In message <1256152779-10054-8-git-send-email-vaurora@xxxxxxxxxx>, Valerie Aurora writes:
> While we can check if a file system is currently read-only, we can't
> guarantee that it will stay read-only. The file system can be
> remounted read-write at any time; it's also conceivable that a file
> system can be mounted a second time and converted to read-write if the
> underlying fs allows it. This is a problem for union mounts, which
> require the underlying file system be read-only. Add a read-only
> users count and don't allow remounts to change the file system to
> read-write or read-write mounts if there are any read-only users.
>
> Signed-off-by: Valerie Aurora <vaurora@xxxxxxxxxx>
> ---
> fs/super.c | 14 ++++++++++++++
> include/linux/fs.h | 5 +++++
> 2 files changed, 19 insertions(+), 0 deletions(-)
>
> diff --git a/fs/super.c b/fs/super.c
> index 2761d3e..c8140ac 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -553,6 +553,15 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
> }
> remount_rw = !(flags & MS_RDONLY) && (sb->s_flags & MS_RDONLY);
>
> + /* If we are remounting read/write, make sure that none of the
> + users require read-only for correct operation (such as
> + union mounts). */
Minor nit: but I think multi-line comments look better like this:
/*
* text
*/
> + if (remount_rw && sb->s_readonly_users) {
> + printk(KERN_INFO "%s: In use by %d read-only user(s)\n",
> + sb->s_id, sb->s_readonly_users);
> + return -EROFS;
> + }
> +
> if (sb->s_op->remount_fs) {
> retval = sb->s_op->remount_fs(sb, &flags, data);
> if (retval)
> @@ -889,6 +898,11 @@ vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void
> if (error)
> goto out_sb;
>
> + error = -EROFS;
> + if (!(flags & MS_RDONLY) &&
> + (mnt->mnt_sb->s_readonly_users))
Minor nit: two parts of '&&' in the above 'if' can go on same line and not
violate checkpatch.
> + goto out_sb;
> +
> mnt->mnt_mountpoint = mnt->mnt_root;
> mnt->mnt_parent = mnt;
> up_write(&mnt->mnt_sb->s_umount);
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 73e9b64..5fb7343 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1379,6 +1379,11 @@ struct super_block {
> * generic_show_options()
> */
> char *s_options;
> +
> + /*
> + * Users who require read-only access - e.g., union mounts
> + */
Minor nit: for short one-line comments I prefer to save LoC:
/* text */
> + int s_readonly_users;
> };
>
> extern struct timespec current_fs_time(struct super_block *sb);
> --
> 1.6.3.3
>
> --
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Erez.
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