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Date: Fri, 12 Jul 2019 13:39:24 +0100
From: Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v9 05/10] namei: O_BENEATH-style path resolution flags
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To: Aleksa Sarai <cyphar@cyphar.com>
Cc: Jeff Layton <jlayton@kernel.org>, "J. Bruce Fields" <bfields@fieldses.org>, Arnd Bergmann <arnd@arndb.de>, David Howells <dhowells@redhat.com>, Shuah Khan <shuah@kernel.org>, Shuah Khan <skhan@linuxfoundation.org>, Christian Brauner <christian@brauner.io>, David Drysdale <drysdale@google.com>, Andy Lutomirski <luto@kernel.org>, Linus Torvalds <torvalds@linux-foundation.org>, Eric Biederman <ebiederm@xmission.com>, Andrew Morton <akpm@linux-foundation.org>, Alexei Starovoitov <ast@kernel.org>, Kees Cook <keescook@chromium.org>, Jann Horn <jannh@google.com>, Tycho Andersen <tycho@tycho.ws>, Chanho Min <chanho.min@lge.com>, Oleg Nesterov <oleg@redhat.com>, Aleksa Sarai <asarai@suse.de>, containers@lists.linux-foundation.org, linux-alpha@vger.kernel.org, linux-api@vger.kernel.org, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-ia64@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, linux-xtensa@linux-xtensa.org, sparclinux@vger.kernel.org

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.