nss_dns: Rewrite _nss_dns_gethostbyaddr2_r and getanswer_ptr

The simplification takes advantage of the split from getanswer_r.
It fixes various aliases issues, and optimizes NSS buffer usage.
The new DNS packet parsing helpers are used, too.

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

1 files changed, 102 insertions, 303 deletions

diff --git a/resolv/nss_dns/dns-host.c b/resolv/nss_dns/dns-host.c
index d384e1f..cd26399 100644
--- a/resolv/nss_dns/dns-host.c
+++ b/resolv/nss_dns/dns-host.c
@@ -69,6 +69,7 @@
  * --Copyright--
  */
 
+#include <alloc_buffer.h>
 #include <assert.h>
 #include <ctype.h>
 #include <errno.h>
@@ -116,10 +117,9 @@ static enum nss_status getanswer_r (struct resolv_context *ctx,
 				    struct hostent *result, char *buffer,
 				    size_t buflen, int *errnop, int *h_errnop,
 				    int map, int32_t *ttlp, char **canonp);
-static enum nss_status getanswer_ptr (const querybuf *answer, int anslen,
-				      const char *qname,
-				      struct hostent *result, char *buffer,
-				      size_t buflen, int *errnop,
+static enum nss_status getanswer_ptr (unsigned char *packet, size_t packetlen,
+				      struct alloc_buffer *abuf,
+				      char **hnamep, int *errnop,
 				      int *h_errnop, int32_t *ttlp);
 
 static enum nss_status gaih_getanswer (const querybuf *answer1, int anslen1,
@@ -456,36 +456,21 @@ _nss_dns_gethostbyaddr2_r (const void *addr, socklen_t len, int af,
   static const u_char tunnelled[] = { 0,0, 0,0, 0,0, 0,0, 0,0, 0,0 };
   static const u_char v6local[] = { 0,0, 0,1 };
   const u_char *uaddr = (const u_char *)addr;
-  struct host_data
-  {
-    char *aliases[MAX_NR_ALIASES];
-    unsigned char host_addr[16];	/* IPv4 or IPv6 */
-    char *h_addr_ptrs[MAX_NR_ADDRS + 1];
-    char linebuffer[0];
-  } *host_data = (struct host_data *) buffer;
-  union
-  {
-    querybuf *buf;
-    u_char *ptr;
-  } host_buffer;
-  querybuf *orig_host_buffer;
   char qbuf[MAXDNAME+1], *qp = NULL;
   size_t size;
   int n, status;
   int olderr = errno;
 
- uintptr_t pad = -(uintptr_t) buffer % __alignof__ (struct host_data);
- buffer += pad;
- buflen = buflen > pad ? buflen - pad : 0;
-
- if (__glibc_unlikely (buflen < sizeof (struct host_data)))
-   {
-     *errnop = ERANGE;
-     *h_errnop = NETDB_INTERNAL;
-     return NSS_STATUS_TRYAGAIN;
-   }
-
- host_data = (struct host_data *) buffer;
+  /* Prepare the allocation buffer.  Store the pointer array first, to
+     benefit from buffer alignment.  */
+  struct alloc_buffer abuf = alloc_buffer_create (buffer, buflen);
+  char **address_array = alloc_buffer_alloc_array (&abuf, char *, 2);
+  if (address_array == NULL)
+    {
+      *errnop = ERANGE;
+      *h_errnop = NETDB_INTERNAL;
+      return NSS_STATUS_TRYAGAIN;
+    }
 
   struct resolv_context *ctx = __resolv_context_get ();
   if (ctx == NULL)
@@ -529,8 +514,6 @@ _nss_dns_gethostbyaddr2_r (const void *addr, socklen_t len, int af,
       return NSS_STATUS_UNAVAIL;
     }
 
-  host_buffer.buf = orig_host_buffer = (querybuf *) alloca (1024);
-
   switch (af)
     {
     case AF_INET:
@@ -554,35 +537,52 @@ _nss_dns_gethostbyaddr2_r (const void *addr, socklen_t len, int af,
       break;
     }
 
-  n = __res_context_query (ctx, qbuf, C_IN, T_PTR, host_buffer.buf->buf,
-			   1024, &host_buffer.ptr, NULL, NULL, NULL, NULL);
+  unsigned char dns_packet_buffer[1024];
+  unsigned char *alt_dns_packet_buffer = dns_packet_buffer;
+  n = __res_context_query (ctx, qbuf, C_IN, T_PTR,
+			   dns_packet_buffer, sizeof (dns_packet_buffer),
+			   &alt_dns_packet_buffer,
+			   NULL, NULL, NULL, NULL);
   if (n < 0)
     {
       *h_errnop = h_errno;
       __set_errno (olderr);
-      if (host_buffer.buf != orig_host_buffer)
-	free (host_buffer.buf);
+      if (alt_dns_packet_buffer != dns_packet_buffer)
+	free (alt_dns_packet_buffer);
       __resolv_context_put (ctx);
       return errno == ECONNREFUSED ? NSS_STATUS_UNAVAIL : NSS_STATUS_NOTFOUND;
     }
 
