de-duplicate dn_expand, fix return value and signature, clean up

the duplicate code in dn_expand and its incorrect return values are
both results of the history of the code: the version in __dns.c was
originally written with no awareness of the legacy resolver API, and
was later copy-and-paste duplicated to provide the legacy API.

this commit is the first of a series that will restructure the
internal dns code to share as much code as possible with the legacy
resolver API functions.

I have also removed the loop detection logic, since the output buffer
length limit naturally prevents loops. in order to avoid long runtime
when encountering a loop if the caller provided a ridiculously long
buffer, the caller-provided length is clamped at the maximum dns name
length.

include/resolv.h

@@ -134,7 +134,7 @@ int res_search(const char *, int, int, unsigned char *, int);
 int res_mkquery(int, const char *, int, int, char *, int, struct rrec *, char *, int);
 int res_send(const char *, int, char *, int);
 int dn_comp(unsigned char *, unsigned char *, int, unsigned char **, unsigned char *, unsigned char **);
-int dn_expand(unsigned char *, unsigned char *, unsigned char *, unsigned char *, int);
+int dn_expand(const unsigned char *, const unsigned char *, const unsigned char *, char *, int);
 int dn_skipname(const unsigned char *, const unsigned char *);
 
 #ifdef __cplusplus

src/network/__dns.c

@@ -195,32 +195,7 @@ int __dns_query(unsigned char *r, const void *a, int family, int ptr)
 	return __dns_doqueries(r, a, rr, rrcnt);
 }
 
-
-#define BITOP(a,b,op) \
- ((a)[(size_t)(b)/(8*sizeof *(a))] op (size_t)1<<((size_t)(b)%(8*sizeof *(a))))
-
-static int decname(char *s, const unsigned char *b, const unsigned char *p)
-{
-	/* Remember jump destinations to detect loops and abort */
-	size_t seen[PACKET_MAX/8/sizeof(size_t)] = { 0 };
-	char *sz = s + HOST_NAME_MAX;
-	const unsigned char *pz = b+512;
-	for (;;) {
-		if (p>=pz) return -1;
-		else if (*p&0xc0) {
-			int j = (p[0]&1) | p[1];
-			if (BITOP(seen, j, &)) return -1;
-			BITOP(seen, j, |=);
-			p = b + j;
-		} else if (*p) {
-			if (p+*p+1>=pz || s+*p>=sz) return -1;
-			memcpy(s, p+1, *p);
-			s += *p+1;
-			p += *p+1;
-			s[-1] = *p ? '.' : 0;
-		} else return 0;
-	}
-}
+int __dn_expand(const unsigned char *, const unsigned char *, const unsigned char *, char *, int);
 
 int __dns_get_rr(void *dest, size_t stride, size_t maxlen, size_t limit, const unsigned char *r, int rr, int dec)
 {
@@ -249,7 +224,8 @@ int __dns_get_rr(void *dest, size_t stride, size_t maxlen, size_t limit, const u
 		len = p[8]*256 + p[9];
 		if (p+len > r+512) return -1;
 		if (p[1]==rr && len <= maxlen) {
-			if (dec && decname(tmp, r, p+10)<0) return -1;
+			if (dec && __dn_expand(r, r+512, p+10, tmp, sizeof tmp)<0)
+				return -1;
 			if (dest && limit) {
 				if (dec) strcpy(dest, tmp);
 				else memcpy(dest, p+10, len);

src/network/dn_expand.c

@@ -1,28 +1,27 @@
 #include <resolv.h>
-#include <string.h>
+#include "libc.h"
 
-#define BITOP(a,b,op) \
- ((a)[(size_t)(b)/(8*sizeof *(a))] op (size_t)1<<((size_t)(b)%(8*sizeof *(a))))
-
-int dn_expand(unsigned char *b, unsigned char *pz, unsigned char *p, unsigned char *s, int outlen)
+int __dn_expand(const unsigned char *base, const unsigned char *end, const unsigned char *src, char *dest, int space)
 {
-	/* Remember jump destinations to detect loops and abort */
-	size_t seen[512/8/sizeof(size_t)] = { 0 };
-	unsigned char *sz = s + outlen;
-	if (pz-b > 512) return -1;
+	const unsigned char *p = src;
+	int len = -1, j;
+	if (space > 256) space = 256;
+	if (p==end) return -1;
 	for (;;) {
-		if (p>=pz) return -1;
-		else if (*p&0xc0) {
-			int j = (p[0]&1) | p[1];
-			if (BITOP(seen, j, &)) return -1;
-			BITOP(seen, j, |=);
-			p = b + j;
+		if (*p & 0xc0) {
+			if (p+1==end) return -1;
+			j = (p[0]&1) | p[1];
+			if (len < 0) len = p+2-src;
+			if (j >= end-base) return -1;
+			p = base+j;
 		} else if (*p) {
-			if (p+*p+1>=pz || s+*p>=sz) return -1;
-			memcpy(s, p+1, *p);
-			s += *p+1;
-			p += *p+1;
-			s[-1] = *p ? '.' : 0;
-		} else return 0;
+			j = *p+1;
+			if (len < 0) len = p+1-src;
+			if (j>=end-p || j>space) return -1;
+			while (--j) *dest++ = *p++;
+			*dest++ = *++p ? '.' : 0;
+		} else return len;
 	}
 }
+
+weak_alias(__dn_expand, dn_expand);