kern/proc: expose reaper PID and subtree root in struct kinfo_proc

Expose process reaper metadata through struct kinfo_proc so userland
can reconstruct reaper hierarchies from kern.proc.all without adding
a new procctl(2) operation.

Two pid_t fields are added by carving 8 bytes from ki_sparestrings
(46 -> 38), restoring KI_NSPARE_INT to 2 and keeping sizeof(struct
kinfo_proc) unchanged:

  ki_reaper: PID of the owning reaper process
  ki_reapsubtree: PID of the direct child of the reaper that roots
                  the subtree the process belongs to

fill_kinfo_proc_pgrp() populates both fields under proctree_lock.
kvm_proclist() is updated for crash dump consumers. The freebsd32
compat struct and freebsd32_kinfo_proc_out() are updated accordingly.

PR:	293871
Reviewed by:	kib
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D56538

lib/libkvm/kvm_proc.c

@@ -121,6 +121,7 @@ kvm_proclist(kvm_t *kd, int what, int arg, struct proc *p,
 	struct thread mtd;
 	struct proc proc;
 	struct proc pproc;
+	struct proc rproc;
 	struct sysentvec sysent;
 	char svname[KI_EMULNAMELEN];
 	struct thread *td = NULL;
@@ -365,6 +366,13 @@ kvm_proclist(kvm_t *kd, int what, int arg, struct proc *p,
 		kp->ki_xstat = KW_EXITCODE(proc.p_xexit, proc.p_xsig);
 		kp->ki_acflag = proc.p_acflag;
 		kp->ki_lock = proc.p_lock;
+		if (KREAD(kd, (u_long)proc.p_reaper, &rproc)) {
+			_kvm_err(kd, kd->program,
+			    "can't read reaper at %p", proc.p_reaper);
+			return (-1);
+		}
+		kp->ki_reaper = rproc.p_pid;
+		kp->ki_reapsubtree = proc.p_reapsubtree;
 		kp->ki_tdev_freebsd11 = kp->ki_tdev; /* truncate */
 
 		/* Per-thread items; iterate as appropriate. */

sys/compat/freebsd32/freebsd32.h

@@ -392,8 +392,10 @@ struct kinfo_proc32 {
 	char	ki_emul[KI_EMULNAMELEN+1];
 	char	ki_loginclass[LOGINCLASSLEN+1];
 	char	ki_moretdname[MAXCOMLEN-TDNAMLEN+1];
-	char	ki_sparestrings[46];
+	char	ki_sparestrings[38];
 	int	ki_spareints[KI_NSPARE_INT];
+	pid_t	ki_reaper;
+	pid_t	ki_reapsubtree;
 	freebsd32_uint64_t ki_tdev;
 	int	ki_oncpu;
 	int	ki_lastcpu;

sys/kern/kern_proc.c

@@ -1235,6 +1235,8 @@ fill_kinfo_proc_pgrp(struct proc *p, struct kinfo_proc *kp)
 		kp->ki_tdev = NODEV;
 		kp->ki_tdev_freebsd11 = kp->ki_tdev; /* truncate */
 	}
+	kp->ki_reaper = p->p_reaper->p_pid;
+	kp->ki_reapsubtree = p->p_reapsubtree;
 }
 
 /*
@@ -1494,6 +1496,8 @@ freebsd32_kinfo_proc_out(const struct kinfo_proc *ki, struct kinfo_proc32 *ki32)
 	CP(*ki, *ki32, ki_fibnum);
 	CP(*ki, *ki32, ki_cr_flags);
 	CP(*ki, *ki32, ki_jid);
+	CP(*ki, *ki32, ki_reaper);
+	CP(*ki, *ki32, ki_reapsubtree);
 	CP(*ki, *ki32, ki_numthreads);
 	CP(*ki, *ki32, ki_tid);
 	CP(*ki, *ki32, ki_pri);

sys/sys/user.h

@@ -186,8 +186,10 @@ struct kinfo_proc {
 	 * front of ki_sparestrings, and ints from the end of ki_spareints.
 	 * That way the spare room from both arrays will remain contiguous.
 	 */
-	char	ki_sparestrings[46];	/* spare string space */
+	char	ki_sparestrings[38];	/* spare string space */
 	int	ki_spareints[KI_NSPARE_INT];	/* spare room for growth */
+	pid_t	ki_reaper;		/* pid of reaper process */
+	pid_t	ki_reapsubtree;		/* reaper subtree id */
 	uint64_t ki_tdev;		/* controlling tty dev */
 	int	ki_oncpu;		/* Which cpu we are on */
 	int	ki_lastcpu;		/* Last cpu we were on */