Re: [PATCH v15 5/9] namei: LOOKUP_IN_ROOT: chroot-like scoped resolution

From: Al Viro <viro@zeniv•linux.org.uk>
To: Aleksa Sarai <cyphar@cyphar•com>
Cc: linux-ia64@vger•kernel.org, linux-sh@vger•kernel.org,
	Peter Zijlstra <peterz@infradead•org>,
	Rasmus Villemoes <linux@rasmusvillemoes•dk>,
	Alexei Starovoitov <ast@kernel•org>,
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	linux-kselftest@vger•kernel.org, sparclinux@vger•kernel.org,
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	Jiri Olsa <jolsa@redhat•com>,
	linux-arch@vger•kernel.org, linux-s390@vger•kernel.org,
	Tycho Andersen <tycho@tycho•ws>, Aleksa Sarai <asarai@suse•de>,
	Shuah Khan <shuah@kernel•org>,
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	Andrew Morton <akpm@linux-foundation•org>,
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Subject: Re: [PATCH v15 5/9] namei: LOOKUP_IN_ROOT: chroot-like scoped resolution
Date: Wed, 13 Nov 2019 02:59:41 +0000	[thread overview]
Message-ID: <20191113025941.GE26530@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20191113024414.wlmvtjstpnkxa36n@yavin.dot.cyphar.com>

On Wed, Nov 13, 2019 at 01:44:14PM +1100, Aleksa Sarai wrote:
> On 2019-11-13, Al Viro <viro@zeniv•linux.org.uk> wrote:
> > On Tue, Nov 05, 2019 at 08:05:49PM +1100, Aleksa Sarai wrote:
> > 
> > > @@ -2277,12 +2277,20 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
> > >  
> > >  	nd->m_seq = read_seqbegin(&mount_lock);
> > >  
> > > -	/* Figure out the starting path and root (if needed). */
> > > -	if (*s == '/') {
> > > +	/* Absolute pathname -- fetch the root. */
> > > +	if (flags & LOOKUP_IN_ROOT) {
> > > +		/* With LOOKUP_IN_ROOT, act as a relative path. */
> > > +		while (*s == '/')
> > > +			s++;
> > 
> > Er...  Why bother skipping slashes?  I mean, not only link_path_walk()
> > will skip them just fine, you are actually risking breakage in this:
> >                 if (*s && unlikely(!d_can_lookup(dentry))) {
> >                         fdput(f);
> >                         return ERR_PTR(-ENOTDIR);
> >                 }
> > which is downstream from there with you patch, AFAICS.
> 
> I switched to stripping the slashes at your suggestion a few revisions
> ago[1], and had (wrongly) assumed we needed to handle "/" somehow in
> path_init(). But you're quite right about link_path_walk() -- and I'd be
> more than happy to drop it.

That, IIRC, was about untangling the weirdness around multiple calls of
dirfd_path_init() and basically went "we might want just strip the slashes
in case of that flag very early in the entire thing, so that later the
normal logics for absolute/relative would DTRT".  Since your check is
right next to checking for absolute pathnames (and not in the very
beginning of path_init()), we might as well turn the check for
absolute pathname into *s == '/' && !(flags & LOOKUP_IN_ROOT) and be
done with that.

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