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

[Aleksa Sarai <asarai@suse.de>]

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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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