nss_dns: Rewrite _nss_dns_gethostbyname4_r using current interfaces

Introduce struct alloc_buffer to this function, and use it and
struct ns_rr_cursor in gaih_getanswer_slice.  Adjust gaih_getanswer
and gaih_getanswer_noaaaa accordingly.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

resolv/nss_dns/dns-host.c

@@ -100,13 +100,6 @@
 #endif
 #define MAXHOSTNAMELEN 256
 
-/* We need this time later.  */
-typedef union querybuf
-{
-  HEADER hdr;
-  u_char buf[MAXPACKET];
-} querybuf;
-
 /* For historic reasons, pointers to IP addresses are char *, so use a
    single list type for addresses and host names.  */
 #define DYNARRAY_STRUCT ptrlist
@@ -125,18 +118,18 @@ static enum nss_status getanswer_ptr (unsigned char *packet, size_t packetlen,
 				      char **hnamep, int *errnop,
 				      int *h_errnop, int32_t *ttlp);
 
-static enum nss_status gaih_getanswer (const querybuf *answer1, int anslen1,
+static enum nss_status gaih_getanswer (unsigned char *packet1,
-				       const querybuf *answer2, int anslen2,
+				       size_t packet1len,
-				       const char *qname,
+				       unsigned char *packet2,
+				       size_t packet2len,
+				       struct alloc_buffer *abuf,
 				       struct gaih_addrtuple **pat,
-				       char *buffer, size_t buflen,
 				       int *errnop, int *h_errnop,
 				       int32_t *ttlp);
-static enum nss_status gaih_getanswer_noaaaa (const querybuf *answer1,
+static enum nss_status gaih_getanswer_noaaaa (unsigned char *packet,
-					      int anslen1,
+					      size_t packetlen,
-					      const char *qname,
+					      struct alloc_buffer *abuf,
 					      struct gaih_addrtuple **pat,
-					      char *buffer, size_t buflen,
 					      int *errnop, int *h_errnop,
 					      int32_t *ttlp);
 
@@ -408,17 +401,13 @@ _nss_dns_gethostbyname4_r (const char *name, struct gaih_addrtuple **pat,
 	name = cp;
     }
 
-  union
+  unsigned char dns_packet_buffer[2048];
-  {
+  unsigned char *alt_dns_packet_buffer = dns_packet_buffer;
-    querybuf *buf;
-    u_char *ptr;
-  } host_buffer;
-  querybuf *orig_host_buffer;
-  host_buffer.buf = orig_host_buffer = (querybuf *) alloca (2048);
   u_char *ans2p = NULL;
   int nans2p = 0;
   int resplen2 = 0;
   int ans2p_malloced = 0;
+  struct alloc_buffer abuf = alloc_buffer_create (buffer, buflen);
 
 
   int olderr = errno;
@@ -427,22 +416,21 @@ _nss_dns_gethostbyname4_r (const char *name, struct gaih_addrtuple **pat,
   if ((ctx->resp->options & RES_NOAAAA) == 0)
     {
       n = __res_context_search (ctx, name, C_IN, T_QUERY_A_AND_AAAA,
-				host_buffer.buf->buf, 2048, &host_buffer.ptr,
+				dns_packet_buffer, sizeof (dns_packet_buffer),
-				&ans2p, &nans2p, &resplen2, &ans2p_malloced);
+				&alt_dns_packet_buffer, &ans2p, &nans2p,
+				&resplen2, &ans2p_malloced);
       if (n >= 0)
-	status = gaih_getanswer (host_buffer.buf, n, (const querybuf *) ans2p,
+	status = gaih_getanswer (alt_dns_packet_buffer, n, ans2p, resplen2,
-				 resplen2, name, pat, buffer, buflen,
+				 &abuf, pat, errnop, herrnop, ttlp);
-				 errnop, herrnop, ttlp);
     }
   else
     {
       n = __res_context_search (ctx, name, C_IN, T_A,
-				host_buffer.buf->buf, 2048, NULL,
+				dns_packet_buffer, sizeof (dns_packet_buffer),
-				NULL, NULL, NULL, NULL);
+				NULL, NULL, NULL, NULL, NULL);
       if (n >= 0)
-	status = gaih_getanswer_noaaaa (host_buffer.buf, n,
+	status = gaih_getanswer_noaaaa (alt_dns_packet_buffer, n,
-					name, pat, buffer, buflen,
+					&abuf, pat, errnop, herrnop, ttlp);
-					errnop, herrnop, ttlp);
     }
   if (n < 0)
     {
@@ -473,12 +461,20 @@ _nss_dns_gethostbyname4_r (const char *name, struct gaih_addrtuple **pat,
 	__set_errno (olderr);
     }
 
