linux-ext4 - [PATCH] ext4: do not update s_last

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Message-Id: <1524413530-23808-1-git-send-email-amir73il@gmail.com>
Date:   Sun, 22 Apr 2018 09:12:10 -0700
From:   Amir Goldstein <amir73il@...il.com>
To:     Theodore Tso <tytso@....edu>
Cc:     Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org
Subject: [PATCH] ext4: do not update s_last_mounted of a frozen fs

If fs is frozen after mount and before the first file open, the
update of s_last_mounted bypasses freeze protection and prints out
a WARNING splat:

$ mount /vdf
$ fsfreeze -f /vdf
$ cat /vdf/foo

[   31.578555] WARNING: CPU: 1 PID: 1415 at
fs/ext4/ext4_jbd2.c:53 ext4_journal_check_start+0x48/0x82

[   31.614016] Call Trace:
[   31.614997]  __ext4_journal_start_sb+0xe4/0x1a4
[   31.616771]  ? ext4_file_open+0xb6/0x189
[   31.618094]  ext4_file_open+0xb6/0x189

This fix might not be free of open vs. freeze race, but it closes
a big hole.

[backport hint: to apply to stable tree, just need to repace
                sb_rdonly(sb) with (sb->s_flags & MS_RDONLY)]

Fixes: bc0b0d6d69ee ("ext4: update the s_last_mounted field in the
superblock")
Cc: <stable@...r.kernel.org>
Signed-off-by: Amir Goldstein <amir73il@...il.com>
---
 fs/ext4/file.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index fb6f023622fe..b78e5e6c0e51 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -392,8 +392,8 @@ static int ext4_file_open(struct inode * inode, struct file * filp)
 	if (unlikely(ext4_forced_shutdown(EXT4_SB(inode->i_sb))))
 		return -EIO;
 
-	if (unlikely(!(sbi->s_mount_flags & EXT4_MF_MNTDIR_SAMPLED) &&
-		     !sb_rdonly(sb))) {
+	if (unlikely(!(sbi->s_mount_flags & EXT4_MF_MNTDIR_SAMPLED)) &&
+	    !sb_rdonly(sb) && sb->s_writers.frozen == SB_UNFROZEN) {
 		sbi->s_mount_flags |= EXT4_MF_MNTDIR_SAMPLED;
 		/*
 		 * Sample where the filesystem has been mounted and
-- 
2.7.4