Re: [PATCH net-next 2/3] net: core: add getsockopt SO_PEERPIDFD

[Christian Brauner <brauner@kernel.org>]

On Thu, Mar 16, 2023 at 02:15:25PM +0100, Alexander Mikhalitsyn wrote:
> Add SO_PEERPIDFD which allows to get pidfd of peer socket holder pidfd.
> This thing is direct analog of SO_PEERCRED which allows to get plain PID.
> 
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: Paolo Abeni <pabeni@redhat.com>
> Cc: Leon Romanovsky <leon@kernel.org>
> Cc: David Ahern <dsahern@kernel.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: linux-kernel@vger.kernel.org
> Cc: netdev@vger.kernel.org
> Cc: linux-arch@vger.kernel.org
> Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
> ---
>  arch/alpha/include/uapi/asm/socket.h    |  1 +
>  arch/mips/include/uapi/asm/socket.h     |  1 +
>  arch/parisc/include/uapi/asm/socket.h   |  1 +
>  arch/sparc/include/uapi/asm/socket.h    |  1 +
>  include/uapi/asm-generic/socket.h       |  1 +
>  net/core/sock.c                         | 24 ++++++++++++++++++++++++
>  tools/include/uapi/asm-generic/socket.h |  1 +
>  7 files changed, 30 insertions(+)
> 
> diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h
> index ff310613ae64..e94f621903fe 100644
> --- a/arch/alpha/include/uapi/asm/socket.h
> +++ b/arch/alpha/include/uapi/asm/socket.h
> @@ -138,6 +138,7 @@
>  #define SO_RCVMARK		75
>  
>  #define SO_PASSPIDFD		76
> +#define SO_PEERPIDFD		77
>  
>  #if !defined(__KERNEL__)
>  
> diff --git a/arch/mips/include/uapi/asm/socket.h b/arch/mips/include/uapi/asm/socket.h
> index 762dcb80e4ec..60ebaed28a4c 100644
> --- a/arch/mips/include/uapi/asm/socket.h
> +++ b/arch/mips/include/uapi/asm/socket.h
> @@ -149,6 +149,7 @@
>  #define SO_RCVMARK		75
>  
>  #define SO_PASSPIDFD		76
> +#define SO_PEERPIDFD		77
>  
>  #if !defined(__KERNEL__)
>  
> diff --git a/arch/parisc/include/uapi/asm/socket.h b/arch/parisc/include/uapi/asm/socket.h
> index df16a3e16d64..be264c2b1a11 100644
> --- a/arch/parisc/include/uapi/asm/socket.h
> +++ b/arch/parisc/include/uapi/asm/socket.h
> @@ -130,6 +130,7 @@
>  #define SO_RCVMARK		0x4049
>  
>  #define SO_PASSPIDFD		0x404A
> +#define SO_PEERPIDFD		0x404B
>  
>  #if !defined(__KERNEL__)
>  
> diff --git a/arch/sparc/include/uapi/asm/socket.h b/arch/sparc/include/uapi/asm/socket.h
> index 6e2847804fea..682da3714686 100644
> --- a/arch/sparc/include/uapi/asm/socket.h
> +++ b/arch/sparc/include/uapi/asm/socket.h
> @@ -131,6 +131,7 @@
>  #define SO_RCVMARK               0x0054
>  
>  #define SO_PASSPIDFD             0x0055
> +#define SO_PEERPIDFD             0x0056
>  
>  #if !defined(__KERNEL__)
>  
> diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h
> index b76169fdb80b..8ce8a39a1e5f 100644
> --- a/include/uapi/asm-generic/socket.h
> +++ b/include/uapi/asm-generic/socket.h
> @@ -133,6 +133,7 @@
>  #define SO_RCVMARK		75
>  
>  #define SO_PASSPIDFD		76
> +#define SO_PEERPIDFD		77
>  
>  #if !defined(__KERNEL__)
>  
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 3f974246ba3e..3aa1ccd4bcf3 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -1763,6 +1763,30 @@ int sk_getsockopt(struct sock *sk, int level, int optname,
>  		goto lenout;
>  	}
>  
> +	case SO_PEERPIDFD:
> +	{
> +		struct pid *peer_pid;
> +		int pidfd;
> +		if (len > sizeof(pidfd))
> +			len = sizeof(pidfd);
> +
> +		spin_lock(&sk->sk_peer_lock);
> +		peer_pid = get_pid(sk->sk_peer_pid);
> +		spin_unlock(&sk->sk_peer_lock);
> +
> +		if (!peer_pid ||
> +		    !pid_has_task(peer_pid, PIDTYPE_TGID))
> +			pidfd = -ESRCH;

Any specific reason you want -ESRCH here?
pidfd_create() returns -EINVAL for exactly this check it performs mainly
because the non-existence of PIDTYPE_TGID could either indicate that
this struct pid isn't used as a thread-group leader or - indeed - that
the process has already been reaped. IOW, if there's no specific reason
I would not deviate from pidfd_create()'s return value.