+  /* Implement the buffer resizing protocol.  */
+  if (alloc_buffer_has_failed (&abuf))
+    {
+      *errnop = ERANGE;
+      *herrnop = NETDB_INTERNAL;
+      status = NSS_STATUS_TRYAGAIN;
+    }
+
   /* Check whether ans2p was separately allocated.  */
   if (ans2p_malloced)
     free (ans2p);
 
-  if (host_buffer.buf != orig_host_buffer)
+  if (alt_dns_packet_buffer != dns_packet_buffer)
-    free (host_buffer.buf);
+    free (alt_dns_packet_buffer);
 
   __resolv_context_put (ctx);
   return status;
@@ -892,259 +888,152 @@ getanswer_ptr (unsigned char *packet, size_t packetlen,
   return NSS_STATUS_TRYAGAIN;
 }
 
+/* Parses DNS data found in PACKETLEN bytes at PACKET in struct
+   gaih_addrtuple address tuples.  The new address tuples are linked
+   from **TAILP, with backing store allocated from ABUF, and *TAILP is
+   updated to point where the next tuple pointer should be stored.  If
+   TTLP is not null, *TTLP is updated to reflect the minimum TTL.  If
+   STORE_CANON is true, the canonical name is stored as part of the
+   first address tuple being written.  */
 static enum nss_status
-gaih_getanswer_slice (const querybuf *answer, int anslen, const char *qname,
+gaih_getanswer_slice (unsigned char *packet, size_t packetlen,
-		      struct gaih_addrtuple ***patp,
+		      struct alloc_buffer *abuf,
-		      char **bufferp, size_t *buflenp,
+		      struct gaih_addrtuple ***tailp,
-		      int *errnop, int *h_errnop, int32_t *ttlp, int *firstp)
+		      int *errnop, int *h_errnop, int32_t *ttlp,
+		      bool store_canon)
 {
-  char *buffer = *bufferp;
+  struct ns_rr_cursor c;
-  size_t buflen = *buflenp;
+  if (!__ns_rr_cursor_init (&c, packet, packetlen))
-
-  struct gaih_addrtuple **pat = *patp;
-  const HEADER *hp = &answer->hdr;
-  int ancount = ntohs (hp->ancount);
-  int qdcount = ntohs (hp->qdcount);
-  const u_char *cp = answer->buf + HFIXEDSZ;
-  const u_char *end_of_message = answer->buf + anslen;
-  if (__glibc_unlikely (qdcount != 1))
     {
+      /* This should not happen because __res_context_query already
+	 perfroms response validation.  */
       *h_errnop = NO_RECOVERY;
       return NSS_STATUS_UNAVAIL;
     }
+  bool haveanswer = false; /* Set to true if at least one address.  */
+  uint16_t qtype = ns_rr_cursor_qtype (&c);
+  int ancount = ns_rr_cursor_ancount (&c);
+  const unsigned char *expected_name = ns_rr_cursor_qname (&c);
+  /* expected_name may be updated to point into this buffer.  */
+  unsigned char name_buffer[NS_MAXCDNAME];
 
