debugfs: add small file operations for most files

As struct file_operations is really big, but (most) debugfs
files only use simple_open, read, write and perhaps seek, and
don't need anything else, this wastes a lot of space for NULL
pointers.

Add a struct debugfs_short_fops and some bookkeeping code in
debugfs so that users can use that with debugfs_create_file()
using _Generic to figure out which function to use.

Converting mac80211 to use it where possible saves quite a
bit of space:

1010127 205064 1220 1216411 128f9b net/mac80211/mac80211.ko (before)
981199 205064 1220 1187483 121e9b net/mac80211/mac80211.ko (after)
-------
-28928 = ~28KiB

With a marginal space cost in debugfs:

8701 550 16 9267 2433 fs/debugfs/inode.o (before)
25233 325 32 25590 63f6 fs/debugfs/file.o (before)
8914 558 16 9488 2510 fs/debugfs/inode.o (after)
25380 325 32 25737 6489 fs/debugfs/file.o (after)
---------------
+360 +8

(All on x86-64)

A simple spatch suggests there are more than 300 instances,
not even counting the ones hidden in macros like in mac80211,
that could be trivially converted, for additional savings of
about 240 bytes for each.

Reviewed-by: Greg Kroah-Hartman <[e-mail]>
Link: https://patch.msgid.link/20241022151838.26f9925fb959.Ia80b55e934bbfc45ce0df42a3233d34b35508046@changeid
Signed-off-by: Johannes Berg <[e-mail]>

fs/debugfs/file.c

···
 		if (!fsd)
 			return -ENOMEM;
 
-		fsd->real_fops = (void *)((unsigned long)d_fsd &
-					~DEBUGFS_FSDATA_IS_REAL_FOPS_BIT);
+		if ((unsigned long)d_fsd & DEBUGFS_FSDATA_IS_SHORT_FOPS_BIT) {
+			fsd->real_fops = NULL;
+			fsd->short_fops = (void *)((unsigned long)d_fsd &
+						~(DEBUGFS_FSDATA_IS_REAL_FOPS_BIT |
+						  DEBUGFS_FSDATA_IS_SHORT_FOPS_BIT));
+		} else {
+			fsd->real_fops = (void *)((unsigned long)d_fsd &
+						~DEBUGFS_FSDATA_IS_REAL_FOPS_BIT);
+			fsd->short_fops = NULL;
+		}
 		refcount_set(&fsd->active_users, 1);
 		init_completion(&fsd->active_users_drained);
 		INIT_LIST_HEAD(&fsd->cancellations);
···
 {
 	if ((inode->i_mode & 07777 & ~0444) == 0 &&
 	    !(filp->f_mode & FMODE_WRITE) &&
-	    !real_fops->unlocked_ioctl &&
-	    !real_fops->compat_ioctl &&
-	    !real_fops->mmap)
+	    (!real_fops ||
+	     (!real_fops->unlocked_ioctl &&
+	      !real_fops->compat_ioctl &&
+	      !real_fops->mmap)))
 		return 0;
 
 	if (security_locked_down(LOCKDOWN_DEBUGFS))
···
 	return r;							\
 }
 
-FULL_PROXY_FUNC(llseek, loff_t, filp,
-		PROTO(struct file *filp, loff_t offset, int whence),
-		ARGS(filp, offset, whence));
+#define FULL_PROXY_FUNC_BOTH(name, ret_type, filp, proto, args)		\
+static ret_type full_proxy_ ## name(proto)				\
+{									\
+	struct dentry *dentry = F_DENTRY(filp);				\
+	struct debugfs_fsdata *fsd;					\
+	ret_type r;							\
+									\
+	r = debugfs_file_get(dentry);					\
+	if (unlikely(r))						\
+		return r;						\
+	fsd = dentry->d_fsdata;						\
+	if (fsd->real_fops)						\
+		r = fsd->real_fops->name(args);				\
+	else								\
+		r = fsd->short_fops->name(args);			\
+	debugfs_file_put(dentry);					\
+	return r;							\
+}
 
