gnu: glibc: Add fix for CVE-2015-7547.

* gnu/packages/patches/glibc-CVE-2015-7547.patch: New file.
* gnu-system.am (dist_patch_DATA): Add it.
* gnu/packages/base.scm (glibc)[source]: Add patch.

1 files changed, 559 insertions, 0 deletions

diff --git a/gnu/packages/patches/glibc-CVE-2015-7547.patch b/gnu/packages/patches/glibc-CVE-2015-7547.patch
new file mode 100644
index 0000000000..9a0909af74
--- /dev/null
+++ b/gnu/packages/patches/glibc-CVE-2015-7547.patch
@@ -0,0 +1,559 @@
+Copied from Fedora:
+http://pkgs.fedoraproject.org/cgit/rpms/glibc.git/tree/glibc-CVE-2015-7547.patch?h=f23&id=9f1734eb6ce3257b788d6e9203572e8204c6c584
+
+Adapted to apply cleanly to glibc-2.22.
+
+Index: b/resolv/nss_dns/dns-host.c
+===================================================================
+--- a/resolv/nss_dns/dns-host.c
++++ b/resolv/nss_dns/dns-host.c
+@@ -1031,7 +1031,10 @@ gaih_getanswer_slice (const querybuf *an
+   int h_namelen = 0;
+ 
+   if (ancount == 0)
+-    return NSS_STATUS_NOTFOUND;
++    {
++      *h_errnop = HOST_NOT_FOUND;
++      return NSS_STATUS_NOTFOUND;
++    }
+ 
+   while (ancount-- > 0 && cp < end_of_message && had_error == 0)
+     {
+@@ -1208,7 +1211,14 @@ gaih_getanswer_slice (const querybuf *an
+   /* 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.  */
+-  return canon == NULL ? NSS_STATUS_TRYAGAIN : NSS_STATUS_NOTFOUND;
++  if (canon != NULL)
++    {
++      *h_errnop = HOST_NOT_FOUND;
++      return NSS_STATUS_NOTFOUND;
++    }
++
++  *h_errnop = NETDB_INTERNAL;
++  return NSS_STATUS_TRYAGAIN;
+ }
+ 
+ 
+@@ -1222,11 +1232,101 @@ gaih_getanswer (const querybuf *answer1,
+ 
+   enum nss_status status = NSS_STATUS_NOTFOUND;
+ 
++  /* Combining the NSS status of two distinct queries requires some
++     compromise and attention to symmetry (A or AAAA queries can be
++     returned in any order).  What follows is a breakdown of how this
++     code is expected to work and why. We discuss only SUCCESS,
++     TRYAGAIN, NOTFOUND and UNAVAIL, since they are the only returns
++     that apply (though RETURN and MERGE exist).  We make a distinction
++     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.
++
++     Lastly, you may be tempted to make significant changes to the
++     conditions in this code to bring about symmetry between responses.
++     Please don't change anything without due consideration for
++     expected application behaviour.  Some of the synthesized responses
++     aren't very well thought out and sometimes appear to imply that
++     IPv4 responses are always answer 1, and IPv6 responses are always
++     answer 2, but that's not true (see the implemetnation of send_dg
++     and send_vc to see response can arrive in any order, particlarly
++     for UDP). However, we expect it holds roughly enough of the time
++     that this code works, but certainly needs to be fixed to make this
++     a more robust implementation.