-  u_char packtmp[NS_MAXCDNAME];
+  /* This is a pointer to a possibly-compressed name in the packet.
-  int n = __ns_name_unpack (answer->buf, end_of_message, cp,
+     Eventually it is equivalent to the canonical name.  If needed, it
-			    packtmp, sizeof packtmp);
+     is uncompressed and translated to text form when the first
-  /* We unpack the name to check it for validity.  But we do not need
+     address tuple is encountered.  */
-     it later.  */
+  const unsigned char *compressed_alias_name = expected_name;
-  if (n != -1 && __ns_name_ntop (packtmp, buffer, buflen) == -1)
-    {
-      if (__glibc_unlikely (errno == EMSGSIZE))
-	{
-	too_small:
-	  *errnop = ERANGE;
-	  *h_errnop = NETDB_INTERNAL;
-	  return NSS_STATUS_TRYAGAIN;
-	}
 
-      n = -1;
+  if (ancount == 0 || !__res_binary_hnok (compressed_alias_name))
-    }
-
-  if (__glibc_unlikely (n < 0))
-    {
-      *errnop = errno;
-      *h_errnop = NO_RECOVERY;
-      return NSS_STATUS_UNAVAIL;
-    }
-  if (__glibc_unlikely (__libc_res_hnok (buffer) == 0))
-    {
-      errno = EBADMSG;
-      *errnop = EBADMSG;
-      *h_errnop = NO_RECOVERY;
-      return NSS_STATUS_UNAVAIL;
-    }
-  cp += n + QFIXEDSZ;
-
-  int haveanswer = 0;
-  int had_error = 0;
-  char *canon = NULL;
-  char *h_name = NULL;
-  int h_namelen = 0;
-
-  if (ancount == 0)
     {
       *h_errnop = HOST_NOT_FOUND;
       return NSS_STATUS_NOTFOUND;
     }
 
-  while (ancount-- > 0 && cp < end_of_message && had_error == 0)
+  for (; ancount > -0; --ancount)
     {
-      n = __ns_name_unpack (answer->buf, end_of_message, cp,
+      struct ns_rr_wire rr;
-			    packtmp, sizeof packtmp);
+      if (!__ns_rr_cursor_next (&c, &rr))
-      if (n != -1 &&
-	  (h_namelen = __ns_name_ntop (packtmp, buffer, buflen)) == -1)
 	{
-	  if (__glibc_unlikely (errno == EMSGSIZE))
+	  *h_errnop = NO_RECOVERY;
-	    goto too_small;
+	  return NSS_STATUS_UNAVAIL;
-
-	  n = -1;
-	}
-      if (__glibc_unlikely (n < 0))
-	{
-	  ++had_error;
-	  continue;
-	}
-      if (*firstp && canon == NULL && __libc_res_hnok (buffer))
-	{
-	  h_name = buffer;
-	  buffer += h_namelen;
-	  buflen -= h_namelen;
 	}
 
-      cp += n;				/* name */
+      /* Update TTL for known record types.  */
+      if ((rr.rtype == T_CNAME || rr.rtype == qtype)
+	  && ttlp != NULL && *ttlp > rr.ttl)
+	*ttlp = rr.ttl;
 
