libc/resolv: Refactor the option parser

Start the loop by finding the end of the option name, the name-value
separator (if any), and the end of the option.  Use those pointers to
simplify matching the option name and parsing the option value.

This means that:

* We no longer accept trailing garbage in an option name or value.  For
  instance, we would previously interpret “edns0123” as “edns0” and
  “timeout:3xyz” as “timeout:3”.  This was actually quite lucky because
  we also failed to recognize the newline at the end of the option line
  as a whitespace character.

* For options that take a numerical value, we would previously happily
  set a numerical option to a negative value and treat non-numerical
  arguments as 0, while a numerical argument that happened to exceed
  the maximum for the option would result in the option being set to its
  default value.  Now, any failure to parse the argument, including
  overflow, results in the option being set to its default value.

MFC after:	1 week

1 files changed, 88 insertions, 77 deletions

diff --git a/lib/libc/resolv/res_init.c b/lib/libc/resolv/res_init.c
index e9e982fe27bb..20f6824857b5 100644
--- a/lib/libc/resolv/res_init.c
+++ b/lib/libc/resolv/res_init.c
@@ -3,6 +3,7 @@
  *
  * Copyright (c) 1985, 1989, 1993
  *    The Regents of the University of California.  All rights reserved.
+ * Copyright (c) 2026 Dag-Erling Smørgrav
  * 
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -80,6 +81,7 @@
 #include <arpa/nameser.h>
 
 #include <ctype.h>
+#include <limits.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -103,9 +105,11 @@ static const char sort_mask[] = "/&";
 static u_int32_t net_mask(struct in_addr);
 #endif
 
-#if !defined(isascii)	/*%< XXX - could be a function */
-# define isascii(c) (!(c & 0200))
-#endif
+#define res_space(ch)							\
+	isspace((unsigned char)(ch))
+#define res_match(str, end, word)					\
+	(strncmp((str), (word), (end) - (str)) == 0 &&			\
+	    (word)[(end) - (str)] == '\0')
 
 /*
  * Resolver state default settings.
@@ -520,129 +524,136 @@ __res_vinit(res_state statp, int preinit) {
 static void
 res_setoptions(res_state statp, const char *options, const char *source)
 {
-	const char *cp = options;
-	int i;
+	const char *cp = options, *sp, *ep;
+	char *numend;
+	long num;
 	struct __res_state_ext *ext = statp->_u._ext.ext;
 
