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

[Li Lingfeng]

Friendly ping ...

Thanks

在 2024/6/14 11:14, Li Lingfeng 写道:
> 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);