-      if (__glibc_unlikely (cp + 10 > end_of_message))
+      if (rr.rtype == T_CNAME)
 	{
-	  ++had_error;
+	  /* NB: No check for owner name match, based on historic
-	  continue;
+	     precedent.  Record the CNAME target as the new expected
-	}
+	     name.  */
-
+	  int n = __ns_name_unpack (c.begin, c.end, rr.rdata,
-      uint16_t type;
+				    name_buffer, sizeof (name_buffer));
-      NS_GET16 (type, cp);
+	  if (n < 0)
-      uint16_t class;
-      NS_GET16 (class, cp);
-      int32_t ttl;
-      NS_GET32 (ttl, cp);
-      NS_GET16 (n, cp);		/* RDATA length.  */
-
-      if (end_of_message - cp < n)
-	{
-	  /* RDATA extends beyond the end of the packet.  */
-	  ++had_error;
-	  continue;
-	}
-
-      if (class != C_IN)
-	{
-	  cp += n;
-	  continue;
-	}
-
-      if (type == T_CNAME)
-	{
-	  char tbuf[MAXDNAME];
-
-	  /* A CNAME could also have a TTL entry.  */
-	  if (ttlp != NULL && ttl < *ttlp)
-	      *ttlp = ttl;
-
-	  n = __libc_dn_expand (answer->buf, end_of_message, cp,
-				tbuf, sizeof tbuf);
-	  if (__glibc_unlikely (n < 0))
 	    {
-	      ++had_error;
+	      *h_errnop = NO_RECOVERY;
-	      continue;
+	      return NSS_STATUS_UNAVAIL;
 	    }
-	  cp += n;
+	  expected_name = name_buffer;
-
+	  if (store_canon && __res_binary_hnok (name_buffer))
-	  if (*firstp && __libc_res_hnok (tbuf))
+	    /* This name can be used as a canonical name.  Do not
+	       translate to text form here to conserve buffer space.
+	       Point to the compressed name because name_buffer can be
+	       overwritten with an unusable name later.  */
+	    compressed_alias_name = rr.rdata;
+	}
+      else if (rr.rtype == qtype
+	       && __ns_samebinaryname (rr.rname, expected_name)
+	       && rr.rdlength == rrtype_to_rdata_length (qtype))
+	{
+	  struct gaih_addrtuple *ntup
+	    = alloc_buffer_alloc (abuf, struct gaih_addrtuple);
+	  /* Delay error reporting to the callers (they implement the
+	     ERANGE buffer resizing handshake).  */
+	  if (ntup != NULL)
 	    {
-	      /* Reclaim buffer space.  */
+	      ntup->next = NULL;
-	      if (h_name + h_namelen == buffer)
+	      if (store_canon && compressed_alias_name != NULL)
 		{
-		  buffer = h_name;
+		  /* This assumes that all the CNAME records come
-		  buflen += h_namelen;
+		     first.  Use MAXHOSTNAMELEN instead of
+		     NS_MAXCDNAME for additional length checking.
+		     However, these checks are not expected to fail
+		     because all size NS_MAXCDNAME names should into
+		     the hname buffer because no escaping is
+		     needed.  */
+		  char unsigned nbuf[NS_MAXCDNAME];
+		  char hname[MAXHOSTNAMELEN + 1];
+		  if (__ns_name_unpack (c.begin, c.end,
+					compressed_alias_name,
+					nbuf, sizeof (nbuf)) >= 0
+		      && __ns_name_ntop (nbuf, hname, sizeof (hname)) >= 0)
+		    /* Space checking is performed by the callers.  */
+		    ntup->name = alloc_buffer_copy_string (abuf, hname);
+		  store_canon = false;
 		}
+	      else
+		ntup->name = NULL;
+	      if (rr.rdlength == 4)
+		ntup->family = AF_INET;
+	      else
+		ntup->family = AF_INET6;
+	      memcpy (ntup->addr, rr.rdata, rr.rdlength);
+	      ntup->scopeid = 0;
 
-	      n = strlen (tbuf) + 1;
+	      /* Link in the new tuple, and update the tail pointer to
-	      if (__glibc_unlikely (n > buflen))
+		 point to its next field.  */
-		goto too_small;
+	      **tailp = ntup;
-	      if (__glibc_unlikely (n >= MAXHOSTNAMELEN))
+	      *tailp = &ntup->next;
-		{
-		  ++had_error;
-		  continue;
-		}
 
