sysfs: Add sysfs_bin_attr_simple_read() helper

When drivers expose a bin_attribute in sysfs which is backed by a buffer
in memory, a common pattern is to set the @private and @size members in
struct bin_attribute to the buffer's location and size.

The ->read() callback then merely consists of a single memcpy() call.
It's not even necessary to perform bounds checks as these are already
handled by sysfs_kf_bin_read().

However each driver is so far providing its own ->read() implementation.
The pattern is sufficiently frequent to merit a public helper, so add
sysfs_bin_attr_simple_read() as well as BIN_ATTR_SIMPLE_RO() and
BIN_ATTR_SIMPLE_ADMIN_RO() macros to ease declaration of such
bin_attributes and reduce LoC and .text section size.

Signed-off-by: Lukas Wunner <[e-mail]>
Reviewed-by: Greg Kroah-Hartman <[e-mail]>
Acked-by: Rafael J. Wysocki <[e-mail]>
Acked-by: Ard Biesheuvel <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

fs/sysfs/file.c

···
 	return len;
 }
 EXPORT_SYMBOL_GPL(sysfs_emit_at);
+
+/**
+ *	sysfs_bin_attr_simple_read - read callback to simply copy from memory.
+ *	@file:	attribute file which is being read.
+ *	@kobj:	object to which the attribute belongs.
+ *	@attr:	attribute descriptor.
+ *	@buf:	destination buffer.
+ *	@off:	offset in bytes from which to read.
+ *	@count:	maximum number of bytes to read.
+ *
+ * Simple ->read() callback for bin_attributes backed by a buffer in memory.
+ * The @private and @size members in struct bin_attribute must be set to the
+ * buffer's location and size before the bin_attribute is created in sysfs.
+ *
+ * Bounds check for @off and @count is done in sysfs_kf_bin_read().
+ * Negative value check for @off is done in vfs_setpos() and default_llseek().
+ *
+ * Returns number of bytes written to @buf.
+ */
+ssize_t sysfs_bin_attr_simple_read(struct file *file, struct kobject *kobj,
+				   struct bin_attribute *attr, char *buf,
+				   loff_t off, size_t count)
+{
+	memcpy(buf, attr->private + off, count);
+	return count;
+}
+EXPORT_SYMBOL_GPL(sysfs_bin_attr_simple_read);

include/linux/sysfs.h

···
 #define BIN_ATTR_ADMIN_RW(_name, _size)					\
 struct bin_attribute bin_attr_##_name = __BIN_ATTR_ADMIN_RW(_name, _size)
 
+#define __BIN_ATTR_SIMPLE_RO(_name, _mode) {				\
+	.attr	= { .name = __stringify(_name), .mode = _mode },	\
+	.read	= sysfs_bin_attr_simple_read,				\
+}
+
+#define BIN_ATTR_SIMPLE_RO(_name)					\
+struct bin_attribute bin_attr_##_name = __BIN_ATTR_SIMPLE_RO(_name, 0444)
+
+#define BIN_ATTR_SIMPLE_ADMIN_RO(_name)					\
+struct bin_attribute bin_attr_##_name = __BIN_ATTR_SIMPLE_RO(_name, 0400)
+
 struct sysfs_ops {
 	ssize_t	(*show)(struct kobject *, struct attribute *, char *);
 	ssize_t	(*store)(struct kobject *, struct attribute *, const char *, size_t);
···
 int sysfs_emit(char *buf, const char *fmt, ...);
 __printf(3, 4)
 int sysfs_emit_at(char *buf, int at, const char *fmt, ...);
+
+ssize_t sysfs_bin_attr_simple_read(struct file *file, struct kobject *kobj,
+				   struct bin_attribute *attr, char *buf,
+				   loff_t off, size_t count);
 
 #else /* CONFIG_SYSFS */