2:
- Update patch to apply cleanly to linux-next. With 8f0a7a2d7bc3
  ("acl: Realign struct posix_acl to save 8 bytes") in -next, this
  patch should not trigger the false-positive buffer overflow anymore
- Link to v1: https://lore.kernel.org/linux-kernel/20240923213809.235128-2-thorsten.blum@linux.dev/
---
 fs/posix_acl.c            | 13 ++++++-------
 include/linux/posix_acl.h |  4 ++--
 2 files changed, 8 insertions(+), 9 deletions(-)

diff --git a/fs/posix_acl.c b/fs/posix_acl.c
index 6c66a37522d0..4050942ab52f 100644
--- a/fs/posix_acl.c
+++ b/fs/posix_acl.c
@@ -200,11 +200,11 @@ EXPORT_SYMBOL(posix_acl_init);
  * Allocate a new ACL with the specified number of entries.
  */
 struct posix_acl *
-posix_acl_alloc(int count, gfp_t flags)
+posix_acl_alloc(unsigned int count, gfp_t flags)
 {
-	const size_t size = sizeof(struct posix_acl) +
-	                    count * sizeof(struct posix_acl_entry);
-	struct posix_acl *acl = kmalloc(size, flags);
+	struct posix_acl *acl;
+
+	acl = kmalloc(struct_size(acl, a_entries, count), flags);
 	if (acl)
 		posix_acl_init(acl, count);
 	return acl;
@@ -220,9 +220,8 @@ posix_acl_clone(const struct posix_acl *acl, gfp_t flags)
 	struct posix_acl *clone = NULL;
 
 	if (acl) {
-		int size = sizeof(struct posix_acl) + acl->a_count *
-		           sizeof(struct posix_acl_entry);
-		clone = kmemdup(acl, size, flags);
+		clone = kmemdup(acl, struct_size(acl, a_entries, acl->a_count),
+				flags);
 		if (clone)
 			refcount_set(&clone->a_refcount, 1);
 	}
diff --git a/include/linux/posix_acl.h b/include/linux/posix_acl.h
index 2d6a4badd306..e2d47eb1a7f3 100644
--- a/include/linux/posix_acl.h
+++ b/include/linux/posix_acl.h
@@ -30,7 +30,7 @@ struct posix_acl {
 	refcount_t		a_refcount;
 	unsigned int		a_count;
 	struct rcu_head		a_rcu;
-	struct posix_acl_entry	a_entries[];
+	struct posix_acl_entry	a_entries[] __counted_by(a_count);
 };
 
 #define FOREACH_ACL_ENTRY(pa, acl, pe) \
@@ -62,7 +62,7 @@ posix_acl_release(struct posix_acl *acl)
 /* posix_acl.c */
 
 extern void posix_acl_init(struct posix_acl *, int);
-extern struct posix_acl *posix_acl_alloc(int, gfp_t);
+extern struct posix_acl *posix_acl_alloc(unsigned int count, gfp_t flags);
 extern struct posix_acl *posix_acl_from_mode(umode_t, gfp_t);
 extern int posix_acl_equiv_mode(const struct posix_acl *, umode_t *);
 extern int __posix_acl_create(struct posix_acl **, gfp_t, umode_t *);
-- 
2.47.0