-	      canon = buffer;
+	      haveanswer = true;
-	      buffer = __mempcpy (buffer, tbuf, n);
-	      buflen -= n;
-	      h_namelen = 0;
-	    }
-	  continue;
-	}
-
-      /* Stop parsing if we encounter a record with incorrect RDATA
-	 length.  */
-      if (type == T_A || type == T_AAAA)
-	{
-	  if (n != rrtype_to_rdata_length (type))
-	    {
-	      ++had_error;
-	      continue;
 	    }
 	}
-      else
-	{
-	  /* Skip unknown records.  */
-	  cp += n;
-	  continue;
-	}
-
-      assert (type == T_A || type == T_AAAA);
-      if (*pat == NULL)
-	{
-	  uintptr_t pad = (-(uintptr_t) buffer
-			   % __alignof__ (struct gaih_addrtuple));
-	  buffer += pad;
-	  buflen = buflen > pad ? buflen - pad : 0;
-
-	  if (__glibc_unlikely (buflen < sizeof (struct gaih_addrtuple)))
-	    goto too_small;
-
-	  *pat = (struct gaih_addrtuple *) buffer;
-	  buffer += sizeof (struct gaih_addrtuple);
-	  buflen -= sizeof (struct gaih_addrtuple);
-	}
-
-      (*pat)->name = NULL;
-      (*pat)->next = NULL;
-
-      if (*firstp)
-	{
-	  /* We compose a single hostent out of the entire chain of
-	     entries, so the TTL of the hostent is essentially the lowest
-	     TTL in the chain.  */
-	  if (ttlp != NULL && ttl < *ttlp)
-	    *ttlp = ttl;
-
-	  (*pat)->name = canon ?: h_name;
-
-	  *firstp = 0;
-	}
-
-      (*pat)->family = type == T_A ? AF_INET : AF_INET6;
-      memcpy ((*pat)->addr, cp, n);
-      cp += n;
-      (*pat)->scopeid = 0;
-
-      pat = &((*pat)->next);
-
-      haveanswer = 1;
     }
 
   if (haveanswer)
     {
-      *patp = pat;
-      *bufferp = buffer;
-      *buflenp = buflen;
-
       *h_errnop = NETDB_SUCCESS;
       return NSS_STATUS_SUCCESS;
     }
-
+  else
-  /* Special case here: if the resolver sent a result but it only
-     contains a CNAME while we are looking for a T_A or T_AAAA record,
-     we fail with NOTFOUND instead of TRYAGAIN.  */
-  if (canon != NULL)
     {
+      /* Special case here: if the resolver sent a result but it only
+	 contains a CNAME while we are looking for a T_A or T_AAAA
+	 record, we fail with NOTFOUND.  */
       *h_errnop = HOST_NOT_FOUND;
       return NSS_STATUS_NOTFOUND;
     }
-
-  *h_errnop = NETDB_INTERNAL;
-  return NSS_STATUS_TRYAGAIN;
 }
 
 
 static enum nss_status
-gaih_getanswer (const querybuf *answer1, int anslen1, const querybuf *answer2,
+gaih_getanswer (unsigned char *packet1, size_t packet1len,
-		int anslen2, const char *qname,
+		unsigned char *packet2, size_t packet2len,
-		struct gaih_addrtuple **pat, char *buffer, size_t buflen,
+		struct alloc_buffer *abuf, struct gaih_addrtuple **pat,
 		int *errnop, int *h_errnop, int32_t *ttlp)
 {
-  int first = 1;
-
   enum nss_status status = NSS_STATUS_NOTFOUND;
 
   /* Combining the NSS status of two distinct queries requires some
@@ -1156,7 +1045,10 @@ gaih_getanswer (const querybuf *answer1, int anslen1, const querybuf *answer2,
      between TRYAGAIN (recoverable) and TRYAGAIN' (not-recoverable).
      A recoverable TRYAGAIN is almost always due to buffer size issues
      and returns ERANGE in errno and the caller is expected to retry
-     with a larger buffer.
+     with a larger buffer.  (The caller, _nss_dns_gethostbyname4_r,
+     ignores the return status if it detects that the result buffer
+     has been exhausted and generates a TRYAGAIN failure with an
+     ERANGE code.)
 
