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

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Message-ID: <20190712105745.nruaftgeat6irhzr@yavin>
Date:   Fri, 12 Jul 2019 20:57:45 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     Al Viro <viro@...iv.linux.org.uk>
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 2019-07-12, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Sun, Jul 07, 2019 at 12:57:32AM +1000, Aleksa Sarai wrote:
> > @@ -1442,8 +1464,11 @@ 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;
> 
> > @@ -1468,6 +1493,8 @@ static int follow_dotdot_rcu(struct nameidata *nd)
> >  				return -ECHILD;
> >  			if (&mparent->mnt == nd->path.mnt)
> >  				break;
> > +			if (unlikely(nd->flags & LOOKUP_XDEV))
> > +				return -EXDEV;
> >  			/* we know that mountpoint was pinned */
> >  			nd->path.dentry = mountpoint;
> >  			nd->path.mnt = &mparent->mnt;
> > @@ -1482,6 +1509,8 @@ static int follow_dotdot_rcu(struct nameidata *nd)
> >  			return -ECHILD;
> >  		if (!mounted)
> >  			break;
> > +		if (unlikely(nd->flags & LOOKUP_XDEV))
> > +			return -EXDEV;
> 
> Are you sure these failure exits in follow_dotdot_rcu() won't give
> suprious hard errors?

I could switch to -ECHILD for the *_rcu() checks if you'd prefer that.
Though, I'd have (probably naively) thought that you'd have already
gotten -ECHILD from the seqlock checks if there was a race during ".."
handling.

> > +	if (unlikely(nd->flags & LOOKUP_BENEATH)) {
> > +		error = dirfd_path_init(nd);
> > +		if (unlikely(error))
> > +			return ERR_PTR(error);
> > +		nd->root = nd->path;
> > +		if (!(nd->flags & LOOKUP_RCU))
> > +			path_get(&nd->root);
> > +	}
> >  	if (*s == '/') {
> >  		if (likely(!nd->root.mnt))
> >  			set_root(nd);
> > @@ -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?

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

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