[PATCH 07/11] ext4: shortcut setting of xattr to the same value

[Thadeu Lima de Souza Cascardo]

From: Jan Kara <jack at suse.cz>

When someone tried to set xattr to the same value (i.e., not changing
anything) we did all the work of removing original xattr, possibly
breaking references to shared xattr block, inserting new xattr, and
merging xattr blocks again. Since this is not so rare operation and it
is relatively cheap for us to detect this case, check for this and
shortcut xattr setting in that case.

Signed-off-by: Jan Kara <jack at suse.cz>
Signed-off-by: Theodore Ts'o <tytso at mit.edu>
(cherry picked from commit 3fd164629d25b04f291a79a013dcc7ce1a301269)
CVE-2015-8952
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---
 fs/ext4/xattr.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
index ba6b8ac5f462..6f8fd0ec76e2 100644
--- a/fs/ext4/xattr.c
+++ b/fs/ext4/xattr.c
@@ -1112,6 +1112,17 @@ static int ext4_xattr_ibody_set(handle_t *handle, struct inode *inode,
 	return 0;
 }
 
+static int ext4_xattr_value_same(struct ext4_xattr_search *s,
+				 struct ext4_xattr_info *i)
+{
+	void *value;
+
+	if (le32_to_cpu(s->here->e_value_size) != i->value_len)
+		return 0;
+	value = ((void *)s->base) + le16_to_cpu(s->here->e_value_offs);
+	return !memcmp(value, i->value, i->value_len);
+}
+
 /*
  * ext4_xattr_set_handle()
  *
@@ -1188,6 +1199,13 @@ ext4_xattr_set_handle(handle_t *handle, struct inode *inode, int name_index,
 		else if (!bs.s.not_found)
 			error = ext4_xattr_block_set(handle, inode, &i, &bs);
 	} else {
+		error = 0;
+		/* Xattr value did not change? Save us some work and bail out */
+		if (!is.s.not_found && ext4_xattr_value_same(&is.s, &i))
+			goto cleanup;
+		if (!bs.s.not_found && ext4_xattr_value_same(&bs.s, &i))
+			goto cleanup;
+
 		error = ext4_xattr_ibody_set(handle, inode, &i, &is);
 		if (!error && !bs.s.not_found) {
 			i.value = NULL;
-- 
2.14.1