      Lastly, you may be tempted to make significant changes to the
      conditions in this code to bring about symmetry between responses.
@@ -1236,36 +1128,30 @@ gaih_getanswer (const querybuf *answer1, int anslen1, const querybuf *answer2,
 	 is a recoverable error we now return TRYAGIN even if the first
 	 response was SUCCESS.  */
 
-  if (anslen1 > 0)
+  if (packet1len > 0)
-    status = gaih_getanswer_slice(answer1, anslen1, qname,
-				  &pat, &buffer, &buflen,
-				  errnop, h_errnop, ttlp,
-				  &first);
-
-  if ((status == NSS_STATUS_SUCCESS || status == NSS_STATUS_NOTFOUND
-       || (status == NSS_STATUS_TRYAGAIN
-	   /* We want to look at the second answer in case of an
-	      NSS_STATUS_TRYAGAIN only if the error is non-recoverable, i.e.
-	      *h_errnop is NO_RECOVERY. If not, and if the failure was due to
-	      an insufficient buffer (ERANGE), then we need to drop the results
-	      and pass on the NSS_STATUS_TRYAGAIN to the caller so that it can
-	      repeat the query with a larger buffer.  */
-	   && (*errnop != ERANGE || *h_errnop == NO_RECOVERY)))
-      && answer2 != NULL && anslen2 > 0)
     {
-      enum nss_status status2 = gaih_getanswer_slice(answer2, anslen2, qname,
+      status = gaih_getanswer_slice (packet1, packet1len,
-						     &pat, &buffer, &buflen,
+				     abuf, &pat, errnop, h_errnop, ttlp, true);
-						     errnop, h_errnop, ttlp,
+      if (alloc_buffer_has_failed (abuf))
-						     &first);
+	/* Do not try parsing the second packet if a larger result
+	   buffer is needed.  The caller implements the resizing
+	   protocol because *abuf has been exhausted.  */
+	return NSS_STATUS_TRYAGAIN; /* Ignored by the caller.  */
+    }
+
+  if ((status == NSS_STATUS_SUCCESS || status == NSS_STATUS_NOTFOUND)
+      && packet2 != NULL && packet2len > 0)
+    {
+      enum nss_status status2
+	= gaih_getanswer_slice (packet2, packet2len,
+				abuf, &pat, errnop, h_errnop, ttlp,
+				/* Success means that data with a
+				   canonical name has already been
+				   stored.  Do not store the name again.  */
+				status != NSS_STATUS_SUCCESS);
       /* Use the second response status in some cases.  */
       if (status != NSS_STATUS_SUCCESS && status2 != NSS_STATUS_NOTFOUND)
 	status = status2;
-      /* Do not return a truncated second response (unless it was
-	 unavoidable e.g. unrecoverable TRYAGAIN).  */
-      if (status == NSS_STATUS_SUCCESS
-	  && (status2 == NSS_STATUS_TRYAGAIN
-	      && *errnop == ERANGE && *h_errnop != NO_RECOVERY))
-	status = NSS_STATUS_TRYAGAIN;
     }
 
   return status;
@@ -1273,18 +1159,13 @@ gaih_getanswer (const querybuf *answer1, int anslen1, const querybuf *answer2,
 
 /* Variant of gaih_getanswer without a second (AAAA) response.  */
 static enum nss_status
-gaih_getanswer_noaaaa (const querybuf *answer1, int anslen1, const char *qname,
+gaih_getanswer_noaaaa (unsigned char *packet, size_t packetlen,
-		       struct gaih_addrtuple **pat,
+		       struct alloc_buffer *abuf, struct gaih_addrtuple **pat,
-		       char *buffer, size_t buflen,
 		       int *errnop, int *h_errnop, int32_t *ttlp)
 {
-  int first = 1;
-
   enum nss_status status = NSS_STATUS_NOTFOUND;
-  if (anslen1 > 0)
+  if (packetlen > 0)
-    status = gaih_getanswer_slice (answer1, anslen1, qname,
+    status = gaih_getanswer_slice (packet, packetlen,
-				   &pat, &buffer, &buflen,
+				   abuf, &pat, errnop, h_errnop, ttlp, true);
-				   errnop, h_errnop, ttlp,
-				   &first);
   return status;
 }