linux-kernel - Re: [PATCH v3 09/25] fs: add is_userns

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Date:   Tue, 18 Feb 2020 20:42:33 -0600
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Stéphane Graber <stgraber@...ntu.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Aleksa Sarai <cyphar@...har.com>, Jann Horn <jannh@...gle.com>,
        smbarber@...omium.org, Seth Forshee <seth.forshee@...onical.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Serge Hallyn <serge@...lyn.com>,
        James Morris <jmorris@...ei.org>,
        Kees Cook <keescook@...omium.org>,
        Jonathan Corbet <corbet@....net>,
        Phil Estes <estesp@...il.com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        containers@...ts.linux-foundation.org,
        linux-security-module@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH v3 09/25] fs: add is_userns_visible() helper

On Tue, Feb 18, 2020 at 03:33:55PM +0100, Christian Brauner wrote:
> Introduce a helper which makes it possible to detect fileystems whose
> superblock is visible in multiple user namespace. This currently only
> means proc and sys. Such filesystems usually have special semantics so their
> behavior will not be changed with the introduction of fsid mappings.

Hi,

I'm afraid I've got a bit of a hangup about the terminology here.  I
*think* what you mean is that SB_I_USERNS_VISIBLE is an fs whose uids are
always translated per the id mappings, not fsid mappings.  But when I see
the name it seems to imply that !SB_I_USERNS_VISIBLE filesystems can't
be seen by other namespaces at all.

Am I right in my first interpretation?  If so, can we talk about the
naming?

> Signed-off-by: Christian Brauner <christian.brauner@...ntu.com>
> ---
> /* v2 */
> unchanged
> 
> /* v3 */
> unchanged
> ---
>  include/linux/fs.h | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 3cd4fe6b845e..fdc8fb2d786b 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -3651,4 +3651,9 @@ static inline int inode_drain_writes(struct inode *inode)
>  	return filemap_write_and_wait(inode->i_mapping);
>  }
>  
> +static inline bool is_userns_visible(unsigned long iflags)
> +{
> +	return (iflags & SB_I_USERNS_VISIBLE);
> +}
> +
>  #endif /* _LINUX_FS_H */
> -- 
> 2.25.0