Add kcmp(2) kernel bits

This is based purely on reading the Linux kcmp(2) man page.
In addition to the Linux set of comparators, I also added KCMP_FILEOBJ to
compare underlying file' objects.

Tested by:	manu
Reviewed by:	brooks, markj
Sponsored by:   The FreeBSD Foundation
MFC after:      1 week
Differential revision:  https://reviews.freebsd.org/D43518

sys/kern/sys_generic.c

@@ -65,6 +65,7 @@
 #include <sys/sysctl.h>
 #include <sys/sysent.h>
 #include <sys/vnode.h>
+#include <sys/unistd.h>
 #include <sys/bio.h>
 #include <sys/buf.h>
 #include <sys/condvar.h>
@@ -2071,3 +2072,89 @@ kern_posix_error(struct thread *td, int error)
 	td->td_retval[0] = error;
 	return (0);
 }
+
+int
+kcmp_cmp(uintptr_t a, uintptr_t b)
+{
+	if (a == b)
+		return (0);
+	else if (a < b)
+		return (1);
+	return (2);
+}
+
+static int
+kcmp_pget(struct thread *td, pid_t pid, struct proc **pp)
+{
+	if (pid == td->td_proc->p_pid) {
+		*pp = td->td_proc;
+		return (0);
+	}
+	return (pget(pid, PGET_CANDEBUG | PGET_NOTWEXIT | PGET_HOLD, pp));
+}
+
+int
+kern_kcmp(struct thread *td, pid_t pid1, pid_t pid2, int type,
+    uintptr_t idx1, uintptr_t idx2)
+{
+	struct proc *p1, *p2;
+	struct file *fp1, *fp2;
+	int error, res;
+
+	res = -1;
+	p1 = p2 = NULL;
+	error = kcmp_pget(td, pid1, &p1);
+	if (error == 0)
+		error = kcmp_pget(td, pid2, &p2);
+	if (error != 0)
+		goto out;
+
+	switch (type) {
+	case KCMP_FILE:
+	case KCMP_FILEOBJ:
+		error = fget_remote(td, p1, idx1, &fp1);
+		if (error == 0) {
+			error = fget_remote(td, p2, idx2, &fp2);
+			if (error == 0) {
+				if (type == KCMP_FILEOBJ)
+					res = fo_cmp(fp1, fp2, td);
+				else
+					res = kcmp_cmp((uintptr_t)fp1,
+					    (uintptr_t)fp2);
+				fdrop(fp2, td);
+			}
+			fdrop(fp1, td);
+		}
+		break;
+	case KCMP_FILES:
+		res = kcmp_cmp((uintptr_t)p1->p_fd, (uintptr_t)p2->p_fd);
+		break;
+	case KCMP_SIGHAND:
+		res = kcmp_cmp((uintptr_t)p1->p_sigacts,
+		    (uintptr_t)p2->p_sigacts);
+		break;
+	case KCMP_VM:
+		res = kcmp_cmp((uintptr_t)p1->p_vmspace,
+		    (uintptr_t)p2->p_vmspace);
+		break;
+	default:
+		error = EINVAL;
+		break;
+	}
+
+out:
+	if (p1 != NULL && p1 != td->td_proc)
+		PRELE(p1);
+	if (p2 != NULL && p2 != td->td_proc)
+		PRELE(p2);
+
+	td->td_retval[0] = res;
+	return (error);
+}
+
+int
+sys_kcmp(struct thread *td, struct kcmp_args *uap)
+{
+	return (kern_kcmp(td, uap->pid1, uap->pid2, uap->type,
+	    uap->idx1, uap->idx2));
+}

sys/kern/syscalls.master

@@ -3329,6 +3329,14 @@
 		    _Out_opt_ _Contains_long_timet_ struct itimerspec *old_value
 		);
 	}
-
+588	AUE_NULL	STD {
+		int kcmp(
+		    pid_t pid1,
+		    pid_t pid2,
+		    int type,
+		    uintptr_t idx1,
+		    uintptr_t idx2
+		);
+	}
 
 ; vim: syntax=off

sys/sys/syscallsubr.h

@@ -198,6 +198,8 @@ int	kern_ioctl(struct thread *td, int fd, u_long com, caddr_t data);
 int	kern_jail(struct thread *td, struct jail *j);
 int	kern_jail_get(struct thread *td, struct uio *options, int flags);
 int	kern_jail_set(struct thread *td, struct uio *options, int flags);
+int	kern_kcmp(struct thread *td, pid_t pid1, pid_t pid2, int type,
+	    uintptr_t idx1, uintptr_t idx2);
 int	kern_kevent(struct thread *td, int fd, int nchanges, int nevents,
 	    struct kevent_copyops *k_ops, const struct timespec *timeout);
 int	kern_kevent_anonymous(struct thread *td, int nevents,

sys/sys/systm.h

@@ -485,6 +485,8 @@ int poll_no_poll(int events);
 /* XXX: Should be void nanodelay(u_int nsec); */
 void	DELAY(int usec);
 
+int kcmp_cmp(uintptr_t a, uintptr_t b);
+
 /* Root mount holdback API */
 struct root_hold_token {
 	int				flags;

sys/sys/unistd.h

@@ -195,6 +195,13 @@
     RFPROCDESC | RFSPAWN | RFPPWAIT)
 #define	RFKERNELONLY	(RFSTOPPED | RFHIGHPID | RFPROCDESC)
 
+/* kcmp() options. */
+#define	KCMP_FILE	100
+#define	KCMP_FILEOBJ	101
+#define	KCMP_FILES	102
+#define	KCMP_SIGHAND	103
+#define	KCMP_VM		104
+
 #define	SWAPOFF_FORCE	0x00000001
 
 /*