-FULL_PROXY_FUNC(read, ssize_t, filp,
-		PROTO(struct file *filp, char __user *buf, size_t size,
-			loff_t *ppos),
-		ARGS(filp, buf, size, ppos));
+FULL_PROXY_FUNC_BOTH(llseek, loff_t, filp,
+		     PROTO(struct file *filp, loff_t offset, int whence),
+		     ARGS(filp, offset, whence));
 
-FULL_PROXY_FUNC(write, ssize_t, filp,
-		PROTO(struct file *filp, const char __user *buf, size_t size,
-			loff_t *ppos),
-		ARGS(filp, buf, size, ppos));
+FULL_PROXY_FUNC_BOTH(read, ssize_t, filp,
+		     PROTO(struct file *filp, char __user *buf, size_t size,
+			   loff_t *ppos),
+		     ARGS(filp, buf, size, ppos));
+
+FULL_PROXY_FUNC_BOTH(write, ssize_t, filp,
+		     PROTO(struct file *filp, const char __user *buf,
+			   size_t size, loff_t *ppos),
+		     ARGS(filp, buf, size, ppos));
 
 FULL_PROXY_FUNC(unlocked_ioctl, long, filp,
 		PROTO(struct file *filp, unsigned int cmd, unsigned long arg),
···
 	 * not to leak any resources. Releasers must not assume that
 	 * ->i_private is still being meaningful here.
 	 */
-	if (real_fops->release)
+	if (real_fops && real_fops->release)
 		r = real_fops->release(inode, filp);
 
 	replace_fops(filp, d_inode(dentry)->i_fop);
···
 }
 
 static void __full_proxy_fops_init(struct file_operations *proxy_fops,
-				const struct file_operations *real_fops)
+				   struct debugfs_fsdata *fsd)
 {
 	proxy_fops->release = full_proxy_release;
-	if (real_fops->llseek)
+
+	if ((fsd->real_fops && fsd->real_fops->llseek) ||
+	    (fsd->short_fops && fsd->short_fops->llseek))
 		proxy_fops->llseek = full_proxy_llseek;
-	if (real_fops->read)
+
+	if ((fsd->real_fops && fsd->real_fops->read) ||
+	    (fsd->short_fops && fsd->short_fops->read))
 		proxy_fops->read = full_proxy_read;
-	if (real_fops->write)
+
+	if ((fsd->real_fops && fsd->real_fops->write) ||
+	    (fsd->short_fops && fsd->short_fops->write))
 		proxy_fops->write = full_proxy_write;
-	if (real_fops->poll)
+
+	if (fsd->real_fops && fsd->real_fops->poll)
 		proxy_fops->poll = full_proxy_poll;
-	if (real_fops->unlocked_ioctl)
+
+	if (fsd->real_fops && fsd->real_fops->unlocked_ioctl)
 		proxy_fops->unlocked_ioctl = full_proxy_unlocked_ioctl;
 }
 
 static int full_proxy_open(struct inode *inode, struct file *filp)
 {
 	struct dentry *dentry = F_DENTRY(filp);
-	const struct file_operations *real_fops = NULL;
+	const struct file_operations *real_fops;
 	struct file_operations *proxy_fops = NULL;
+	struct debugfs_fsdata *fsd;
 	int r;
 
 	r = debugfs_file_get(dentry);
 	if (r)
 		return r == -EIO ? -ENOENT : r;
 
-	real_fops = debugfs_real_fops(filp);
-
+	fsd = dentry->d_fsdata;
+	real_fops = fsd->real_fops;
 	r = debugfs_locked_down(inode, filp, real_fops);
 	if (r)
 		goto out;
 
-	if (!fops_get(real_fops)) {
+	if (real_fops && !fops_get(real_fops)) {
 #ifdef CONFIG_MODULES
 		if (real_fops->owner &&
 		    real_fops->owner->state == MODULE_STATE_GOING) {
···
 		r = -ENOMEM;
 		goto free_proxy;
 	}
-	__full_proxy_fops_init(proxy_fops, real_fops);
+	__full_proxy_fops_init(proxy_fops, fsd);
 	replace_fops(filp, proxy_fops);
 
