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Date:   Fri, 12 Jul 2019 13:39:24 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
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>,
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Subject: Re: [PATCH v9 05/10] namei: O_BENEATH-style path resolution flags
Message-ID: <20190712123924.GK17978@ZenIV.linux.org.uk>
References: <20190706145737.5299-1-cyphar@cyphar.com>
 <20190706145737.5299-6-cyphar@cyphar.com>
 <20190712043341.GI17978@ZenIV.linux.org.uk>
 <20190712105745.nruaftgeat6irhzr@yavin>
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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.