 #ifdef DEBUG
-	if (statp->options & RES_DEBUG)
+	if (statp->options & RES_DEBUG) {
 		printf(";; res_setoptions(\"%s\", \"%s\")...\n",
 		       options, source);
+	}
 #endif
-	while (*cp) {
+	for (;;) {
 		/* skip leading and inner runs of spaces */
-		while (*cp == ' ' || *cp == '\t')
+		while (res_space(*cp))
 			cp++;
+		/* find the separator if there is one */
+		sp = cp;
+		while (*sp != '\0' && !res_space(*sp) && *sp != ':')
+			sp++;
+		/* find the end of the option */
+		ep = sp;
+		while (*ep != '\0' && !res_space(*ep))
+			ep++;
+		/* end of option line */
+		if (ep == cp)
+			break;
 		/* search for and process individual options */
-		if (!strncmp(cp, "ndots:", sizeof("ndots:") - 1)) {
-			i = atoi(cp + sizeof("ndots:") - 1);
-			if (i <= RES_MAXNDOTS)
-				statp->ndots = i;
+		if (res_match(cp, sp, "ndots") && sp < ep) {
+			num = strtol(sp + 1, &numend, 10);
+			if (numend == ep && num <= RES_MAXNDOTS)
+				statp->ndots = num;
 			else
 				statp->ndots = RES_MAXNDOTS;
 #ifdef DEBUG
 			if (statp->options & RES_DEBUG)
 				printf(";;\tndots=%d\n", statp->ndots);
 #endif
-		} else if (!strncmp(cp, "timeout:", sizeof("timeout:") - 1)) {
-			i = atoi(cp + sizeof("timeout:") - 1);
-			if (i <= RES_MAXRETRANS)
-				statp->retrans = i;
+		} else if (res_match(cp, sp, "timeout") && sp < ep) {
+			num = strtol(sp + 1, &numend, 10);
+			if (numend == ep && num > 0 && num <= RES_MAXRETRANS)
+				statp->retrans = num;
 			else
 				statp->retrans = RES_MAXRETRANS;
 #ifdef DEBUG
 			if (statp->options & RES_DEBUG)
 				printf(";;\ttimeout=%d\n", statp->retrans);
 #endif
-		} else if (!strncmp(cp, "attempts:", sizeof("attempts:") - 1)){
-			i = atoi(cp + sizeof("attempts:") - 1);
-			if (i <= RES_MAXRETRY)
-				statp->retry = i;
+		} else if (res_match(cp, sp, "attempts") && sp < ep) {
+			num = strtol(sp + 1, &numend, 10);
+			if (numend == ep && num > 0 && num <= RES_MAXRETRY)
+				statp->retry = num;
 			else
 				statp->retry = RES_MAXRETRY;
 #ifdef DEBUG
 			if (statp->options & RES_DEBUG)
 				printf(";;\tattempts=%d\n", statp->retry);
 #endif
-		} else if (!strncmp(cp, "debug", sizeof("debug") - 1)) {
+		} else if (res_match(cp, ep, "debug")) {
 #ifdef DEBUG
 			if (!(statp->options & RES_DEBUG)) {
 				printf(";; res_setoptions(\"%s\", \"%s\")..\n",
 				       options, source);
-				statp->options |= RES_DEBUG;
 			}
+#endif
+			statp->options |= RES_DEBUG;
+#ifdef DEBUG
 			printf(";;\tdebug\n");
 #endif
-		} else if (!strncmp(cp, "no_tld_query",
-				    sizeof("no_tld_query") - 1) ||
-			   !strncmp(cp, "no-tld-query",
-				    sizeof("no-tld-query") - 1)) {
+		} else if (res_match(cp, ep, "no-tld-query") ||
+		    res_match(cp, ep, "no_tld_query")) {
 			statp->options |= RES_NOTLDQUERY;
-		} else if (!strncmp(cp, "inet6", sizeof("inet6") - 1)) {
+		} else if (res_match(cp, ep, "inet6")) {
 			statp->options |= RES_USE_INET6;
-		} else if (!strncmp(cp, "insecure1", sizeof("insecure1") - 1)) {
-		       statp->options |= RES_INSECURE1;
-		} else if (!strncmp(cp, "insecure2", sizeof("insecure2") - 1)) {
-		       statp->options |= RES_INSECURE2;
-		} else if (!strncmp(cp, "rotate", sizeof("rotate") - 1)) {
+		} else if (res_match(cp, ep, "insecure1")) {
+			statp->options |= RES_INSECURE1;
