[patch] Initialize "cur" to avoid undefined behavior is dmesg.c

[Nan Xiao]

Hi tech@,

This patch fixes the extreme case in dmesg.c: if memf or nlistf is not
NULL, and "NOKVM" macro is defined.

Current code in dmesg.c:

        struct msgbuf cur;
        
Since "cur" is not initialized, so the following code has undefined
behavior:

        if (cur.msg_bufx >= cur.msg_bufs)
                cur.msg_bufx = 0;
        /*
         * The message buffer is circular; start at the read pointer, and
         * go to the write pointer - 1.
         */
        for (newl = skip = i = 0, p = bufdata + cur.msg_bufx;
            i < cur.msg_bufs; i++, p++) {
        .....
        }

My patch can skip the whole loop, and the "dmesg" program just prints
a newline:

        if (!newl)
                putchar('\n');

Best Regards
Nan Xiao

Index: dmesg.c
===================================================================
RCS file: /cvs/src/sbin/dmesg/dmesg.c,v
retrieving revision 1.29
diff -u -p -r1.29 dmesg.c
--- dmesg.c     1 Sep 2017 07:31:45 -0000       1.29
+++ dmesg.c     4 Sep 2017 08:55:50 -0000
@@ -65,12 +65,12 @@ main(int argc, char *argv[])
        int ch, newl, skip, i;
        char *p;
        struct msgbuf cur;
-       char *memf, *nlistf, *bufdata = NULL;
+       char *memf = NULL, *nlistf = NULL, *bufdata = NULL;
        char *allocated = NULL;
        int startupmsgs = 0;
        char buf[5];

-       memf = nlistf = NULL;
+       memset(&cur, 0, sizeof(cur));
        while ((ch = getopt(argc, argv, "sM:N:")) != -1)
                switch(ch) {
                case 's':
@@ -184,7 +184,8 @@ main(int argc, char *argv[])
        }
        if (!newl)
                putchar('\n');
-       free(allocated);
+       if (allocated)
+               free(allocated);
        return (0);
 }