struct filename: saner handling of long names

Always allocate struct filename from names_cachep, long name or short;
short names would be embedded into struct filename.  Longer ones do
not cannibalize the original struct filename - put them into kmalloc'ed
buffers (PATH_MAX-sized for import from userland, strlen() + 1 - for
ones originating kernel-side, where we know the length beforehand).

Cutoff length for short names is chosen so that struct filename would be
192 bytes long - that's both a multiple of 64 and large enough to cover
the majority of real-world uses.

Simplifies logics in getname()/putname() and friends.

[fixed an embarrassing braino in EMBEDDED_NAME_MAX, first reported by
Dan Carpenter]

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 8 insertions, 2 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 997d515bab32..f0f1e8034539 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2409,13 +2409,19 @@ extern struct kobject *fs_kobj;
 
 /* fs/open.c */
 struct audit_names;
-struct filename {
+
+struct __filename_head {
 	const char		*name;	/* pointer to actual string */
 	atomic_t		refcnt;
 	struct audit_names	*aname;
-	const char		iname[];
+};
+#define EMBEDDED_NAME_MAX	(192 - sizeof(struct __filename_head))
+struct filename {
+	struct __filename_head;
+	const char		iname[EMBEDDED_NAME_MAX];
 };
 static_assert(offsetof(struct filename, iname) % sizeof(long) == 0);
+static_assert(sizeof(struct filename) % 64 == 0);
 
 static inline struct mnt_idmap *file_mnt_idmap(const struct file *file)
 {