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 <lukas@wunner.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/5ed62b197a442ec6db53d8746d9d806dd0576e2d.1712410202.git.lukas@wunner.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
index 6b7652fb805057b8a155a0c94a3746fcb72eb5c7..289b57d32c89da0e46f3e259d91bf3ccfc1a9bad 100644 (file)
--- a/fs/sysfs/file.c
+++ b/fs/sysfs/file.c
@@ -783,3 +783,30 @@ int sysfs_emit_at(char *buf, int at, const char *fmt, ...)
        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);

diff --git a/include/linux/sysfs.h b/include/linux/sysfs.h
index 326341c623857e2afbb9b0c20a5f9b9ae4a54293..a7d725fbf7393723d96d4068605883316f95f8fd 100644 (file)
--- a/include/linux/sysfs.h
+++ b/include/linux/sysfs.h
@@ -371,6 +371,17 @@ struct bin_attribute bin_attr_##_name = __BIN_ATTR_ADMIN_RO(_name, _size)
 #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);
@@ -478,6 +489,10 @@ 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 */
 
 static inline int sysfs_create_dir_ns(struct kobject *kobj, const void *ns)