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

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Message-ID: <20190712125552.GL17978@ZenIV.linux.org.uk>
Date:   Fri, 12 Jul 2019 13:55:52 +0100
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>,
        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 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...