++
++     ----------------------------------------------
++     | Answer 1 Status /   | Synthesized | Reason |
++     | Answer 2 Status     | Status      |        |
++     |--------------------------------------------|
++     | SUCCESS/SUCCESS     | SUCCESS     | [1]    |
++     | SUCCESS/TRYAGAIN    | TRYAGAIN    | [5]    |
++     | SUCCESS/TRYAGAIN'   | SUCCESS     | [1]    |
++     | SUCCESS/NOTFOUND    | SUCCESS     | [1]    |
++     | SUCCESS/UNAVAIL     | SUCCESS     | [1]    |
++     | TRYAGAIN/SUCCESS    | TRYAGAIN    | [2]    |
++     | TRYAGAIN/TRYAGAIN   | TRYAGAIN    | [2]    |
++     | TRYAGAIN/TRYAGAIN'  | TRYAGAIN    | [2]    |
++     | TRYAGAIN/NOTFOUND   | TRYAGAIN    | [2]    |
++     | TRYAGAIN/UNAVAIL    | TRYAGAIN    | [2]    |
++     | TRYAGAIN'/SUCCESS   | SUCCESS     | [3]    |
++     | TRYAGAIN'/TRYAGAIN  | TRYAGAIN    | [3]    |
++     | TRYAGAIN'/TRYAGAIN' | TRYAGAIN'   | [3]    |
++     | TRYAGAIN'/NOTFOUND  | TRYAGAIN'   | [3]    |
++     | TRYAGAIN'/UNAVAIL   | UNAVAIL     | [3]    |
++     | NOTFOUND/SUCCESS    | SUCCESS     | [3]    |
++     | NOTFOUND/TRYAGAIN   | TRYAGAIN    | [3]    |
++     | NOTFOUND/TRYAGAIN'  | TRYAGAIN'   | [3]    |
++     | NOTFOUND/NOTFOUND   | NOTFOUND    | [3]    |
++     | NOTFOUND/UNAVAIL    | UNAVAIL     | [3]    |
++     | UNAVAIL/SUCCESS     | UNAVAIL     | [4]    |
++     | UNAVAIL/TRYAGAIN    | UNAVAIL     | [4]    |
++     | UNAVAIL/TRYAGAIN'   | UNAVAIL     | [4]    |
++     | UNAVAIL/NOTFOUND    | UNAVAIL     | [4]    |
++     | UNAVAIL/UNAVAIL     | UNAVAIL     | [4]    |
++     ----------------------------------------------
++
++     [1] If the first response is a success we return success.
++         This ignores the state of the second answer and in fact
++         incorrectly sets errno and h_errno to that of the second
++	 answer.  However because the response is a success we ignore
++	 *errnop and *h_errnop (though that means you touched errno on
++         success).  We are being conservative here and returning the
++         likely IPv4 response in the first answer as a success.
++
++     [2] If the first response is a recoverable TRYAGAIN we return
++	 that instead of looking at the second response.  The
++	 expectation here is that we have failed to get an IPv4 response
++	 and should retry both queries.
++
++     [3] If the first response was not a SUCCESS and the second
++	 response is not NOTFOUND (had a SUCCESS, need to TRYAGAIN,
++	 or failed entirely e.g. TRYAGAIN' and UNAVAIL) then use the
++	 result from the second response, otherwise the first responses
++	 status is used.  Again we have some odd side-effects when the
++	 second response is NOTFOUND because we overwrite *errnop and
++	 *h_errnop that means that a first answer of NOTFOUND might see
++	 its *errnop and *h_errnop values altered.  Whether it matters
++	 in practice that a first response NOTFOUND has the wrong
++	 *errnop and *h_errnop is undecided.
++
++     [4] If the first response is UNAVAIL we return that instead of
++	 looking at the second response.  The expectation here is that
++	 it will have failed similarly e.g. configuration failure.
++
++     [5] Testing this code is complicated by the fact that truncated
++	 second response buffers might be returned as SUCCESS if the
++	 first answer is a SUCCESS.  To fix this we add symmetry to
++	 TRYAGAIN with the second response.  If the second response
++	 is a recoverable error we now return TRYAGIN even if the first
++	 response was SUCCESS.  */
++
+   if (anslen1 > 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
+@@ -1242,8 +1342,15 @@ gaih_getanswer (const querybuf *answer1,
+ 						     &pat, &buffer, &buflen,
+ 						     errnop, h_errnop, ttlp,
+ 						     &first);
++      /* 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;
+Index: b/resolv/res_query.c
+===================================================================
+--- a/resolv/res_query.c
++++ b/resolv/res_query.c
+@@ -396,6 +396,7 @@ __libc_res_nsearch(res_state statp,
+ 		  {
+ 		    free (*answerp2);
+ 		    *answerp2 = NULL;
++		    *nanswerp2 = 0;
+ 		    *answerp2_malloced = 0;
+ 		  }
+ 	}
+@@ -447,6 +448,7 @@ __libc_res_nsearch(res_state statp,
+ 			  {
+ 			    free (*answerp2);
+ 			    *answerp2 = NULL;
++			    *nanswerp2 = 0;
+ 			    *answerp2_malloced = 0;
+ 			  }
+ 
+@@ -521,6 +523,7 @@ __libc_res_nsearch(res_state statp,
+ 	  {
+ 	    free (*answerp2);
+ 	    *answerp2 = NULL;
++	    *nanswerp2 = 0;
+ 	    *answerp2_malloced = 0;
+ 	  }
+ 	if (saved_herrno != -1)
+Index: b/resolv/res_send.c
+===================================================================
+--- a/resolv/res_send.c
++++ b/resolv/res_send.c
+@@ -1,3 +1,20 @@
++/* Copyright (C) 2016 Free Software Foundation, Inc.
