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From: Aleksa Sarai <asarai@suse.de>
To: Andy Lutomirski <luto@kernel.org>
Cc: j@bitron.ch, Aleksa Sarai <cyphar@cyphar.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Jeff Layton <jlayton@kernel.org>,
	"J. Bruce Fields" <bfields@fieldses.org>,
	Arnd Bergmann <arnd@arndb.de>,
	David Howells <dhowells@redhat.com>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Christian Brauner <christian@brauner.io>,
	Linux API <linux-api@vger.kernel.org>,
	Jann Horn <jannh@google.com>,
	David Drysdale <drysdale@google.com>,
	Linux Containers <containers@lists.linux-foundation.org>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>
Subject: Re: [PATCH v4 2/4] namei: O_BENEATH-style path resolution flags
Date: Sat, 24 Nov 2018 03:48:19 +1100
Message-ID: <20181123164819.skruu23wjwzf47f5@mikami> (raw)
In-Reply-To: <CALCETrURCXjNMBR6DjujVc6HHHS=CbET81kHS+odur+Bm-z7Tg@mail.gmail.com>

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On 2018-11-23, Andy Lutomirski <luto@kernel.org> wrote:
> > On Nov 23, 2018, at 5:10 AM, Jürg Billeter <j@bitron.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 agree that there is something quite weird about having path resolution
flags in an *open* syscall. The main reason why it's linked to open is
because that's the only way to get O_PATH descriptors (which is what you
would use for most of the extended operations we need -- as well as
reading+writing to files which is what most users would do with this).

And I think O_PATH is another example of an open flag that is just odd
in how it changes the semantics of using open(2).

One of the ideas I pitched in an earlier mail was a hypothetical
resolveat(2) -- which would just be a new way of getting an O_PATH
descriptor. This way, we wouldn't be using up more O_* flag bits with
this feature and we'd be able to play with more radical semantic changes
in the future. I can outline these here if you like, but it's a bit of a
long discussion and I'd prefer not to derail things too much if
resolveat(2) is definitely out of the question.

> 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.

I also completely agree. I think that we should have a discussion about
the long-term plan of syscall flags because it's starting to get a
little bit crazy:

 * Every "get an fd" syscall has its own brand of O_CLOEXEC. Thankfully,
   many of them use the same value (except for memfd_create(2) and a few
   other examples).
 * AT_* was supposed to be generic across all *at(2) syscalls, but there
   are several cases where this isn't really true anymore.

   * renameat2(2) only supports RENAME_* flags.
   * openat(2) supports only O_* flags.
   * Most AT_* flags have O_* counterparts (or are even more of a mess
	 such as with {AT_SYMLINK_FOLLOW,AT_SYMLINK_NOFOLLOW,O_NOFOLLOW}).
   * statx(2) added AT_STATX_* flags, making AT_* no longer generic.

(Also I just want to mention something I noticed while writing this
patch -- openat(2) violates one of the kernel "golden rules" -- that you
reject unknown flags. openat(2) will silently ignore unknown flag bits.
I'm sure there's a really good reason for this, but it's another flag
oddity that I felt fit here.)

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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  reply index

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-12 14:26 [PATCH v4 0/4] namei: O_* flags to restrict path resolution Aleksa Sarai
2018-11-12 14:26 ` [PATCH v4 1/4] namei: split out nd->dfd handling to dirfd_path_init Aleksa Sarai
2018-11-12 14:26 ` [PATCH v4 2/4] namei: O_BENEATH-style path resolution flags Aleksa Sarai
2018-11-23 12:10   ` Jürg Billeter
2018-11-23 16:07     ` Andy Lutomirski
2018-11-23 16:48       ` Aleksa Sarai [this message]
2018-11-23 16:52         ` Aleksa Sarai
2018-11-23 16:58     ` Aleksa Sarai
2018-11-12 14:26 ` [PATCH v4 3/4] namei: O_THISROOT: chroot-like path resolution Aleksa Sarai
2018-11-12 14:26 ` [PATCH v4 4/4] namei: aggressively check for nd->root escape on ".." resolution Aleksa Sarai

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