linux-ext4 - Re: [PATCH v5 15/42] fs: add file_user

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Date:   Tue, 19 Jan 2021 16:05:00 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        John Johansen <john.johansen@...onical.com>,
        James Morris <jmorris@...ei.org>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
        Geoffrey Thomas <geofft@...reload.com>,
        Mrunal Patel <mpatel@...hat.com>,
        Josh Triplett <josh@...htriplett.org>,
        Andy Lutomirski <luto@...nel.org>,
        Theodore Tso <tytso@....edu>, Alban Crequy <alban@...volk.io>,
        Tycho Andersen <tycho@...ho.ws>,
        David Howells <dhowells@...hat.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Seth Forshee <seth.forshee@...onical.com>,
        Stéphane Graber <stgraber@...ntu.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Lennart Poettering <lennart@...ttering.net>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Stephen Barber <smbarber@...omium.org>,
        Phil Estes <estesp@...il.com>, Serge Hallyn <serge@...lyn.com>,
        Kees Cook <keescook@...omium.org>,
        Todd Kjos <tkjos@...gle.com>, Paul Moore <paul@...l-moore.com>,
        Jonathan Corbet <corbet@....net>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        linux-xfs@...r.kernel.org, linux-integrity@...r.kernel.org,
        SElinux list <selinux@...r.kernel.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v5 15/42] fs: add file_user_ns() helper

On Wed, Jan 13, 2021 at 1:52 AM Christian Brauner
<christian.brauner@...ntu.com> wrote:
> Add a simple helper to retrieve the user namespace associated with the
> vfsmount of a file. Christoph correctly points out that this makes
> codepaths (e.g. ioctls) way easier to follow that would otherwise
> dereference via mnt_user_ns(file->f_path.mnt).
>
> In order to make file_user_ns() static inline we'd need to include
> mount.h in either file.h or fs.h which seems undesirable so let's simply
> not force file_user_ns() to be inline.
[...]
> +struct user_namespace *file_user_ns(struct file *file)
> +{
> +       return mnt_user_ns(file->f_path.mnt);
> +}

That name is confusing to me, because when I think of "the userns of a
file", it's file->f_cred->user_ns. There are a bunch of places that
look at that, as you can see from grepping for "f_cred->user_ns".

If you really want this to be a separate helper, can you maybe give it
a clearer name? file_mnt_user_ns(), or something like that, idk.