Fix coverity unbound buffer issues

During coverity scan, there are reported four issues
with unbounded source buffer for each usage of input arg
directly with syslog function.

Sample coverity test report for chsh.c file:

 1. string_size_argv: argv contains strings with unknown size.
 int main (int argc, char **argv)
[...]
 4. var_assign_var: Assigning: user = argv[optind]. Both are now tainted.
 user = argv[optind];
[...]
CID 5771784: (shadow-maint#1 of 1): Unbounded source buffer (STRING_SIZE)
15. string_size: Passing string user of unknown size to syslog.
 SYSLOG ((LOG_INFO, "changed user '%s' shell to '%s'", user, loginsh));

Similar issue is reported three times more:
File: chfn.c, function: main, variable: user
File: passwd.c, function: main, variable: name
File: newgrp.c, function: main, variable: group

The proposed commit is a try to fix the reported issues.

src/chfn.c

@@ -34,6 +34,7 @@
 #include "string/sprintf/snprintf.h"
 #include "string/strcpy/strtcpy.h"
 #include "string/strdup/xstrdup.h"
+#include "chkname.h"
 
 
 /*
@@ -643,6 +644,10 @@ int main (int argc, char **argv)
 	 * name, or the name getlogin() returns.
 	 */
 	if (optind < argc) {
+		if (!is_valid_user_name (argv[optind])) {
+			fprintf (stderr, _("%s: Provided user name is not a valid name\n"), Prog);
+			fail_exit (E_NOPERM);
+		}
 		user = argv[optind];
 		pw = xgetpwnam (user);
 		if (NULL == pw) {

src/chsh.c

@@ -32,6 +32,8 @@
 #include "shadowlog.h"
 #include "string/strcpy/strtcpy.h"
 #include "string/strdup/xstrdup.h"
+#include "chkname.h"
+
 
 #ifndef SHELLS_FILE
 #define SHELLS_FILE "/etc/shells"
@@ -499,6 +501,10 @@ int main (int argc, char **argv)
 	 * name, or the name getlogin() returns.
 	 */
 	if (optind < argc) {
+		if (!is_valid_user_name (argv[optind])) {
+			fprintf (stderr, _("%s: Provided user name is not a valid name\n"), Prog);
+			fail_exit (E_NOPERM);
+		}
 		user = argv[optind];
 		pw = xgetpwnam (user);
 		if (NULL == pw) {

src/newgrp.c

@@ -27,6 +27,7 @@
 #include "shadowlog.h"
 #include "string/sprintf/snprintf.h"
 #include "string/strdup/xstrdup.h"
+#include "chkname.h"
 
 
 /*
@@ -483,6 +484,12 @@ int main (int argc, char **argv)
 		 * not "newgrp".
 		 */
 		if ((argc > 0) && (argv[0][0] != '-')) {
+			if (!is_valid_group_name (argv[0])) {
+				fprintf (
+					stderr, _("%s: provided group is not a valid group name\n"),
+					Prog);
+				goto failure;
+			}
 			group = argv[0];
 			argc--;
 			argv++;
@@ -514,6 +521,12 @@ int main (int argc, char **argv)
 			usage ();
 			goto failure;
 		} else if (argv[0] != NULL) {
+			if (!is_valid_group_name (argv[0])) {
+				fprintf (
+					stderr, _("%s: provided group is not a valid group name\n"),
+					Prog);
+				goto failure;
+			}
 			group = argv[0];
 		} else {
 			/*

src/passwd.c

@@ -36,6 +36,7 @@
 #include "string/strcpy/strtcpy.h"
 #include "string/strdup/xstrdup.h"
 #include "time/day_to_str.h"
+#include "chkname.h"
 
 
 /*
@@ -910,6 +911,10 @@ main(int argc, char **argv)
 	}
 	myname = xstrdup (pw->pw_name);
 	if (optind < argc) {
+		if (!is_valid_user_name (argv[optind])) {
+			fprintf (stderr, _("%s: Provided user name is not a valid name\n"), Prog);
+			fail_exit (E_NOPERM);
+		}
 		name = argv[optind];
 	} else {
 		name = myname;