linux-kernel - Re: [PATCH 1/1 linux-next] ext4: add compatibility flag check

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Date:   Thu, 24 Nov 2016 22:43:19 +0100 (CET)
From:   Fabian Frederick <fabf@...net.be>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     linux-kernel@...r.kernel.org, Jan Kara <jack@...e.com>,
        tytso@....edu, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/1 linux-next] ext4: add compatibility flag check



> On 24 November 2016 at 21:39 Al Viro <viro@...IV.linux.org.uk> wrote:
>
>
> On Thu, Nov 24, 2016 at 08:47:41PM +0100, Fabian Frederick wrote:
> > data=journal mount option should disable O_DIRECT access
> > (See Documentation/filesystems/ext4.txt) but open operations
> > using O_CREAT|O_RDWR|O_DIRECT|O_SYNC have no warning in return and file is
> > being
> > created. This patch adds vfs super_operations compatibility flag function
> > returning -EPERM in such a case.
>
> Why not simply check it in ->open()?  Occam's Razor and all such...
Thanks for the advice Al but I already tried that solution (see below) and had
an empty file created
(write operation is of course avoided).

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 2a822d3..ec414b5 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -323,6 +323,10 @@ static int ext4_file_open(struct inode * inode, struct file
* filp)
        char buf[64], *cp;
        int ret;
 
+       if ((test_opt(inode->i_sb, DATA_FLAGS) == EXT4_MOUNT_JOURNAL_DATA) &&
+          (filp->f_flags & O_DIRECT ))
+               return -EPERM;
+
        if (unlikely(!(sbi->s_mount_flags & EXT4_MF_MNTDIR_SAMPLED) &&
                     !(sb->s_flags & MS_RDONLY))) {
                sbi->s_mount_flags |= EXT4_MF_MNTDIR_SAMPLED;