-  status = getanswer_ptr (host_buffer.buf, n, qbuf, result,
-			  buffer, buflen, errnop, h_errnop, ttlp);
-  if (host_buffer.buf != orig_host_buffer)
-    free (host_buffer.buf);
+  status = getanswer_ptr (alt_dns_packet_buffer, n,
+			  &abuf, &result->h_name, errnop, h_errnop, ttlp);
+
+  if (alt_dns_packet_buffer != dns_packet_buffer)
+    free (alt_dns_packet_buffer);
+  __resolv_context_put (ctx);
+
   if (status != NSS_STATUS_SUCCESS)
-    {
-      __resolv_context_put (ctx);
-      return status;
-    }
+    return status;
 
+  /* result->h_name has already been set by getanswer_ptr.  */
   result->h_addrtype = af;
   result->h_length = len;
-  memcpy (host_data->host_addr, addr, len);
-  host_data->h_addr_ptrs[0] = (char *) host_data->host_addr;
-  host_data->h_addr_ptrs[1] = NULL;
+  /* Increase the alignment to 4, in case there are applications out
+     there that expect at least this level of address alignment.  */
+  address_array[0] = (char *) alloc_buffer_next (&abuf, uint32_t);
+  alloc_buffer_copy_bytes (&abuf, uaddr, len);
+  address_array[1] = NULL;
+
+  /* This check also covers allocation failure in getanswer_ptr.  */
+  if (alloc_buffer_has_failed (&abuf))
+    {
+      *errnop = ERANGE;
+      *h_errnop = NETDB_INTERNAL;
+      return NSS_STATUS_TRYAGAIN;
+    }
+  result->h_addr_list = address_array;
+  result->h_aliases = &address_array[1]; /* Points to NULL.  */
+
   *h_errnop = NETDB_SUCCESS;
-  __resolv_context_put (ctx);
   return NSS_STATUS_SUCCESS;
 }
 libc_hidden_def (_nss_dns_gethostbyaddr2_r)
@@ -961,287 +961,86 @@ getanswer_r (struct resolv_context *ctx,
 }
 
 static enum nss_status
-getanswer_ptr (const querybuf *answer, int anslen, const char *qname,
-	       struct hostent *result, char *buffer, size_t buflen,
+getanswer_ptr (unsigned char *packet, size_t packetlen,
+	       struct alloc_buffer *abuf, char **hnamep,
 	       int *errnop, int *h_errnop, int32_t *ttlp)
 {
-  struct host_data
-  {
-    char *aliases[MAX_NR_ALIASES];
-    unsigned char host_addr[16];	/* IPv4 or IPv6 */
-    char *h_addr_ptrs[0];
-  } *host_data;
-  int linebuflen;
-  const HEADER *hp;
-  const u_char *end_of_message, *cp;
-  int n, ancount, qdcount;
-  int haveanswer, had_error;
-  char *bp, **ap, **hap;
-  char tbuf[MAXDNAME];
-  const char *tname;
-  u_char packtmp[NS_MAXCDNAME];
-  uintptr_t pad = -(uintptr_t) buffer % __alignof__ (struct host_data);
-  buffer += pad;
-  buflen = buflen > pad ? buflen - pad : 0;
-  if (__glibc_unlikely (buflen < sizeof (struct host_data)))
-    {
-      /* The buffer is too small.  */
-    too_small:
-      *errnop = ERANGE;
-      *h_errnop = NETDB_INTERNAL;
-      return NSS_STATUS_TRYAGAIN;
-    }
-  host_data = (struct host_data *) buffer;
-  linebuflen = buflen - sizeof (struct host_data);
-  if (buflen - sizeof (struct host_data) != linebuflen)
-    linebuflen = INT_MAX;
-
-  tname = qname;
-  result->h_name = NULL;
-  end_of_message = answer->buf + anslen;
-
-  /*
-   * find first satisfactory answer
-   */
-  hp = &answer->hdr;
-  ancount = ntohs (hp->ancount);
-  qdcount = ntohs (hp->qdcount);
-  cp = answer->buf + HFIXEDSZ;
-  if (__glibc_unlikely (qdcount != 1))
-    {
-      *h_errnop = NO_RECOVERY;
-      return NSS_STATUS_UNAVAIL;
-    }
-  if (sizeof (struct host_data) + (ancount + 1) * sizeof (char *) >= buflen)
-    goto too_small;
-  bp = (char *) &host_data->h_addr_ptrs[ancount + 1];
-  linebuflen -= (ancount + 1) * sizeof (char *);
-
-  n = __ns_name_unpack (answer->buf, end_of_message, cp,
-			packtmp, sizeof packtmp);
-  if (n != -1 && __ns_name_ntop (packtmp, bp, linebuflen) == -1)
+  struct ns_rr_cursor c;
+  if (!__ns_rr_cursor_init (&c, packet, packetlen))
     {
-      if (__glibc_unlikely (errno == EMSGSIZE))
-	goto too_small;
-
-      n = -1;
-    }
-
-  if (__glibc_unlikely (n < 0))
-    {
-      *errnop = errno;
-      *h_errnop = NO_RECOVERY;
-      return NSS_STATUS_UNAVAIL;
-    }
-  if (__glibc_unlikely (__libc_res_dnok (bp) == 0))
-    {
-      errno = EBADMSG;
-      *errnop = EBADMSG;
+      /* This should not happen because __res_context_query already
+	 perfroms response validation.  */
       *h_errnop = NO_RECOVERY;
       return NSS_STATUS_UNAVAIL;
     }
-  cp += n + QFIXEDSZ;
+  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];
 
