Subject: Re: [PATCH net-next v7 1/4] scm: add SO_PASSPIDFD and SCM_PIDFD

From: Heiko Carstens <[email-masked]>
Date: Fri, 1 Sep 2023 22:05:17 +0200

On Thu, Jun 08, 2023 at 10:26:25PM +0200, Alexander Mikhalitsyn wrote:
> Implement SCM_PIDFD, a new type of CMSG type analogical to SCM_CREDENTIALS,
> but it contains pidfd instead of plain pid, which allows programmers not
> to care about PID reuse problem.
> 
> We mask SO_PASSPIDFD feature if CONFIG_UNIX is not builtin because
> it depends on a pidfd_prepare() API which is not exported to the kernel
> modules.
> 
> Idea comes from UAPI kernel group:
> https://uapi-group.org/kernel-features/
> 
> Big thanks to Christian Brauner and Lennart Poettering for productive
> discussions about this.
> 
> Cc: "David S. Miller" <[email-masked]>
> Cc: Eric Dumazet <[email-masked]>
> Cc: Jakub Kicinski <[email-masked]>
> Cc: Paolo Abeni <[email-masked]>
> Cc: Leon Romanovsky <[email-masked]>
> Cc: David Ahern <[email-masked]>
> Cc: Arnd Bergmann <[email-masked]>
> Cc: Kees Cook <[email-masked]>
> Cc: Christian Brauner <[email-masked]>
> Cc: Kuniyuki Iwashima <[email-masked]>
> Cc: Lennart Poettering <[email-masked]>
> Cc: Luca Boccassi <[email-masked]>
> Cc: [email-masked]
> Cc: [email-masked]
> Cc: [email-masked]
> Tested-by: Luca Boccassi <[email-masked]>
> Reviewed-by: Kuniyuki Iwashima <[email-masked]>
> Reviewed-by: Christian Brauner <[email-masked]>
> Signed-off-by: Alexander Mikhalitsyn <[email-masked]>
> ---
>  arch/alpha/include/uapi/asm/socket.h    |  2 ++
>  arch/mips/include/uapi/asm/socket.h     |  2 ++
>  arch/parisc/include/uapi/asm/socket.h   |  2 ++
>  arch/sparc/include/uapi/asm/socket.h    |  2 ++
>  include/linux/net.h                     |  1 +
>  include/linux/socket.h                  |  1 +
>  include/net/scm.h                       | 39 +++++++++++++++++++++++--
>  include/uapi/asm-generic/socket.h       |  2 ++
>  net/core/sock.c                         | 11 +++++++
>  net/mptcp/sockopt.c                     |  1 +
>  net/unix/af_unix.c                      | 18 ++++++++----
>  tools/include/uapi/asm-generic/socket.h |  2 ++
>  12 files changed, 76 insertions(+), 7 deletions(-)
...
> +static __inline__ void scm_pidfd_recv(struct msghdr *msg, struct scm_cookie *scm)
> +{
> +	struct file *pidfd_file = NULL;
> +	int pidfd;
> +
> +	/*
> +	 * put_cmsg() doesn't return an error if CMSG is truncated,
> +	 * that's why we need to opencode these checks here.
> +	 */
> +	if ((msg->msg_controllen <= sizeof(struct cmsghdr)) ||
> +	    (msg->msg_controllen - sizeof(struct cmsghdr)) < sizeof(int)) {
> +		msg->msg_flags |= MSG_CTRUNC;
> +		return;
> +	}

This does not work for compat tasks since the size of struct cmsghdr (aka
struct compat_cmsghdr) is differently. If the check from put_cmsg() is
open-coded here, then also a different check for compat tasks needs to be
added.

Discovered this because I was wondering why strace compat tests fail; it
seems because of this.

See https://github.com/strace/strace/blob/master/tests/scm_pidfd.c

For compat tasks recvmsg() returns with msg_flags=MSG_CTRUNC since the
above code expects a larger buffer than is necessary.