EVFILT_PROCDESC: support NOTE_FORK

Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D58292

4 files changed, 32 insertions, 1 deletions

diff --git a/sys/kern/kern_fork.c b/sys/kern/kern_fork.c
index 26e2510698e2..60d89c03f865 100644
--- a/sys/kern/kern_fork.c
+++ b/sys/kern/kern_fork.c
@@ -825,6 +825,15 @@ do_fork(struct thread *td, struct fork_req *fr, struct proc *p2, struct thread *
 		sx_xunlock(&proctree_lock);
 	}
 
+	/*
+	 * Activate procdesc NOTE_FORK after we attached the debugger
+	 * to the child.  This guarantees that a debugger which does
+	 * kevent() on the process descriptor to get notifications of
+	 * fork events, can properly observe the child right after the
+	 * notification fired.
+	 */
+	procdesc_fork(p1, p2->p_pid);
+
 	racct_proc_fork_done(p2);
 
 	if ((fr->fr_flags & RFSTOPPED) == 0) {
diff --git a/sys/kern/sys_procdesc.c b/sys/kern/sys_procdesc.c
index 152f24c219c9..fd4b660d6afa 100644
--- a/sys/kern/sys_procdesc.c
+++ b/sys/kern/sys_procdesc.c
@@ -335,6 +335,22 @@ procdesc_jobstate(struct proc *p)
 	wakeup(&p->p_procdesc);
 }
 
+void
+procdesc_fork(struct proc *p, pid_t child_pid)
+{
+	struct procdesc *pd;
+
+	PROC_LOCK(p);
+	pd = p->p_procdesc;
+	if (pd != NULL) {
+		PROCDESC_LOCK(pd);
+		pd->pd_last_child = child_pid;
+		KNOTE_LOCKED(&pd->pd_selinfo.si_note, NOTE_FORK);
+		PROCDESC_UNLOCK(pd);
+	}
+	PROC_UNLOCK(p);
+}
+
 /*
  * When a process descriptor is reaped, perhaps as a result of close(), release
  * the process's reference on the process descriptor.
@@ -506,6 +522,9 @@ procdesc_kqops_event(struct knote *kn, long hint)
 		return (1);
 	}
 
+	if ((kn->kn_fflags & NOTE_FORK) != 0)
+		kn->kn_data = pd->pd_last_child;
+
 	return (kn->kn_fflags != 0);
 }
 
diff --git a/sys/sys/event.h b/sys/sys/event.h
index ed215453a325..9b8a3ba2bcf1 100644
--- a/sys/sys/event.h
+++ b/sys/sys/event.h
@@ -204,7 +204,8 @@ struct freebsd11_kevent32 {
  */
 #define	NOTE_EXIT	0x80000000		/* proc/procdesc: process
 						   exited */
-#define	NOTE_FORK	0x40000000		/* proc: process forked */
+#define	NOTE_FORK	0x40000000		/* proc/procdesc: process
+						   forked */
 #define	NOTE_EXEC	0x20000000		/* proc: process exec'd */
 #define	NOTE_PDSIGCHLD	0x10000000		/* procdesc: pdwait() info
 						   available */
diff --git a/sys/sys/procdesc.h b/sys/sys/procdesc.h
index 5ca93d3f4646..ac23dbdcb53b 100644
--- a/sys/sys/procdesc.h
+++ b/sys/sys/procdesc.h
@@ -64,6 +64,7 @@ struct procdesc {
 	 */
 	struct proc	*pd_proc;		/* (t) Process. */
 	pid_t		 pd_pid;		/* (c) Cached pid. */
+	pid_t		 pd_last_child;		/* (p) Pid of the last child. */
 	u_int		 pd_refcount;		/* (r) Reference count. */
 	u_int		 pd_fpcount;		/* (p|t) files referencing me */
 
@@ -102,6 +103,7 @@ struct procdesc {
  * In-kernel interfaces to process descriptors.
  */
 bool	 procdesc_exit(struct proc *);
+void	 procdesc_fork(struct proc *p, pid_t child_pid);
 void	 procdesc_jobstate(struct proc *p);
 int	 kern_pdgetpid(struct thread *, int fd, const cap_rights_t *,
 	    pid_t *pidp);