-	if (real_fops->open) {
-		r = real_fops->open(inode, filp);
+	if (!real_fops || real_fops->open) {
+		if (real_fops)
+			r = real_fops->open(inode, filp);
+		else
+			r = simple_open(inode, filp);
 		if (r) {
 			replace_fops(filp, d_inode(dentry)->i_fop);
 			goto free_proxy;

fs/debugfs/inode.c

···
 static struct dentry *__debugfs_create_file(const char *name, umode_t mode,
 				struct dentry *parent, void *data,
 				const struct file_operations *proxy_fops,
-				const struct file_operations *real_fops)
+				const void *real_fops)
 {
 	struct dentry *dentry;
 	struct inode *inode;
···
 	return end_creating(dentry);
 }
 
-/**
- * debugfs_create_file - create a file in the debugfs filesystem
- * @name: a pointer to a string containing the name of the file to create.
- * @mode: the permission that the file should have.
- * @parent: a pointer to the parent dentry for this file.  This should be a
- *          directory dentry if set.  If this parameter is NULL, then the
- *          file will be created in the root of the debugfs filesystem.
- * @data: a pointer to something that the caller will want to get to later
- *        on.  The inode.i_private pointer will point to this value on
- *        the open() call.
- * @fops: a pointer to a struct file_operations that should be used for
- *        this file.
- *
- * This is the basic "create a file" function for debugfs.  It allows for a
- * wide range of flexibility in creating a file, or a directory (if you want
- * to create a directory, the debugfs_create_dir() function is
- * recommended to be used instead.)
- *
- * This function will return a pointer to a dentry if it succeeds.  This
- * pointer must be passed to the debugfs_remove() function when the file is
- * to be removed (no automatic cleanup happens if your module is unloaded,
- * you are responsible here.)  If an error occurs, ERR_PTR(-ERROR) will be
- * returned.
- *
- * If debugfs is not enabled in the kernel, the value -%ENODEV will be
- * returned.
- *
- * NOTE: it's expected that most callers should _ignore_ the errors returned
- * by this function. Other debugfs functions handle the fact that the "dentry"
- * passed to them could be an error and they don't crash in that case.
- * Drivers should generally work fine even if debugfs fails to init anyway.
- */
-struct dentry *debugfs_create_file(const char *name, umode_t mode,
-				   struct dentry *parent, void *data,
-				   const struct file_operations *fops)
+struct dentry *debugfs_create_file_full(const char *name, umode_t mode,
+					struct dentry *parent, void *data,
+					const struct file_operations *fops)
 {
+	if (WARN_ON((unsigned long)fops &
+		    (DEBUGFS_FSDATA_IS_SHORT_FOPS_BIT |
+		     DEBUGFS_FSDATA_IS_REAL_FOPS_BIT)))
+		return ERR_PTR(-EINVAL);
 
 	return __debugfs_create_file(name, mode, parent, data,
 				fops ? &debugfs_full_proxy_file_operations :
 					&debugfs_noop_file_operations,
 				fops);
 }
-EXPORT_SYMBOL_GPL(debugfs_create_file);
+EXPORT_SYMBOL_GPL(debugfs_create_file_full);
+
+struct dentry *debugfs_create_file_short(const char *name, umode_t mode,
+					 struct dentry *parent, void *data,
+					 const struct debugfs_short_fops *fops)
+{
+	if (WARN_ON((unsigned long)fops &
+		    (DEBUGFS_FSDATA_IS_SHORT_FOPS_BIT |
+		     DEBUGFS_FSDATA_IS_REAL_FOPS_BIT)))
+		return ERR_PTR(-EINVAL);
+
+	return __debugfs_create_file(name, mode, parent, data,
+				fops ? &debugfs_full_proxy_file_operations :
+					&debugfs_noop_file_operations,
+				(const void *)((unsigned long)fops |
+					       DEBUGFS_FSDATA_IS_SHORT_FOPS_BIT));
+}
+EXPORT_SYMBOL_GPL(debugfs_create_file_short);
 
