Re: [PATCH 03/10] net: add SO_PASSPIDFD_THREAD to get a thread-specific SCM_PIDFD
From: Alexander Mikhalitsyn <hidden>
Date: 2026-09-07 10:02:56
Also in:
linux-fsdevel, lkml
Am Mo., 31. Aug. 2026 um 13:21 Uhr schrieb Christian Brauner [off-list ref]:
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. Either option makes recvmsg() deliver an SCM_PIDFD. SO_PASSPIDFD sends a pidfd for the thread-group leader and SO_PASSPIDFD_THREAD sends a pidfd for the specific thread. The two options are mutually exclusive. Enabling one switches the other off, so getsockopt() always reports which of the two is active. 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) <brauner@kernel.org>
LGTM Reviewed-by: Alexander Mikhalitsyn <redacted>
quoted hunk ↗ jump to hunk
--- 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/net/sock.h | 10 +++++++++- include/uapi/asm-generic/socket.h | 2 ++ net/core/scm.c | 19 +++++++++++++------ net/core/sock.c | 26 ++++++++++++++++++++++++-- net/unix/af_unix.c | 11 ++++++++--- 9 files changed, 64 insertions(+), 12 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 == 64diff --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 == 64diff --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 == 64diff --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/net/sock.h b/include/net/sock.h index 51185222aac2..fc09c92e8a83 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;@@ -2986,6 +2988,12 @@ static inline bool sk_is_stream_unix(const struct sock *sk) return sk_is_unix(sk) && sk->sk_type == SOCK_STREAM; } +/* SO_PASSPIDFD or SO_PASSPIDFD_THREAD asked for an SCM_PIDFD. */ +static inline bool sk_scm_pidfd_wanted(const struct sock *sk) +{ + return sk->sk_scm_pidfd || sk->sk_scm_pidfd_thread; +} + static inline bool sk_is_vsock(const struct sock *sk) { return sk->sk_family == AF_VSOCK;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..d69768414af4 100644 --- a/net/core/scm.c +++ b/net/core/scm.c@@ -499,9 +499,13 @@ 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 sock *sk, struct msghdr *msg, + struct scm_cookie *scm) { + enum pid_type type = sk->sk_scm_pidfd_thread ? PIDTYPE_PID : PIDTYPE_TGID; + struct pid *pid = scm->pid[type]; struct file *pidfd_file = NULL; + unsigned int flags = PIDFD_STALE; int len, pidfd; /* put_cmsg() doesn't return an error if CMSG is truncated,@@ -517,10 +521,13 @@ 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; + + pidfd = pidfd_prepare(pid, flags, &pidfd_file); if (put_cmsg(msg, SOL_SOCKET, SCM_PIDFD, sizeof(int), &pidfd)) { if (pidfd_file) {@@ -539,7 +546,7 @@ static bool __scm_recv_common(struct sock *sk, struct msghdr *msg, struct scm_cookie *scm, int flags) { if (!msg->msg_control) { - if (sk->sk_scm_credentials || sk->sk_scm_pidfd || + if (sk->sk_scm_credentials || sk_scm_pidfd_wanted(sk) || scm->fp || scm_has_secdata(sk)) msg->msg_flags |= MSG_CTRUNC;@@ -586,8 +593,8 @@ void scm_recv_unix(struct socket *sock, struct msghdr *msg, scm_detach_fds(msg, scm, READ_ONCE(u->scm_rights_notrunc)); } - if (sock->sk->sk_scm_pidfd) - scm_pidfd_recv(msg, scm); + if (sk_scm_pidfd_wanted(sock->sk)) + scm_pidfd_recv(sock->sk, msg, scm); scm_destroy_cred(scm); }diff --git a/net/core/sock.c b/net/core/sock.c index 1ad41904db25..f9615b0de10e 100644 --- a/net/core/sock.c +++ b/net/core/sock.c@@ -1571,10 +1571,25 @@ int sk_setsockopt(struct sock *sk, int level, int optname, break; case SO_PASSPIDFD: - if (sk_is_unix(sk)) + if (sk_is_unix(sk)) { + /* Mutually exclusive with SO_PASSPIDFD_THREAD. */ sk->sk_scm_pidfd = valbool; - else + if (valbool) + sk->sk_scm_pidfd_thread = 0; + } else { + ret = -EOPNOTSUPP; + } + break; + + case SO_PASSPIDFD_THREAD: + if (sk_is_unix(sk)) { + /* Mutually exclusive with SO_PASSPIDFD. */ + sk->sk_scm_pidfd_thread = valbool; + if (valbool) + sk->sk_scm_pidfd = 0; + } else { ret = -EOPNOTSUPP; + } break; case SO_PASSRIGHTS:@@ -1892,6 +1907,13 @@ 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 011af84e3626..468a9c479b87 100644 --- a/net/unix/af_unix.c +++ b/net/unix/af_unix.c@@ -803,7 +803,7 @@ 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_scm_pidfd_wanted(sk); } static int unix_listen(struct socket *sock, int backlog)@@ -2048,9 +2048,14 @@ 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) &&@@ -3029,7 +3034,7 @@ 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 */ --2.53.0