-  ap = host_data->aliases;
-  *ap = NULL;
-  result->h_aliases = host_data->aliases;
-  hap = host_data->h_addr_ptrs;
-  *hap = NULL;
-  result->h_addr_list = host_data->h_addr_ptrs;
-  haveanswer = 0;
-  had_error = 0;
-
-  while (ancount-- > 0 && cp < end_of_message && had_error == 0)
+  while (ancount > 0)
     {
-      int type, class;
-
-      n = __ns_name_unpack (answer->buf, end_of_message, cp,
-			    packtmp, sizeof packtmp);
-      if (n != -1 && __ns_name_ntop (packtmp, bp, linebuflen) == -1)
+      struct ns_rr_wire rr;
+      if (!__ns_rr_cursor_next (&c, &rr))
 	{
-	  if (__glibc_unlikely (errno == EMSGSIZE))
-	    goto too_small;
-
-	  n = -1;
-	}
-
-      if (__glibc_unlikely (n < 0 || __libc_res_dnok (bp) == 0))
-	{
-	  ++had_error;
-	  continue;
-	}
-      cp += n;				/* name */
-
-      if (__glibc_unlikely (cp + 10 > end_of_message))
-	{
-	  ++had_error;
-	  continue;
+	  *h_errnop = NO_RECOVERY;
+	  return NSS_STATUS_UNAVAIL;
 	}
 
-      NS_GET16 (type, cp);
-      NS_GET16 (class, cp);
-      int32_t ttl;
-      NS_GET32 (ttl, cp);
-      NS_GET16 (n, cp);		/* RDATA length.  */
+      /* Skip over records with the wrong class.  */
+      if (rr.rclass != C_IN)
+	continue;
 
-      if (end_of_message - cp < n)
-	{
-	  /* RDATA extends beyond the end of the packet.  */
-	  ++had_error;
-	  continue;
-	}
-
-      if (__glibc_unlikely (class != C_IN))
-	{
-	  /* XXX - debug? syslog? */
-	  cp += n;
-	  continue;			/* XXX - had_error++ ? */
-	}
+      /* Update TTL for known record types.  */
+      if ((rr.rtype == T_CNAME || rr.rtype == T_PTR)
+	  && ttlp != NULL && *ttlp > rr.ttl)
+	*ttlp = rr.ttl;
 
