f2fs: restrict setting up inode.i_advise

In order to give advise to f2fs to recognize hot/cold file, it is possible
that we can set specific bit in inode.i_advise through setxattr(), but
there are several bits which are used internally, such as encrypt_bit,
keep_size_bit, they should never be changed through setxattr().

So that this patch 1) adds FADVISE_MODIFIABLE_BITS to filter modifiable
bits user given, 2) supports to clear {hot,cold}_file bits.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/xattr.c

@@ -94,12 +94,22 @@ static int f2fs_xattr_advise_set(const struct xattr_handler *handler,
 		const char *name, const void *value,
 		size_t size, int flags)
 {
+	unsigned char old_advise = F2FS_I(inode)->i_advise;
+	unsigned char new_advise;
+
 	if (!inode_owner_or_capable(inode))
 		return -EPERM;
 	if (value == NULL)
 		return -EINVAL;
 
-	F2FS_I(inode)->i_advise |= *(char *)value;
+	new_advise = *(char *)value;
+	if (new_advise & ~FADVISE_MODIFIABLE_BITS)
+		return -EINVAL;
+
+	new_advise = new_advise & FADVISE_MODIFIABLE_BITS;
+	new_advise |= old_advise & ~FADVISE_MODIFIABLE_BITS;
+
+	F2FS_I(inode)->i_advise = new_advise;
 	f2fs_mark_inode_dirty_sync(inode, true);
 	return 0;
 }