Restrict -a to root only.

PR:		bin/25337

sbin/dmesg/dmesg.8

@@ -54,6 +54,7 @@ Show all data in the message buffer.
 This includes any syslog records and
 .Pa /dev/console
 output.
+Only root is allowed to use this option.
 .It Fl M
 Extract values associated with the name list from the specified core
 instead of the default

sbin/dmesg/dmesg.c

@@ -45,6 +45,7 @@ static const char rcsid[] =
   "$FreeBSD$";
 #endif /* not lint */
 
+#include <sys/types.h>
 #include <sys/msgbuf.h>
 
 #include <err.h>
@@ -53,9 +54,10 @@ static const char rcsid[] =
 #include <locale.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <sysexits.h>
+#include <syslog.h>
 #include <unistd.h>
 #include <vis.h>
-#include <sys/syslog.h>
 
 struct nlist nl[] = {
 #define	X_MSGBUF	0
@@ -109,6 +111,9 @@ main(argc, argv)
 	if (memf != NULL || nlistf != NULL)
 		setgid(getgid());
 
+	if (all && getuid())
+		errx(EX_NOPERM, "must be root to use -a");
+
 	/* Read in kernel message buffer, do sanity checks. */
 	if ((kd = kvm_open(nlistf, memf, NULL, O_RDONLY, "dmesg")) == NULL)
 		exit (1);