fs: Annotate struct file_handle with __counted_by() and use struct_size()

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time via CONFIG_UBSAN_BOUNDS (for
array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

While there, use struct_size() helper, instead of the open-coded
version.

[brauner@kernel.org: contains a fix by Edward for an OOB access]
Reported-by: syzbot+4139435cb1b34cf759c2@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
Link: https://lore.kernel.org/r/tencent_A7845DD769577306D813742365E976E3A205@qq.comSigned-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/ZgImCXTdGDTeBvSS@neatReviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/fhandle.c

@@ -36,7 +36,7 @@ static long do_sys_name_to_handle(const struct path *path,
 	if (f_handle.handle_bytes > MAX_HANDLE_SZ)
 		return -EINVAL;
 
-	handle = kzalloc(sizeof(struct file_handle) + f_handle.handle_bytes,
+	handle = kzalloc(struct_size(handle, f_handle, f_handle.handle_bytes),
 			 GFP_KERNEL);
 	if (!handle)
 		return -ENOMEM;
@@ -71,7 +71,7 @@ static long do_sys_name_to_handle(const struct path *path,
 	/* copy the mount id */
 	if (put_user(real_mount(path->mnt)->mnt_id, mnt_id) ||
 	    copy_to_user(ufh, handle,
-			 sizeof(struct file_handle) + handle_bytes))
+			 struct_size(handle, f_handle, handle_bytes)))
 		retval = -EFAULT;
 	kfree(handle);
 	return retval;
@@ -192,7 +192,7 @@ static int handle_to_path(int mountdirfd, struct file_handle __user *ufh,
 		retval = -EINVAL;
 		goto out_err;
 	}
-	handle = kmalloc(sizeof(struct file_handle) + f_handle.handle_bytes,
+	handle = kmalloc(struct_size(handle, f_handle, f_handle.handle_bytes),
 			 GFP_KERNEL);
 	if (!handle) {
 		retval = -ENOMEM;

include/linux/fs.h

@@ -1033,7 +1033,7 @@ struct file_handle {
 	__u32 handle_bytes;
 	int handle_type;
 	/* file identifier */
-	unsigned char f_handle[];
+	unsigned char f_handle[] __counted_by(handle_bytes);
 };
 
 static inline struct file *get_file(struct file *f)