From c8134b5f13ae959de2b3c8cc278e2602b0857345 Mon Sep 17 00:00:00 2001 From: Christian Brauner Date: Thu, 26 Feb 2026 14:51:01 +0100 Subject: pidfd: add CLONE_PIDFD_AUTOKILL Add a new clone3() flag CLONE_PIDFD_AUTOKILL that ties a child's lifetime to the pidfd returned from clone3(). When the last reference to the struct file created by clone3() is closed the kernel sends SIGKILL to the child. A pidfd obtained via pidfd_open() for the same process does not keep the child alive and does not trigger autokill - only the specific struct file from clone3() has this property. This is useful for container runtimes, service managers, and sandboxed subprocess execution - any scenario where the child must die if the parent crashes or abandons the pidfd. CLONE_PIDFD_AUTOKILL requires both CLONE_PIDFD (the whole point is tying lifetime to the pidfd file) and CLONE_AUTOREAP (a killed child with no one to reap it would become a zombie). CLONE_THREAD is rejected because autokill targets a process not a thread. The clone3 pidfd is identified by the PIDFD_AUTOKILL file flag set on the struct file at clone3() time. The pidfs .release handler checks this flag and sends SIGKILL via do_send_sig_info(SIGKILL, SEND_SIG_PRIV, ...) only when it is set. Files from pidfd_open() or open_by_handle_at() are distinct struct files that do not carry this flag. dup()/fork() share the same struct file so they extend the child's lifetime until the last reference drops. CLONE_PIDFD_AUTOKILL uses a privilege model based on CLONE_NNP: without CLONE_NNP the child could escalate privileges via setuid/setgid exec after being spawned, so the caller must have CAP_SYS_ADMIN in its user namespace. With CLONE_NNP the child can never gain new privileges so unprivileged usage is allowed. This is a deliberate departure from the pdeath_signal model which is reset during secureexec and commit_creds() rendering it useless for container runtimes that need to deprivilege themselves. Link: https://patch.msgid.link/20260226-work-pidfs-autoreap-v5-3-d148b984a989@kernel.org Reviewed-by: Oleg Nesterov Signed-off-by: Christian Brauner --- include/uapi/linux/pidfd.h | 1 + include/uapi/linux/sched.h | 1 + 2 files changed, 2 insertions(+) (limited to 'include/uapi/linux') diff --git a/include/uapi/linux/pidfd.h b/include/uapi/linux/pidfd.h index ea9a6811fc76..9281956a9f32 100644 --- a/include/uapi/linux/pidfd.h +++ b/include/uapi/linux/pidfd.h @@ -13,6 +13,7 @@ #ifdef __KERNEL__ #include #define PIDFD_STALE CLONE_PIDFD +#define PIDFD_AUTOKILL O_TRUNC #endif /* Flags for pidfd_send_signal(). */ diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h index 386c8d7e89cb..149dbc64923b 100644 --- a/include/uapi/linux/sched.h +++ b/include/uapi/linux/sched.h @@ -38,6 +38,7 @@ #define CLONE_INTO_CGROUP (1ULL << 33) /* Clone into a specific cgroup given the right permissions. */ #define CLONE_AUTOREAP (1ULL << 34) /* Auto-reap child on exit. */ #define CLONE_NNP (1ULL << 35) /* Set no_new_privs on child. */ +#define CLONE_PIDFD_AUTOKILL (1ULL << 36) /* Kill child when clone pidfd closes. */ /* * cloning flags intersect with CSIGNAL so can be used with unshare and clone3 -- cgit v1.2.3