From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro@zeniv.linux.org.uk>
Date: Wed, 13 Nov 2019 02:03:07 +0000
Subject: Re: [PATCH v15 5/9] namei: LOOKUP_IN_ROOT: chroot-like scoped resolution
Message-Id: <20191113020307.GB26530@ZenIV.linux.org.uk>
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References: <20191105090553.6350-1-cyphar@cyphar.com>
 <20191105090553.6350-6-cyphar@cyphar.com>
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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>, Ingo Molnar <mingo@redhat.com>, Peter Zijlstra <peterz@infradead.org>, Christian Brauner <christian.brauner@ubuntu.com>, Eric Biederman <ebiederm@xmission.com>, Andy Lutomirski <luto@kernel.org>, 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>, David Drysdale <drysdale@google.com>, Chanho Min <chanho.min@lge.com>, Oleg Nesterov <oleg@redhat.com>, Rasmus Villemoes <linux@rasmusvillemoes.dk>, Alexan

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.