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

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Date:   Wed, 24 Jun 2020 13:44:37 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Christian Brauner <christian.brauner@...ntu.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

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].

Regards,
Michal


[1] https://github.com/systemd/systemd/blob/a1ba8c5b71164665ccb53c9cec384e5eef7d3689/src/test/test-seccomp.c#L246

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