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[104.53.165.62]) by smtp.gmail.com with ESMTPSA id 00721157ae682-8ae33fe81fdsm41210467b3.47.2026.10.05.08.09.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 05 Oct 2026 08:09:50 -0700 (PDT) From: Andrew Halaney Date: Mon, 05 Oct 2026 10:09:33 -0500 Subject: [PATCH v4 03/10] net: add SO_PASSPIDFD_THREAD to get a thread-specific SCM_PIDFD Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20261005-work-unix-passpidfd-v4-3-350183b6e02a@amutable.com> References: <20261005-work-unix-passpidfd-v4-0-350183b6e02a@amutable.com> In-Reply-To: <20261005-work-unix-passpidfd-v4-0-350183b6e02a@amutable.com> To: Jakub Kicinski , Kuniyuki Iwashima , Oleg Nesterov Cc: "David S. Miller" , Paolo Abeni , Simon Horman , Willem de Bruijn , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Alexander Viro , Jan Kara , linux-fsdevel@vger.kernel.org, Alexander Mikhalitsyn , "Christian Brauner (Amutable)" , Andrew Halaney , Eric Dumazet X-Mailer: b4 0.14.3 From: Christian Brauner Currently, SCM_PIDFD carries a pidfd for the thread-group leader. A broker or the coredump server cannot learn the identity of the specific thread that sent a given message. Now that both struct pids are recorded a receiver can ask for the specific identity it needs. So add SO_PASSPIDFD_THREAD as a sibling of SO_PASSPIDFD. SO_PASSPIDFD_THREAD delivers an SCM_PIDFD_THREAD, i.e. scoped to the specific thread sending. This is completely independent of SO_PASSPIDFD, meaning you can use both options and get both cmsgs. On SOCK_STREAM sockets recvmsg() only stops merging data at a thread boundary when the receiver asked for a thread pidfd. For SO_PASSCRED and SO_PASSPIDFD receivers all threads of one process remain a single writer. Signed-off-by: Christian Brauner (Amutable) Co-developed-by: Andrew Halaney Signed-off-by: Andrew Halaney --- 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/socket.h | 9 +++++---- include/net/sock.h | 4 +++- include/uapi/asm-generic/socket.h | 2 ++ net/core/scm.c | 23 +++++++++++++++++------ net/core/sock.c | 14 ++++++++++++++ net/unix/af_unix.c | 12 +++++++++--- 10 files changed, 58 insertions(+), 14 deletions(-) diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h index 946a5fad2691..bb3d534826bb 100644 --- a/arch/alpha/include/uapi/asm/socket.h +++ b/arch/alpha/include/uapi/asm/socket.h @@ -157,6 +157,8 @@ #define SO_RIGHTS_NOTRUNC 85 +#define SO_PASSPIDFD_THREAD 86 + #if !defined(__KERNEL__) #if __BITS_PER_LONG == 64 diff --git a/arch/mips/include/uapi/asm/socket.h b/arch/mips/include/uapi/asm/socket.h index f1641dde135f..269badcaa086 100644 --- a/arch/mips/include/uapi/asm/socket.h +++ b/arch/mips/include/uapi/asm/socket.h @@ -168,6 +168,8 @@ #define SO_RIGHTS_NOTRUNC 85 +#define SO_PASSPIDFD_THREAD 86 + #if !defined(__KERNEL__) #if __BITS_PER_LONG == 64 diff --git a/arch/parisc/include/uapi/asm/socket.h b/arch/parisc/include/uapi/asm/socket.h index f3a3815c7dc2..313aee10a52c 100644 --- a/arch/parisc/include/uapi/asm/socket.h +++ b/arch/parisc/include/uapi/asm/socket.h @@ -149,6 +149,8 @@ #define SO_RIGHTS_NOTRUNC 0x4053 +#define SO_PASSPIDFD_THREAD 0x4054 + #if !defined(__KERNEL__) #if __BITS_PER_LONG == 64 diff --git a/arch/sparc/include/uapi/asm/socket.h b/arch/sparc/include/uapi/asm/socket.h index 7907f3b1f0ee..bd3e69bcce7a 100644 --- a/arch/sparc/include/uapi/asm/socket.h +++ b/arch/sparc/include/uapi/asm/socket.h @@ -150,6 +150,8 @@ #define SO_RIGHTS_NOTRUNC 0x005e +#define SO_PASSPIDFD_THREAD 0x005f + #if !defined(__KERNEL__) diff --git a/include/linux/socket.h b/include/linux/socket.h index 5a5eb1250103..b17fdb7d38a7 100644 --- a/include/linux/socket.h +++ b/include/linux/socket.h @@ -189,10 +189,11 @@ static inline size_t msg_data_left(const struct msghdr *msg) /* "Socket"-level control message types: */ -#define SCM_RIGHTS 0x01 /* rw: access rights (array of int) */ -#define SCM_CREDENTIALS 0x02 /* rw: struct ucred */ -#define SCM_SECURITY 0x03 /* rw: security label */ -#define SCM_PIDFD 0x04 /* ro: pidfd (int) */ +#define SCM_RIGHTS 0x01 /* rw: access rights (array of int) */ +#define SCM_CREDENTIALS 0x02 /* rw: struct ucred */ +#define SCM_SECURITY 0x03 /* rw: security label */ +#define SCM_PIDFD 0x04 /* ro: pidfd (int) */ +#define SCM_PIDFD_THREAD 0x05 /* ro: thread pidfd (int) */ struct ucred { __u32 pid; diff --git a/include/net/sock.h b/include/net/sock.h index 60ea55dc1885..2c4f754b498b 100644 --- a/include/net/sock.h +++ b/include/net/sock.h @@ -356,6 +356,7 @@ struct sk_filter; * @sk_scm_security: flagged by SO_PASSSEC to recv SCM_SECURITY * @sk_scm_pidfd: flagged by SO_PASSPIDFD to recv SCM_PIDFD * @sk_scm_rights: flagged by SO_PASSRIGHTS to recv SCM_RIGHTS + * @sk_scm_pidfd_thread: flagged by SO_PASSPIDFD_THREAD to recv a thread SCM_PIDFD * @sk_scm_unused: unused flags for scm_recv() * @ns_tracker: tracker for netns reference * @sk_user_frags: xarray of pages the user is holding a reference on. @@ -562,7 +563,8 @@ struct sock { sk_scm_security : 1, sk_scm_pidfd : 1, sk_scm_rights : 1, - sk_scm_unused : 4; + sk_scm_pidfd_thread : 1, + sk_scm_unused : 3; }; }; u8 sk_clockid; diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h index 84ea7b92936e..d1e5c6de146d 100644 --- a/include/uapi/asm-generic/socket.h +++ b/include/uapi/asm-generic/socket.h @@ -152,6 +152,8 @@ #define SO_RIGHTS_NOTRUNC 85 +#define SO_PASSPIDFD_THREAD 86 + #if !defined(__KERNEL__) #if __BITS_PER_LONG == 64 || (defined(__x86_64__) && defined(__ILP32__)) diff --git a/net/core/scm.c b/net/core/scm.c index 9b9e119c353a..00025579d53e 100644 --- a/net/core/scm.c +++ b/net/core/scm.c @@ -499,11 +499,16 @@ static bool scm_has_secdata(struct sock *sk) } #endif -static void scm_pidfd_recv(struct msghdr *msg, struct scm_cookie *scm) +static void scm_pidfd_recv(struct msghdr *msg, struct scm_cookie *scm, + enum pid_type type, int cmsg_type) { + unsigned int flags = PIDFD_STALE; struct file *pidfd_file = NULL; + struct pid *pid; int len, pidfd; + pid = scm->pid[type]; + /* put_cmsg() doesn't return an error if CMSG is truncated, * that's why we need to opencode these checks here. */ @@ -517,12 +522,15 @@ static void scm_pidfd_recv(struct msghdr *msg, struct scm_cookie *scm) return; } - if (!scm->pid[PIDTYPE_TGID]) + if (!pid) return; - pidfd = pidfd_prepare(scm->pid[PIDTYPE_TGID], PIDFD_STALE, &pidfd_file); + if (type == PIDTYPE_PID) + flags |= PIDFD_THREAD; - if (put_cmsg(msg, SOL_SOCKET, SCM_PIDFD, sizeof(int), &pidfd)) { + pidfd = pidfd_prepare(pid, flags, &pidfd_file); + + if (put_cmsg(msg, SOL_SOCKET, cmsg_type, sizeof(int), &pidfd)) { if (pidfd_file) { put_unused_fd(pidfd); fput(pidfd_file); @@ -540,7 +548,7 @@ static bool __scm_recv_common(struct sock *sk, struct msghdr *msg, { if (!msg->msg_control) { if (sk->sk_scm_credentials || sk->sk_scm_pidfd || - scm->fp || scm_has_secdata(sk)) + sk->sk_scm_pidfd_thread || scm->fp || scm_has_secdata(sk)) msg->msg_flags |= MSG_CTRUNC; scm_destroy(scm); @@ -587,7 +595,10 @@ void scm_recv_unix(struct socket *sock, struct msghdr *msg, } if (sock->sk->sk_scm_pidfd) - scm_pidfd_recv(msg, scm); + scm_pidfd_recv(msg, scm, PIDTYPE_TGID, SCM_PIDFD); + + if (sock->sk->sk_scm_pidfd_thread) + scm_pidfd_recv(msg, scm, PIDTYPE_PID, SCM_PIDFD_THREAD); scm_destroy_cred(scm); } diff --git a/net/core/sock.c b/net/core/sock.c index e8551df8330f..c6f33fcbea43 100644 --- a/net/core/sock.c +++ b/net/core/sock.c @@ -1578,6 +1578,13 @@ int sk_setsockopt(struct sock *sk, int level, int optname, ret = -EOPNOTSUPP; break; + case SO_PASSPIDFD_THREAD: + if (sk_is_unix(sk)) + sk->sk_scm_pidfd_thread = valbool; + else + ret = -EOPNOTSUPP; + break; + case SO_PASSRIGHTS: if (sk_is_unix(sk)) sk->sk_scm_rights = valbool; @@ -1893,6 +1900,13 @@ int sk_getsockopt(struct sock *sk, int level, int optname, v.val = sk->sk_scm_pidfd; break; + case SO_PASSPIDFD_THREAD: + if (!sk_is_unix(sk)) + return -EOPNOTSUPP; + + v.val = sk->sk_scm_pidfd_thread; + break; + case SO_PASSRIGHTS: if (!sk_is_unix(sk)) return -EOPNOTSUPP; diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c index 64e720bd289b..e44afb059abf 100644 --- a/net/unix/af_unix.c +++ b/net/unix/af_unix.c @@ -803,7 +803,8 @@ static void copy_peercred(struct sock *sk, struct sock *peersk) static bool unix_may_passcred(const struct sock *sk) { - return sk->sk_scm_credentials || sk->sk_scm_pidfd; + return sk->sk_scm_credentials || sk->sk_scm_pidfd || + sk->sk_scm_pidfd_thread; } static int unix_listen(struct socket *sock, int backlog) @@ -2046,9 +2047,14 @@ static int unix_maybe_add_creds(struct sk_buff *skb, const struct sock *sk, return 0; } -static bool unix_skb_scm_eq(struct sk_buff *skb, +static bool unix_skb_scm_eq(const struct sock *sk, struct sk_buff *skb, struct scm_cookie *scm) { + /* Only a thread pidfd receiver can tell threads of one process apart. */ + if (sk->sk_scm_pidfd_thread && + UNIXCB(skb).pid[PIDTYPE_PID] != scm->pid[PIDTYPE_PID]) + return false; + return UNIXCB(skb).pid[PIDTYPE_TGID] == scm->pid[PIDTYPE_TGID] && uid_eq(UNIXCB(skb).uid, scm->creds.uid) && gid_eq(UNIXCB(skb).gid, scm->creds.gid) && @@ -3031,7 +3037,7 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state, if (check_creds) { /* Never glue messages from different writers */ - if (!unix_skb_scm_eq(skb, &scm)) + if (!unix_skb_scm_eq(sk, skb, &scm)) break; } else if (unix_may_passcred(sk)) { /* Copy credentials */ -- 2.55.0