linux-kernel - Re: [PATCH v3 1/3] namei: implement O_BENEATH-style AT

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Message-ID: <20181013080511.GT32577@ZenIV.linux.org.uk>
Date:   Sat, 13 Oct 2018 09:05:11 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Aleksa Sarai <cyphar@...har.com>
Cc:     Eric Biederman <ebiederm@...ssion.com>,
        Christian Brauner <christian@...uner.io>,
        Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        David Howells <dhowells@...hat.com>,
        Jann Horn <jannh@...gle.com>, Tycho Andersen <tycho@...ho.ws>,
        David Drysdale <drysdale@...gle.com>, dev@...ncontainers.org,
        containers@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH v3 1/3] namei: implement O_BENEATH-style AT_* flags

On Sat, Oct 13, 2018 at 08:33:19AM +0100, Al Viro wrote:

> Pardon me, but... huh?  The reason for your two calls of dirfd_path_init() is,
> AFAICS, the combination of absolute pathname with both LOOKUP_XDEV and
> LOOKUP_BENEATH at the same time.  That combination is treated as if the pathname
> had been relative.  Note that LOOKUP_BENEATH alone is ignored for absolute ones
> (and with a good reason - it's a no-op on path_init() level in that case).
> 
> What the hell?  It complicates your code and doesn't seem to provide any benefits
> whatsoever -- you could bloody well have passed the relative pathname to start with.
> 
> IDGI...  Without that kludge it becomes simply "do as we currently do for absolute
> pathnames, call dirfd_path_init() for relative ones".  And I would argue that
> taking LOOKUP_BENEATH handling out of dirfd_path_init() into path_init() (relative)
> case would be a good idea.
> 
> As it is, the logics is very hard to follow.

	... and it fails on LOOKUP_BENEATH anyway.  Egads...  So that's for your
LOOKUP_CHROOT ;-/  IMO that's awful, especially with the way you've spread those
LOOKUP_CHROOT cases between these two.

	Why not simply have O_THISROOT pick root by dirfd and call file_open_root()?
And if something wants it for stat(), etc. just have them use it combined with
O_PATH and pass the result to ...at()...