[3.5.y.z extended stable] Patch "ext4: atomically set inode->i_flags in ext4_set_inode_flags()" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    ext4: atomically set inode->i_flags in ext4_set_inode_flags()

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From aaee5d1df501dd338761846588ff3daca967b113 Mon Sep 17 00:00:00 2001
From: Theodore Ts'o <tytso at mit.edu>
Date: Sun, 30 Mar 2014 10:20:01 -0400
Subject: ext4: atomically set inode->i_flags in ext4_set_inode_flags()

commit 00a1a053ebe5febcfc2ec498bd894f035ad2aa06 upstream.

Use cmpxchg() to atomically set i_flags instead of clearing out the
S_IMMUTABLE, S_APPEND, etc. flags and then setting them from the
EXT4_IMMUTABLE_FL, EXT4_APPEND_FL flags, since this opens up a race
where an immutable file has the immutable flag cleared for a brief
window of time.

Reported-by: John Sullivan <jsrhbz at kanargh.force9.co.uk>
Signed-off-by: "Theodore Ts'o" <tytso at mit.edu>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
[ luis: backported to 3.5: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 fs/ext4/inode.c        | 15 +++++++++------
 include/linux/bitops.h | 15 +++++++++++++++
 2 files changed, 24 insertions(+), 6 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index efecfb7..070bdbb 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -37,6 +37,7 @@
 #include <linux/printk.h>
 #include <linux/slab.h>
 #include <linux/ratelimit.h>
+#include <linux/bitops.h>

 #include "ext4_jbd2.h"
 #include "xattr.h"
@@ -3640,18 +3641,20 @@ int ext4_get_inode_loc(struct inode *inode, struct ext4_iloc *iloc)
 void ext4_set_inode_flags(struct inode *inode)
 {
 	unsigned int flags = EXT4_I(inode)->i_flags;
+	unsigned int new_fl = 0;

-	inode->i_flags &= ~(S_SYNC|S_APPEND|S_IMMUTABLE|S_NOATIME|S_DIRSYNC);
 	if (flags & EXT4_SYNC_FL)
-		inode->i_flags |= S_SYNC;
+		new_fl |= S_SYNC;
 	if (flags & EXT4_APPEND_FL)
-		inode->i_flags |= S_APPEND;
+		new_fl |= S_APPEND;
 	if (flags & EXT4_IMMUTABLE_FL)
-		inode->i_flags |= S_IMMUTABLE;
+		new_fl |= S_IMMUTABLE;
 	if (flags & EXT4_NOATIME_FL)
-		inode->i_flags |= S_NOATIME;
+		new_fl |= S_NOATIME;
 	if (flags & EXT4_DIRSYNC_FL)
-		inode->i_flags |= S_DIRSYNC;
+		new_fl |= S_DIRSYNC;
+	set_mask_bits(&inode->i_flags,
+		      S_SYNC|S_APPEND|S_IMMUTABLE|S_NOATIME|S_DIRSYNC, new_fl);
 }

 /* Propagate flags from i_flags to EXT4_I(inode)->i_flags */
diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index a3b6b82..c1dde8e 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -185,6 +185,21 @@ static inline unsigned long __ffs64(u64 word)

 #ifdef __KERNEL__

+#ifndef set_mask_bits
+#define set_mask_bits(ptr, _mask, _bits)	\
+({								\
+	const typeof(*ptr) mask = (_mask), bits = (_bits);	\
+	typeof(*ptr) old, new;					\
+								\
+	do {							\
+		old = ACCESS_ONCE(*ptr);			\
+		new = (old & ~mask) | bits;			\
+	} while (cmpxchg(ptr, old, new) != old);		\
+								\
+	new;							\
+})
+#endif
+
 #ifndef find_last_bit
 /**
  * find_last_bit - find the last set bit in a memory region
--
1.9.1