f2fs: use generic checking and prep function for FS_IOC_SETFLAGS

Make the f2fs implementation of FS_IOC_SETFLAGS use the new VFS helper
function vfs_ioc_setflags_prepare().

This is based on a patch from Darrick Wong, but reworked to apply after
commit 360985573b55 ("f2fs: separate f2fs i_flags from fs_flags and ext4
i_flags").

Originally-from: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/file.c

@@ -1770,7 +1770,8 @@ static int f2fs_ioc_getflags(struct file *filp, unsigned long arg)
 static int f2fs_ioc_setflags(struct file *filp, unsigned long arg)
 {
 	struct inode *inode = file_inode(filp);
-	u32 fsflags;
+	struct f2fs_inode_info *fi = F2FS_I(inode);
+	u32 fsflags, old_fsflags;
 	u32 iflags;
 	int ret;
 
@@ -1794,8 +1795,14 @@ static int f2fs_ioc_setflags(struct file *filp, unsigned long arg)
 
 	inode_lock(inode);
 
+	old_fsflags = f2fs_iflags_to_fsflags(fi->i_flags);
+	ret = vfs_ioc_setflags_prepare(inode, old_fsflags, fsflags);
+	if (ret)
+		goto out;
+
 	ret = f2fs_setflags_common(inode, iflags,
 			f2fs_fsflags_to_iflags(F2FS_SETTABLE_FS_FL));
+out:
 	inode_unlock(inode);
 	mnt_drop_write_file(filp);
 	return ret;

fs/inode.c

@@ -2125,3 +2125,27 @@ struct timespec current_time(struct inode *inode)
 	return timespec_trunc(now, inode->i_sb->s_time_gran);
 }
 EXPORT_SYMBOL(current_time);
+
+/*
+ * Generic function to check FS_IOC_SETFLAGS values and reject any invalid
+ * configurations.
+ *
+ * Note: the caller should be holding i_mutex, or else be sure that they have
+ * exclusive access to the inode structure.
+ */
+int vfs_ioc_setflags_prepare(struct inode *inode, unsigned int oldflags,
+			     unsigned int flags)
+{
+	/*
+	 * The IMMUTABLE and APPEND_ONLY flags can only be changed by
+	 * the relevant capability.
+	 *
+	 * This test looks nicer. Thanks to Pauline Middelink
+	 */
+	if ((flags ^ oldflags) & (FS_APPEND_FL | FS_IMMUTABLE_FL) &&
+	    !capable(CAP_LINUX_IMMUTABLE))
+		return -EPERM;
+
+	return 0;
+}
+EXPORT_SYMBOL(vfs_ioc_setflags_prepare);

include/linux/fs.h

@@ -3394,4 +3394,7 @@ static inline bool dir_relax_shared(struct inode *inode)
 extern bool path_noexec(const struct path *path);
 extern void inode_nohighmem(struct inode *inode);
 
+int vfs_ioc_setflags_prepare(struct inode *inode, unsigned int oldflags,
+			     unsigned int flags);
+
 #endif /* _LINUX_FS_H */