++   This file is part of the GNU C Library.
++
++   The GNU C Library is free software; you can redistribute it and/or
++   modify it under the terms of the GNU Lesser General Public
++   License as published by the Free Software Foundation; either
++   version 2.1 of the License, or (at your option) any later version.
++
++   The GNU C Library is distributed in the hope that it will be useful,
++   but WITHOUT ANY WARRANTY; without even the implied warranty of
++   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
++   Lesser General Public License for more details.
++
++   You should have received a copy of the GNU Lesser General Public
++   License along with the GNU C Library; if not, see
++   <http://www.gnu.org/licenses/>.  */
++
+ /*
+  * Copyright (c) 1985, 1989, 1993
+  *    The Regents of the University of California.  All rights reserved.
+@@ -361,6 +378,8 @@ __libc_res_nsend(res_state statp, const
+ #ifdef USE_HOOKS
+ 	if (__glibc_unlikely (statp->qhook || statp->rhook))       {
+ 		if (anssiz < MAXPACKET && ansp) {
++			/* Always allocate MAXPACKET, callers expect
++			   this specific size.  */
+ 			u_char *buf = malloc (MAXPACKET);
+ 			if (buf == NULL)
+ 				return (-1);
+@@ -660,6 +679,77 @@ libresolv_hidden_def (res_nsend)
+ 
+ /* Private */
+ 
++/* The send_vc function is responsible for sending a DNS query over TCP
++   to the nameserver numbered NS from the res_state STATP i.e.
++   EXT(statp).nssocks[ns].  The function supports sending both IPv4 and
++   IPv6 queries at the same serially on the same socket.
++
++   Please note that for TCP there is no way to disable sending both
++   queries, unlike UDP, which honours RES_SNGLKUP and RES_SNGLKUPREOP
++   and sends the queries serially and waits for the result after each
++   sent query.  This implemetnation should be corrected to honour these
++   options.
++
++   Please also note that for TCP we send both queries over the same
++   socket one after another.  This technically violates best practice
++   since the server is allowed to read the first query, respond, and
++   then close the socket (to service another client).  If the server
++   does this, then the remaining second query in the socket data buffer
++   will cause the server to send the client an RST which will arrive
++   asynchronously and the client's OS will likely tear down the socket
++   receive buffer resulting in a potentially short read and lost
++   response data.  This will force the client to retry the query again,
++   and this process may repeat until all servers and connection resets
++   are exhausted and then the query will fail.  It's not known if this
++   happens with any frequency in real DNS server implementations.  This
++   implementation should be corrected to use two sockets by default for
++   parallel queries.
++
++   The query stored in BUF of BUFLEN length is sent first followed by
++   the query stored in BUF2 of BUFLEN2 length.  Queries are sent
++   serially on the same socket.
++
++   Answers to the query are stored firstly in *ANSP up to a max of
++   *ANSSIZP bytes.  If more than *ANSSIZP bytes are needed and ANSCP
++   is non-NULL (to indicate that modifying the answer buffer is allowed)
++   then malloc is used to allocate a new response buffer and ANSCP and
++   ANSP will both point to the new buffer.  If more than *ANSSIZP bytes
++   are needed but ANSCP is NULL, then as much of the response as
++   possible is read into the buffer, but the results will be truncated.
++   When truncation happens because of a small answer buffer the DNS
++   packets header feild TC will bet set to 1, indicating a truncated
++   message and the rest of the socket data will be read and discarded.
