commit d7ff1f5448437c7afe491dc0caf01ea418e7ae46
Author: Mikolaj Golub <trociny@freebsd.org>
Date:   Sun Oct 16 19:25:41 2011 +0300

    Make kvm_uread() read from user space using ptrace(2). With this
    change 'ps -e' does not require procfs(5).

diff --git a/lib/libkvm/kvm_proc.c b/lib/libkvm/kvm_proc.c
index 876af50..71809ee 100644
--- a/lib/libkvm/kvm_proc.c
+++ b/lib/libkvm/kvm_proc.c
@@ -57,6 +57,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/cpuset.h>
 #include <sys/user.h>
 #include <sys/proc.h>
+#include <sys/ptrace.h>
 #define	_WANT_PRISON	/* make jail.h give us 'struct prison' */
 #include <sys/jail.h>
 #include <sys/exec.h>
@@ -66,6 +67,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/tty.h>
 #include <sys/file.h>
 #include <sys/conf.h>
+#include <sys/wait.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
@@ -958,10 +960,7 @@ ssize_t
 kvm_uread(kvm_t *kd, const struct kinfo_proc *kp, u_long uva, char *buf,
 	size_t len)
 {
-	char *cp;
-	char procfile[MAXPATHLEN];
-	ssize_t amount;
-	int fd;
+	struct ptrace_io_desc ioreq;
 
 	if (!ISALIVE(kd)) {
 		_kvm_err(kd, kd->program,
@@ -969,36 +968,34 @@ kvm_uread(kvm_t *kd, const struct kinfo_proc *kp, u_long uva, char *buf,
 		return (0);
 	}
 
-	sprintf(procfile, "/proc/%d/mem", kp->ki_pid);
-	fd = open(procfile, O_RDONLY, 0);
-	if (fd < 0) {
-		_kvm_err(kd, kd->program, "cannot open %s", procfile);
+	if (getpid() == kp->ki_pid) {
+		bcopy((void *)uva, buf, len);
+		return ((ssize_t)len);
+	}
+
+	errno = 0;
+	ptrace(PT_ATTACH, kp->ki_pid, NULL, 0);
+	if (errno != 0) {
+		_kvm_err(kd, kd->program, "cannot attach to the process %d", kp->ki_pid);
 		return (0);
 	}
 
-	cp = buf;
-	while (len > 0) {
-		errno = 0;
-		if (lseek(fd, (off_t)uva, 0) == -1 && errno != 0) {
-			_kvm_err(kd, kd->program, "invalid address (%lx) in %s",
-			    uva, procfile);
-			break;
-		}
-		amount = read(fd, cp, len);
-		if (amount < 0) {
-			_kvm_syserr(kd, kd->program, "error reading %s",
-			    procfile);
-			break;
-		}
-		if (amount == 0) {
-			_kvm_err(kd, kd->program, "EOF reading %s", procfile);
-			break;
-		}
-		cp += amount;
-		uva += amount;
-		len -= amount;
+	if (waitpid(kp->ki_pid, NULL, 0) == -1)
+		_kvm_err(kd, kd->program, "unexpect stop in waitpid");
+
+	ioreq.piod_op = PIOD_READ_D;
+	ioreq.piod_offs = (caddr_t)uva;
+	ioreq.piod_addr = buf;
+	ioreq.piod_len = len;
+
+	ptrace(PT_IO, kp->ki_pid, (caddr_t)&ioreq, 0);
+
+	if ((size_t)ioreq.piod_len < len) {
+		_kvm_err(kd, kd->program, "short read wanted %zu, got %zd",
+		    len, ioreq.piod_len);
 	}
 
-	close(fd);
-	return ((ssize_t)(cp - buf));
+	ptrace(PT_DETACH, kp->ki_pid, (caddr_t)1, 0);
+
+	return ((ssize_t)ioreq.piod_len);
 }