linux-kernel - Re: [PATCH v4 2/3] nsproxy: attach to namespaces via pidfds

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Date:   Wed, 24 Jun 2020 13:54:56 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Stéphane Graber <stgraber@...ntu.com>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Serge Hallyn <serge@...lyn.com>, Jann Horn <jannh@...gle.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Aleksa Sarai <cyphar@...har.com>, linux-api@...r.kernel.org,
        systemd-devel@...ts.freedesktop.org
Subject: Re: [PATCH v4 2/3] nsproxy: attach to namespaces via pidfds

On Wed, Jun 24, 2020 at 01:44:37PM +0200, Michal Koutný wrote:
> Hi.
> 
> On Tue, May 05, 2020 at 04:04:31PM +0200, Christian Brauner <christian.brauner@...ntu.com> wrote:
> > -SYSCALL_DEFINE2(setns, int, fd, int, nstype)
> > +SYSCALL_DEFINE2(setns, int, fd, int, flags)
> > [...]
> > -	file = proc_ns_fget(fd);
> > -	if (IS_ERR(file))
> > -		return PTR_ERR(file);
> > +	int err = 0;
> >  
> > -	err = -EINVAL;
> > -	ns = get_proc_ns(file_inode(file));
> > -	if (nstype && (ns->ops->type != nstype))
> > +	file = fget(fd);
> > +	if (!file)
> > +		return -EBADF;
> > +
> > +	if (proc_ns_file(file)) {
> > +		ns = get_proc_ns(file_inode(file));
> > +		if (flags && (ns->ops->type != flags))
> > +			err = -EINVAL;
> > +		flags = ns->ops->type;
> > +	} else if (pidfd_pid(file)) {
> > +		err = check_setns_flags(flags);
> > +	} else {
> > +		err = -EBADF;
> > +	}
> > +	if (err)
> >  		goto out;
> >  
> > -	err = prepare_nsset(ns->ops->type, &nsset);
> > +	err = prepare_nsset(flags, &nsset);
> >  	if (err)
> >  		goto out;
> This modification changed the returned error when a valid file
> descriptor is passed but it doesn't represent a namespace (nor pidfd).
> The error is now EBADF although originally and per man page it
> was/should be EINVAL.
> 
> A change like below would restore it, however, I see it may be less
> consistent with other pidfd calls(?), then I'd suggest updating the
> manpage to capture this.
> 
> --- a/kernel/nsproxy.c
> +++ b/kernel/nsproxy.c
> @@ -531,7 +531,7 @@ SYSCALL_DEFINE2(setns, int, fd, int, flags)
>         } else if (!IS_ERR(pidfd_pid(file))) {
>                 err = check_setns_flags(flags);
>         } else {
> -               err = -EBADF;
> +               err = -EINVAL;
>         }
>         if (err)
>                 goto out;
> 
> I noticed this breaks systemd self tests [1].

Yep, I already have a fix for this in my tree based on a previous report
from LTP. It's sitting in linux-next:
https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/commit/?h=fixes&id=e571d4ee334719727f22cce30c4c74471d4ef68a
with selftests:
https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/commit/?h=fixes&id=86f56395feb2b106b125c47e72192e37da5dd088

I'll send it to Linus this week.

Thanks!
Christian