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

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Message-ID: <20190712150026.GO17978@ZenIV.linux.org.uk>
Date:   Fri, 12 Jul 2019 16:00:27 +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 02:25:53PM +0100, Al Viro wrote:

> 	if (flags & LOOKUP_BENEATH) {
> 		nd->root = nd->path;
> 		if (!(flags & LOOKUP_RCU))
> 			path_get(&nd->root);
> 		else
> 			nd->root_seq = nd->seq;

BTW, this assignment is needed for LOOKUP_RCU case.  Without it
you are pretty much guaranteed that lazy pathwalk will fail,
when it comes to complete_walk().

Speaking of which, what would happen if LOOKUP_ROOT/LOOKUP_BENEATH
combination would someday get passed?