Re: [PATCH 4.19 013/247] cifs: Set CIFS_MOUNT_USE_PREFIX_PATH flag on setting cifs_sb->prepath.

[Salvatore Bonaccorso]

Hi,

On Mon, Mar 01, 2021 at 05:10:33PM +0100, Greg Kroah-Hartman wrote:
> From: Shyam Prasad N <sprasad@xxxxxxxxxxxxx>
> 
> [ Upstream commit a738c93fb1c17e386a09304b517b1c6b2a6a5a8b ]
> 
> While debugging another issue today, Steve and I noticed that if a
> subdir for a file share is already mounted on the client, any new
> mount of any other subdir (or the file share root) of the same share
> results in sharing the cifs superblock, which e.g. can result in
> incorrect device name.
> 
> While setting prefix path for the root of a cifs_sb,
> CIFS_MOUNT_USE_PREFIX_PATH flag should also be set.
> Without it, prepath is not even considered in some places,
> and output of "mount" and various /proc/<>/*mount* related
> options can be missing part of the device name.
> 
> Signed-off-by: Shyam Prasad N <sprasad@xxxxxxxxxxxxx>
> Reviewed-by: Aurelien Aptel <aaptel@xxxxxxxx>
> Signed-off-by: Steve French <stfrench@xxxxxxxxxxxxx>
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> ---
>  fs/cifs/connect.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/fs/cifs/connect.c b/fs/cifs/connect.c
> index 6285085195c15..632249ce61eba 100644
> --- a/fs/cifs/connect.c
> +++ b/fs/cifs/connect.c
> @@ -3882,6 +3882,7 @@ int cifs_setup_cifs_sb(struct smb_vol *pvolume_info,
>  		cifs_sb->prepath = kstrdup(pvolume_info->prepath, GFP_KERNEL);
>  		if (cifs_sb->prepath == NULL)
>  			return -ENOMEM;
> +		cifs_sb->mnt_cifs_flags |= CIFS_MOUNT_USE_PREFIX_PATH;
>  	}
>  
>  	return 0;

A user in Debian reported an issue with mounts of DFS shares after an
update in Debian from 4.19.177 to 4.181:

https://lists.debian.org/debian-user/2021/04/msg00062.html

In a test setup i was able to reproduce the issue with 4.19.184 itself
(but interestingly not withing the 5.10.y series, checked 5.10.26)
which both contain the above commit.

4.19.184 with a738c93fb1c1 ("cifs: Set CIFS_MOUNT_USE_PREFIX_PATH flag
on setting cifs_sb->prepath.") reverted fixes the issue.

Is there probably some missing prerequisites missing in the 4.19.y
brach? I could not test othr versions, but maybe other versions are
affected as well as before 4.19.y, as the commit was backported to
4.14.223 as well.

Regards,
Salvatore