# Re: \[PATCH 03/10\] net: add SO\_PASSPIDFD\_THREAD to get a thread-specific SCM\_PIDFD - From: Alexander Mikhalitsyn \ - Date: 2026-09-07 10:02:56 - List: netdev - Page: https://ratatoskr.run/netdev/2026/09/17524117 - Thread: https://ratatoskr.run/netdev/2026/08/17483571/t.md ```text Am Mo., 31. Aug. 2026 um 13:21 Uhr schrieb Christian Brauner [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) LGTM Reviewed-by: Alexander Mikhalitsyn > --- > 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 == 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/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 > ```