linux-kernel - Re: [PATCH v9 05/10] namei: O_BENEATH-style path resolution flags

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Message-ID: <20190714100825.umyql5op6cxhmpqa@yavin>
Date:   Sun, 14 Jul 2019 20:31:04 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Christian Brauner <christian@...uner.io>,
        David Drysdale <drysdale@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, Tycho Andersen <tycho@...ho.ws>,
        Chanho Min <chanho.min@....com>,
        Oleg Nesterov <oleg@...hat.com>, Aleksa Sarai <asarai@...e.de>,
        containers@...ts.linux-foundation.org, linux-alpha@...r.kernel.org,
        linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, sparclinux@...r.kernel.org
Subject: Re: [PATCH v9 05/10] namei: O_BENEATH-style path resolution flags

On 2019-07-12, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Fri, Jul 12, 2019 at 01:55:52PM +0100, Al Viro wrote:
> > On Fri, Jul 12, 2019 at 01:39:24PM +0100, Al Viro wrote:
> > > On Fri, Jul 12, 2019 at 08:57:45PM +1000, Aleksa Sarai wrote:
> > > 
> > > > > > @@ -2350,9 +2400,11 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
> > > > > >  			s = ERR_PTR(error);
> > > > > >  		return s;
> > > > > >  	}
> > > > > > -	error = dirfd_path_init(nd);
> > > > > > -	if (unlikely(error))
> > > > > > -		return ERR_PTR(error);
> > > > > > +	if (likely(!nd->path.mnt)) {
> > > > > 
> > > > > Is that a weird way of saying "if we hadn't already called dirfd_path_init()"?
> > > > 
> > > > Yes. I did it to be more consistent with the other "have we got the
> > > > root" checks elsewhere. Is there another way you'd prefer I do it?
> > > 
> > > "Have we got the root" checks are inevitable evil; here you are making the
> > > control flow in a single function hard to follow.
> > > 
> > > I *think* what you are doing is
> > > 	absolute pathname, no LOOKUP_BENEATH:
> > > 		set_root
> > > 		error = nd_jump_root(nd)
> > > 	else
> > > 		error = dirfd_path_init(nd)
> > > 	return unlikely(error) ? ERR_PTR(error) : s;
> > > which should be a lot easier to follow (not to mention shorter), but I might
> > > be missing something in all of that.
> > 
> > PS: if that's what's going on, I would be tempted to turn the entire
> > path_init() part into this:
> > 	if (flags & LOOKUP_BENEATH)
> > 		while (*s == '/')
> > 			s++;
> > in the very beginning (plus the handling of nd_jump_root() prototype
> > change, but that belongs with nd_jump_root() change itself, obviously).
> > Again, I might be missing something here...
> 
> Argh... I am, at that - you have setting path->root (and grabbing it)
> in LOOKUP_BENEATH cases and you do it after dirfd_path_init().  So
> how about
> 	if (flags & LOOKUP_BENEATH)
> 		while (*s == '/')
> 			s++;

I can do this for LOOKUP_IN_ROOT, but currently the semantics for
LOOKUP_BENEATH is that absolute paths will return -EXDEV
indiscriminately (nd_jump_root() errors out with LOOKUP_BENEATH). To be
honest, the check could actually just be:

  if (flags & LOOKUP_BENEATH)
    if (*s == '/')
	  return ERR_PTR(-EXDEV);

(Though we'd still need -EXDEV in nd_jump_root() for obvious reasons.)

The logic being that an absolute path means that the resolution starts
out without being "beneath" the starting point -- thus violating the
contract of LOOKUP_BENEATH. And since the "handle absolute paths like
they're scoped to the root" is only implemented for LOOKUP_IN_ROOT, I'd
think it's a bit odd to have LOOKUP_BENEATH do it too for absolute
paths.

I'll be honest, this patchset is more confusing to both of us because of
LOOKUP_BENEATH -- I've only kept it since it was part of the original
patchset (O_BENEATH). Personally I think more people will be far more
interested in LOOKUP_IN_ROOT. Does anyone mind if I drop the
LOOKUP_BENEATH parts of this series, and only keep LOOKUP_NO_* and
LOOKUP_IN_ROOT?

I make a change as you outlined for LOOKUP_IN_ROOT, though.

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

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