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

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Message-ID: <20191113025941.GE26530@ZenIV.linux.org.uk>
Date:   Wed, 13 Nov 2019 02:59:41 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Aleksa Sarai <cyphar@...har.com>
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>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
        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>,
        David Drysdale <drysdale@...gle.com>,
        Chanho Min <chanho.min@....com>,
        Oleg Nesterov <oleg@...hat.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Christian Brauner <christian@...uner.io>,
        Aleksa Sarai <asarai@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        containers@...ts.linux-foundation.org, linux-alpha@...r.kernel.org,
        linux-api@...r.kernel.org, libc-alpha@...rceware.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 v15 5/9] namei: LOOKUP_IN_ROOT: chroot-like scoped
 resolution

On Wed, Nov 13, 2019 at 01:44:14PM +1100, Aleksa Sarai wrote:
> On 2019-11-13, Al Viro <viro@...iv.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.