linux-kernel - Re: [PATCH v15 4/9] namei: LOOKUP_BENEATH: O

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Date:   Thu, 14 Nov 2019 15:57:44 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     linux-ia64@...r.kernel.org, linux-sh@...r.kernel.org,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Alexei Starovoitov <ast@...nel.org>,
        linux-kernel@...r.kernel.org, David Howells <dhowells@...hat.com>,
        linux-kselftest@...r.kernel.org, sparclinux@...r.kernel.org,
        Christian Brauner <christian.brauner@...ntu.com>,
        Shuah Khan <shuah@...nel.org>, linux-arch@...r.kernel.org,
        linux-s390@...r.kernel.org, Jiri Olsa <jolsa@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, linux-mips@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, Kees Cook <keescook@...omium.org>,
        Arnd Bergmann <arnd@...db.de>, Jann Horn <jannh@...gle.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-m68k@...ts.linux-m68k.org,
        Andy Lutomirski <luto@...nel.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Namhyung Kim <namhyung@...nel.org>,
        David Drysdale <drysdale@...gle.com>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        libc-alpha@...rceware.org, linux-parisc@...r.kernel.org,
        linux-api@...r.kernel.org, Chanho Min <chanho.min@....com>,
        Jeff Layton <jlayton@...nel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        linux-alpha@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        containers@...ts.linux-foundation.org
Subject: Re: [PATCH v15 4/9] namei: LOOKUP_BENEATH: O_BENEATH-like scoped
 resolution

On 2019-11-13, Aleksa Sarai <cyphar@...har.com> wrote:
> On 2019-11-13, Al Viro <viro@...iv.linux.org.uk> wrote:
> > Minor nit here - I'd split "move the conditional call of set_root()
> > into nd_jump_root()" into a separate patch before that one.  Makes
> > for fewer distractions in this one.  I'd probably fold "and be
> > ready for errors other than -ECHILD" into the same preliminary
> > patch.
> 
> Will do.
> 
> > > +			/* Not currently safe for scoped-lookups. */
> > > +			if (unlikely(nd->flags & LOOKUP_IS_SCOPED))
> > > +				return ERR_PTR(-EXDEV);
> > 
> > Also a candidate for doing in nd_jump_link()...
> > 
> > > @@ -1373,8 +1403,11 @@ static int follow_dotdot_rcu(struct nameidata *nd)
> > >  	struct inode *inode = nd->inode;
> > >  
> > >  	while (1) {
> > > -		if (path_equal(&nd->path, &nd->root))
> > > +		if (path_equal(&nd->path, &nd->root)) {
> > > +			if (unlikely(nd->flags & LOOKUP_BENEATH))
> > > +				return -EXDEV;
> > 
> > Umm...  Are you sure it's not -ECHILD?
> 
> It wouldn't hurt to be -ECHILD -- though it's not clear to me how likely
> a success would be in REF-walk if the parent components didn't already
> trigger an unlazy_walk() in RCU-walk.
> 
> I guess that also means LOOKUP_NO_XDEV should trigger -ECHILD in
> follow_dotdot_rcu()?

Scratch the last question -- AFAICS we don't need to do that for
LOOKUP_NO_XDEV because we check against mount_lock so it's very unlikely
that -ECHILD will have any benefit.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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