netdev - Re: [PATCH bpf-next v10 2/4] bpf: new helper to obtain namespace data from current task New bpf helper bpf_get_current_pidns

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Date:   Wed, 11 Sep 2019 00:15:06 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Yonghong Song <yhs@...com>
Cc:     Carlos Antonio Neira Bustos <cneirabustos@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ebiederm@...ssion.com" <ebiederm@...ssion.com>,
        "brouer@...hat.com" <brouer@...hat.com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v10 2/4] bpf: new helper to obtain namespace
 data from current task New bpf helper bpf_get_current_pidns_info.

On Tue, Sep 10, 2019 at 10:35:09PM +0000, Yonghong Song wrote:
> 
> Carlos,
> 
> Discussed with Eric today for what is the best way to get
> the device number for a namespace. The following patch seems
> a reasonable start although Eric would like to see
> how the helper is used in order to decide whether the
> interface looks right.
> 
> commit bb00fc36d5d263047a8bceb3e51e969d7fbce7db (HEAD -> fs2)
> Author: Yonghong Song <yhs@...com>
> Date:   Mon Sep 9 21:50:51 2019 -0700
> 
>      nsfs: add an interface function ns_get_inum_dev()
> 
>      This patch added an interface function
>      ns_get_inum_dev(). Given a ns_common structure,
>      the function returns the inode and device
>      numbers. The function will be used later
>      by a newly added bpf helper.
> 
>      Signed-off-by: Yonghong Song <yhs@...com>
> 
> diff --git a/fs/nsfs.c b/fs/nsfs.c
> index a0431642c6b5..a603c6fc3f54 100644
> --- a/fs/nsfs.c
> +++ b/fs/nsfs.c
> @@ -245,6 +245,14 @@ struct file *proc_ns_fget(int fd)
>          return ERR_PTR(-EINVAL);
>   }
> 
> +/* Get the device number for the current task pidns.
> + */
> +void ns_get_inum_dev(struct ns_common *ns, u32 *inum, dev_t *dev)
> +{
> +       *inum = ns->inum;
> +       *dev = nsfs_mnt->mnt_sb->s_dev;
> +}

Umm...  Where would it get the device number once we get (hell knows
what for) multiple nsfs instances?  I still don't understand what
would that be about, TBH...  Is it really per-userns?  Or something
else entirely?  Eric, could you give some context?