[PATCH RFC 2/2] NFSv4: set sb_flags to second superblock

[Li Lingfeng]

I think this may be a problem, but I'm unable to come up with a suitable 
solution. Would you mind providing some suggestions?

在 2024/6/4 19:26, Li Lingfeng 写道:
> From: Li Lingfeng <lilingfeng3 at huawei.com>
>
> During the process of mounting an NFSv4 client, two superblocks will be
> created in sequence. The first superblock corresponds to the root
> directory exported by the server, and the second superblock corresponds to
> the directory that will be actually mounted. The first superblock will
> eventually be destroyed.
> The flag passed from user mode will only be passed to the first
> superblock, resulting in the actual used superblock not carrying the flag
> passed from user mode(fs_context_for_submount() will set sb_flags as 0).
>
> If the 'ro' parameter is used in two consecutive mount commands, only the
> first execution will create a new vfsmount, and the kernel will return
> EBUSY on the second execution. However, if a remount command with the 'ro'
> parameter is executed between the two mount commands, both mount commands
> will create new vfsmounts.
>
> The superblock generated after the first mount command does not have the
> 'ro' flag, and the read-only status of the file system is implemented by
> checking the read-only flag of the vfsmount. After executing the remount
> command, the 'ro' flag will be added to the superblock. When the second
> mount command is executed, the comparison result between the superblock
> with the 'ro' flag and the fs_context without the flag in the
> nfs_compare_mount_options() function will be different, resulting in the
> creation of a new vfsmount.
>
> This problem can be reproduced by performing the following operations:
> mount -t nfs -o ro,vers=4.0 192.168.240.250:/sdb /mnt/sdb
> mount -t nfs -o remount,ro,vers=4.0 192.168.240.250:/sdb /mnt/sdb
> mount -t nfs -o ro,vers=4.0 192.168.240.250:/sdb /mnt/sdb
> Two vfsmounts are generated:
> [root at localhost ~]# mount | grep nfs
> 192.168.240.250:/sdb on /mnt/sdb type nfs4 (ro,relatime,vers=4.0,
> rsize=1048576,wsize=1048576,namlen=255,hard,proto=tcp,timeo=600,retrans=2,
> sec=sys,clientaddr=192.168.240.251,local_lock=none,addr=192.168.240.250)
> 192.168.240.250:/sdb on /mnt/sdb type nfs4 (ro,relatime,vers=4.0,
> rsize=1048576,wsize=1048576,namlen=255,hard,proto=tcp,timeo=600,retrans=2,
> sec=sys,clientaddr=192.168.240.251,local_lock=none,addr=192.168.240.250)
>
> Fix this by setting sb_flags to second superblock.
>
> Signed-off-by: Li Lingfeng <lilingfeng3 at huawei.com>
> ---
>   fs/nfs/namespace.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/nfs/namespace.c b/fs/nfs/namespace.c
> index 887aeacedebd..8b3d75af60d4 100644
> --- a/fs/nfs/namespace.c
> +++ b/fs/nfs/namespace.c
> @@ -158,7 +158,7 @@ struct vfsmount *nfs_d_automount(struct path *path, unsigned int sb_flags)
>   	/* Open a new filesystem context, transferring parameters from the
>   	 * parent superblock, including the network namespace.
>   	 */
> -	fc = fs_context_for_submount(path->mnt->mnt_sb->s_type, path->dentry, 0);
> +	fc = fs_context_for_submount(path->mnt->mnt_sb->s_type, path->dentry, sb_flags);
>   	if (IS_ERR(fc))
>   		return ERR_CAST(fc);
>