+		} else if (res_match(cp, ep, "insecure2")) {
+			statp->options |= RES_INSECURE2;
+		} else if (res_match(cp, ep, "blast")) {
+			statp->options |= RES_BLAST;
+		} else if (res_match(cp, ep, "rotate")) {
 			statp->options |= RES_ROTATE;
-		} else if (!strncmp(cp, "usevc", sizeof("usevc") - 1)) {
+		} else if (res_match(cp, ep, "usevc")) {
 			statp->options |= RES_USEVC;
-		} else if (!strncmp(cp, "no-check-names",
-				    sizeof("no-check-names") - 1)) {
+		} else if (res_match(cp, ep, "no-check-names")) {
 			statp->options |= RES_NOCHECKNAME;
-		} else if (!strncmp(cp, "reload-period:",
-				    sizeof("reload-period:") - 1)) {
-			if (ext != NULL) {
-				ext->reload_period = (u_short)
-				    atoi(cp + sizeof("reload-period:") - 1);
+		} else if (res_match(cp, sp, "reload-period") && sp < ep) {
+			num = strtol(sp + 1, &numend, 10);
+			if (numend == ep && num > 0 && num <= USHRT_MAX &&
+			    ext != NULL) {
+				ext->reload_period = (u_short)num;
 			}
-		}
 #ifdef RES_USE_EDNS0
-		else if (!strncmp(cp, "edns0", sizeof("edns0") - 1)) {
+		} else if (res_match(cp, ep, "edns0")) {
 			statp->options |= RES_USE_EDNS0;
-		}
 #endif
 #ifndef _LIBC
-		else if (!strncmp(cp, "dname", sizeof("dname") - 1)) {
+		} else if (res_match(cp, ep, "dname")) {
 			statp->options |= RES_USE_DNAME;
-		}
-		else if (!strncmp(cp, "nibble:", sizeof("nibble:") - 1)) {
-			if (ext == NULL)
-				goto skip;
-			cp += sizeof("nibble:") - 1;
-			i = MIN(strcspn(cp, " \t"), sizeof(ext->nsuffix) - 1);
-			strncpy(ext->nsuffix, cp, i);
-			ext->nsuffix[i] = '\0';
-		}
-		else if (!strncmp(cp, "nibble2:", sizeof("nibble2:") - 1)) {
-			if (ext == NULL)
-				goto skip;
-			cp += sizeof("nibble2:") - 1;
-			i = MIN(strcspn(cp, " \t"), sizeof(ext->nsuffix2) - 1);
-			strncpy(ext->nsuffix2, cp, i);
-			ext->nsuffix2[i] = '\0';
-		}
-		else if (!strncmp(cp, "v6revmode:", sizeof("v6revmode:") - 1)) {
-			cp += sizeof("v6revmode:") - 1;
+		} else if (res_match(cp, sp, "nibble") && sp < ep) {
+			sp++;
+			if (ext != NULL && ep - sp < sizeof(ext->nsuffix)) {
+				strncpy(ext->nsuffix, sp, ep - sp);
+				ext->nsuffix[ep - sp] = '\0';
+			}
+		} else if (res_match(cp, sp, "nibble2") && sp < ep) {
+			sp++;
+			if (ext != NULL && ep - sp < sizeof(ext->nsuffix2)) {
+				strncpy(ext->nsuffix2, sp, ep - sp);
+				ext->nsuffix2[ep - sp] = '\0';
+			}
+		} else if (res_match(cp, sp, "v6revmode") && sp < ep) {
 			/* "nibble" and "bitstring" used to be valid */
-			if (!strncmp(cp, "single", sizeof("single") - 1)) {
+			if (res_match(sp + 1, ep, "single"))
 				statp->options |= RES_NO_NIBBLE2;
-			} else if (!strncmp(cp, "both", sizeof("both") - 1)) {
-				statp->options &=
-					 ~RES_NO_NIBBLE2;
-			}
-		}
+			else if (res_match(sp + 1, ep, "both"))
+				statp->options &= ~RES_NO_NIBBLE2;
+#ifdef DEBUG
+			else
+				printf(";;\t%.*s: unrecognized value: %.*s\n",
+				    (int)(sp - cp), cp,
+				    (int)(ep - sp - 1), sp + 1);
 #endif
-		else {
-			/* XXX - print a warning here? */
-		}
-#ifndef _LIBC
-   skip:
+#endif /* !_LIBC */
+		} else {
+#ifdef DEBUG
+			printf(";;\tunrecognized option: %.*s\n",
+			    (int)(ep - cp), cp);
 #endif
-		/* skip to next run of spaces */
-		while (*cp && *cp != ' ' && *cp != '\t')
-			cp++;
+		}
+		cp = ep;
 	}
 }