-      if (type == T_CNAME)
+      if (rr.rtype == T_CNAME)
 	{
-	  /* 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 || __libc_res_dnok (tbuf) == 0))
-	    {
-	      ++had_error;
-	      continue;
-	    }
-	  cp += n;
-	  /* Get canonical name.  */
-	  n = strlen (tbuf) + 1;   /* For the \0.  */
-	  if (__glibc_unlikely (n > linebuflen))
-	    goto too_small;
-	  if (__glibc_unlikely (n >= MAXHOSTNAMELEN))
+	  /* NB: No check for owner name match, based on historic
+	     precedent.  Record the CNAME target as the new expected
+	     name.  */
+	  int n = __ns_name_unpack (c.begin, c.end, rr.rdata,
+				    name_buffer, sizeof (name_buffer));
+	  if (n < 0)
 	    {
-	      ++had_error;
-	      continue;
+	      *h_errnop = NO_RECOVERY;
+	      return NSS_STATUS_UNAVAIL;
 	    }
-	  tname = bp;
-	  bp = __mempcpy (bp, tbuf, n);	/* Cannot overflow.  */
-	  linebuflen -= n;
-	  continue;
+	  expected_name = name_buffer;
 	}
-
-      switch (type)
+      else if (rr.rtype == T_PTR
+	       && __ns_samebinaryname (rr.rname, expected_name))
 	{
-	case T_PTR:
-	  if (__glibc_unlikely (__strcasecmp (tname, bp) != 0))
-	    {
-	      cp += n;
-	      continue;			/* XXX - had_error++ ? */
-	    }
-
-	  n = __ns_name_unpack (answer->buf, end_of_message, cp,
-				packtmp, sizeof packtmp);
-	  if (n != -1 && __ns_name_ntop (packtmp, bp, linebuflen) == -1)
-	    {
-	      if (__glibc_unlikely (errno == EMSGSIZE))
-		goto too_small;
-
-	      n = -1;
-	    }
-
-	  if (__glibc_unlikely (n < 0 || __libc_res_hnok (bp) == 0))
+	  /* Decompress the target of the PTR record.  This is the
+	     host name we are looking for.  We can only use it if it
+	     is syntactically valid.  Historically, only one host name
+	     is returned here.  If the recursive resolver performs DNS
+	     record rotation, the returned host name is essentially
+	     random, which is why multiple PTR records are rarely
+	     used.  Use MAXHOSTNAMELEN instead of NS_MAXCDNAME for
+	     additional length checking.  */
+	  char hname[MAXHOSTNAMELEN + 1];
+	  if (__ns_name_unpack (c.begin, c.end, rr.rdata,
+				name_buffer, sizeof (name_buffer)) < 0
+	      || !__res_binary_hnok (expected_name)
+	      || __ns_name_ntop (name_buffer, hname, sizeof (hname)) < 0)
 	    {
-	      ++had_error;
-	      break;
+	      *h_errnop = NO_RECOVERY;
+	      return NSS_STATUS_UNAVAIL;
 	    }
-	  if (ttlp != NULL && ttl < *ttlp)
-	      *ttlp = ttl;
-	  /* bind would put multiple PTR records as aliases, but we don't do
-	     that.  */
-	  result->h_name = bp;
-	  *h_errnop = NETDB_SUCCESS;
+	  /* Successful allocation is checked by the caller.  */
+	  *hnamep = alloc_buffer_copy_string (abuf, hname);
 	  return NSS_STATUS_SUCCESS;
-	case T_A:
-	case T_AAAA:
-	  if (__glibc_unlikely (__strcasecmp (result->h_name, bp) != 0))
-	    {
-	      cp += n;
-	      continue;			/* XXX - had_error++ ? */
-	    }
-
-	  /* Stop parsing at a record whose length is incorrect.  */
-	  if (n != rrtype_to_rdata_length (type))
-	    {
-	      ++had_error;
-	      break;
-	    }
-
-	  /* Skip records of the wrong type.  */
-	  if (n != result->h_length)
-	    {
-	      cp += n;
-	      continue;
-	    }
-	  if (!haveanswer)
-	    {
-	      int nn;
-
-	      /* 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;
-	      result->h_name = bp;
-	      nn = strlen (bp) + 1;	/* for the \0 */
-	      bp += nn;
-	      linebuflen -= nn;
-	    }
-
-	  /* Provide sufficient alignment for both address
-	     families.  */
-	  enum { align = 4 };
-	  _Static_assert ((align % __alignof__ (struct in_addr)) == 0,
-			  "struct in_addr alignment");
-	  _Static_assert ((align % __alignof__ (struct in6_addr)) == 0,
-			  "struct in6_addr alignment");
-	  {
-	    char *new_bp = PTR_ALIGN_UP (bp, align);
-	    linebuflen -= new_bp - bp;
-	    bp = new_bp;
-	  }
-
-	  if (__glibc_unlikely (n > linebuflen))
-	    goto too_small;
-	  bp = __mempcpy (*hap++ = bp, cp, n);
-	  cp += n;
-	  linebuflen -= n;
-	  break;
-	default:
-	  cp += n;
-	  continue;			/* XXX - had_error++ ? */
 	}
-      if (had_error == 0)
-	++haveanswer;
     }
 
-  if (haveanswer > 0)
-    {
-      *ap = NULL;
-      *hap = NULL;
-
-      if (result->h_name == NULL)
-	{
-	  n = strlen (qname) + 1;	/* For the \0.  */
-	  if (n > linebuflen)
-	    goto too_small;
-	  if (n >= MAXHOSTNAMELEN)
-	    goto no_recovery;
-	  result->h_name = bp;
-	  bp = __mempcpy (bp, qname, n);	/* Cannot overflow.  */
-	  linebuflen -= n;
-	}
+  /* No PTR record found.  */
+  if (ttlp != NULL)
+    /* No caching of negative responses.  */
+    *ttlp = 0;
 
-      *h_errnop = NETDB_SUCCESS;
-      return NSS_STATUS_SUCCESS;
-    }
- no_recovery:
   *h_errnop = NO_RECOVERY;
   *errnop = ENOENT;
   return NSS_STATUS_TRYAGAIN;