linux-kernel - Re: [PATCH v4 2/4] namei: O_BENEATH-style path resolution flags

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Date:   Fri, 23 Nov 2018 08:07:03 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     j@...ron.ch
Cc:     Aleksa Sarai <cyphar@...har.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Christian Brauner <christian@...uner.io>,
        Linux API <linux-api@...r.kernel.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Jann Horn <jannh@...gle.com>,
        David Drysdale <drysdale@...gle.com>,
        Aleksa Sarai <asarai@...e.de>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH v4 2/4] namei: O_BENEATH-style path resolution flags

> On Nov 23, 2018, at 5:10 AM, Jürg Billeter <j@...ron.ch> wrote:
>
> Hi Aleksa,
>
>> On Tue, 2018-11-13 at 01:26 +1100, Aleksa Sarai wrote:
>> * O_BENEATH: Disallow "escapes" from the starting point of the
>>  filesystem tree during resolution (you must stay "beneath" the
>>  starting point at all times). Currently this is done by disallowing
>>  ".." and absolute paths (either in the given path or found during
>>  symlink resolution) entirely, as well as all "magic link" jumping.
>
> With open_tree(2) and OPEN_TREE_CLONE, will O_BENEATH still be
> necessary?

This discussion reminds me of something I’m uncomfortable with in the
current patches: currently, most of the O_ flags determine some
property of the returned opened file.  The new O_ flags you're adding
don't -- instead, they affect the lookup of the file.  So O_BENEATH
doesn't return a descriptor that can only be used to loop up files
beneath it -- it just controls whether open(2) succeeds or fails.  It
might be nice for the naming of the flags to reflect this.  I also
don't love that we have some magic AT_ flags that work with some
syscalls and some magic O_ flags that work with others.

In this regard, I liked the AT_ naming better.  Although I don't love
adding AT_ flags because the restrict our ability to usefully use the
high bits of the fd in the future.

--Andy