 /**
  * debugfs_create_file_unsafe - create a file in the debugfs filesystem

fs/debugfs/internal.h

···
 
 struct debugfs_fsdata {
 	const struct file_operations *real_fops;
+	const struct debugfs_short_fops *short_fops;
 	union {
 		/* automount_fn is used when real_fops is NULL */
 		debugfs_automount_t automount;
···
  * pointer gets its lowest bit set.
  */
 #define DEBUGFS_FSDATA_IS_REAL_FOPS_BIT BIT(0)
+/*
+ * A dentry's ->d_fsdata, when pointing to real fops, is with
+ * short fops instead of full fops.
+ */
+#define DEBUGFS_FSDATA_IS_SHORT_FOPS_BIT BIT(1)
 
 /* Access BITS */
 #define DEBUGFS_ALLOW_API	BIT(0)

include/linux/debugfs.h

···
 
 struct dentry *debugfs_lookup(const char *name, struct dentry *parent);
 
-struct dentry *debugfs_create_file(const char *name, umode_t mode,
-				   struct dentry *parent, void *data,
-				   const struct file_operations *fops);
+struct debugfs_short_fops {
+	ssize_t (*read)(struct file *, char __user *, size_t, loff_t *);
+	ssize_t (*write)(struct file *, const char __user *, size_t, loff_t *);
+	loff_t (*llseek) (struct file *, loff_t, int);
+};
+
+struct dentry *debugfs_create_file_full(const char *name, umode_t mode,
+					struct dentry *parent, void *data,
+					const struct file_operations *fops);
+struct dentry *debugfs_create_file_short(const char *name, umode_t mode,
+					 struct dentry *parent, void *data,
+					 const struct debugfs_short_fops *fops);
+
+/**
+ * debugfs_create_file - create a file in the debugfs filesystem
+ * @name: a pointer to a string containing the name of the file to create.
+ * @mode: the permission that the file should have.
+ * @parent: a pointer to the parent dentry for this file.  This should be a
+ *          directory dentry if set.  If this parameter is NULL, then the
+ *          file will be created in the root of the debugfs filesystem.
+ * @data: a pointer to something that the caller will want to get to later
+ *        on.  The inode.i_private pointer will point to this value on
+ *        the open() call.
+ * @fops: a pointer to a struct file_operations or struct debugfs_short_fops that
+ *        should be used for this file.
+ *
+ * This is the basic "create a file" function for debugfs.  It allows for a
+ * wide range of flexibility in creating a file, or a directory (if you want
+ * to create a directory, the debugfs_create_dir() function is
+ * recommended to be used instead.)
+ *
+ * This function will return a pointer to a dentry if it succeeds.  This
+ * pointer must be passed to the debugfs_remove() function when the file is
+ * to be removed (no automatic cleanup happens if your module is unloaded,
+ * you are responsible here.)  If an error occurs, ERR_PTR(-ERROR) will be
+ * returned.
+ *
+ * If debugfs is not enabled in the kernel, the value -%ENODEV will be
+ * returned.
+ *
+ * If fops points to a struct debugfs_short_fops, then simple_open() will be
+ * used for the open, and only read/write/llseek are supported and are proxied,
+ * so no module reference or release are needed.
+ *
+ * NOTE: it's expected that most callers should _ignore_ the errors returned
+ * by this function. Other debugfs functions handle the fact that the "dentry"
+ * passed to them could be an error and they don't crash in that case.
+ * Drivers should generally work fine even if debugfs fails to init anyway.
+ */
+#define debugfs_create_file(name, mode, parent, data, fops)			\
+	_Generic(fops,								\
+		 const struct file_operations *: debugfs_create_file_full,	\
+		 const struct debugfs_short_fops *: debugfs_create_file_short,	\
+		 struct file_operations *: debugfs_create_file_full,		\
+		 struct debugfs_short_fops *: debugfs_create_file_short)	\
+		(name, mode, parent, data, fops)
+
 struct dentry *debugfs_create_file_unsafe(const char *name, umode_t mode,
 				   struct dentry *parent, void *data,
 				   const struct file_operations *fops);
···
 
 static inline struct dentry *debugfs_create_file(const char *name, umode_t mode,
 					struct dentry *parent, void *data,
-					const struct file_operations *fops)
+					const void *fops)
 {
 	return ERR_PTR(-ENODEV);
 }