++
++   Answers to the query are stored secondly in *ANSP2 up to a max of
++   *ANSSIZP2 bytes, with the actual response length stored in
++   *RESPLEN2.  If more than *ANSSIZP bytes are needed and ANSP2
++   is non-NULL (required for a second query) then malloc is used to
++   allocate a new response buffer, *ANSSIZP2 is set to the new buffer
++   size and *ANSP2_MALLOCED is set to 1.
++
++   The ANSP2_MALLOCED argument will eventually be removed as the
++   change in buffer pointer can be used to detect the buffer has
++   changed and that the caller should use free on the new buffer.
++
++   Note that the answers may arrive in any order from the server and
++   therefore the first and second answer buffers may not correspond to
++   the first and second queries.
++
++   It is not supported to call this function with a non-NULL ANSP2
++   but a NULL ANSCP.  Put another way, you can call send_vc with a
++   single unmodifiable buffer or two modifiable buffers, but no other
++   combination is supported.
++
++   It is the caller's responsibility to free the malloc allocated
++   buffers by detecting that the pointers have changed from their
++   original values i.e. *ANSCP or *ANSP2 has changed.
++
++   If errors are encountered then *TERRNO is set to an appropriate
++   errno value and a zero result is returned for a recoverable error,
++   and a less-than zero result is returned for a non-recoverable error.
++
++   If no errors are encountered then *TERRNO is left unmodified and
++   a the length of the first response in bytes is returned.  */
+ static int
+ send_vc(res_state statp,
+ 	const u_char *buf, int buflen, const u_char *buf2, int buflen2,
+@@ -669,11 +759,7 @@ send_vc(res_state statp,
+ {
+ 	const HEADER *hp = (HEADER *) buf;
+ 	const HEADER *hp2 = (HEADER *) buf2;
+-	u_char *ans = *ansp;
+-	int orig_anssizp = *anssizp;
+-	// XXX REMOVE
+-	// int anssiz = *anssizp;
+-	HEADER *anhp = (HEADER *) ans;
++	HEADER *anhp = (HEADER *) *ansp;
+ 	struct sockaddr *nsap = get_nsaddr (statp, ns);
+ 	int truncating, connreset, n;
+ 	/* On some architectures compiler might emit a warning indicating
+@@ -766,6 +852,8 @@ send_vc(res_state statp,
+ 	 * Receive length & response
+ 	 */
+ 	int recvresp1 = 0;
++	/* Skip the second response if there is no second query.
++           To do that we mark the second response as received.  */
+ 	int recvresp2 = buf2 == NULL;
+ 	uint16_t rlen16;
+  read_len:
+@@ -802,40 +890,14 @@ send_vc(res_state statp,
+ 	u_char **thisansp;
+ 	int *thisresplenp;
+ 	if ((recvresp1 | recvresp2) == 0 || buf2 == NULL) {
++		/* We have not received any responses
++		   yet or we only have one response to
++		   receive.  */
+ 		thisanssizp = anssizp;
+ 		thisansp = anscp ?: ansp;
+ 		assert (anscp != NULL || ansp2 == NULL);
+ 		thisresplenp = &resplen;
+ 	} else {
+-		if (*anssizp != MAXPACKET) {
+-			/* No buffer allocated for the first
+-			   reply.  We can try to use the rest
+-			   of the user-provided buffer.  */
+-#if __GNUC_PREREQ (4, 7)
+-			DIAG_PUSH_NEEDS_COMMENT;
+-			DIAG_IGNORE_NEEDS_COMMENT (5, "-Wmaybe-uninitialized");
+-#endif
+-#if _STRING_ARCH_unaligned
+-			*anssizp2 = orig_anssizp - resplen;
+-			*ansp2 = *ansp + resplen;
+-#else
+-			int aligned_resplen
+-			  = ((resplen + __alignof__ (HEADER) - 1)
+-			     & ~(__alignof__ (HEADER) - 1));
+-			*anssizp2 = orig_anssizp - aligned_resplen;
+-			*ansp2 = *ansp + aligned_resplen;
+-#endif
+-#if __GNUC_PREREQ (4, 7)
+-			DIAG_POP_NEEDS_COMMENT;
+-#endif
+-		} else {
+-			/* The first reply did not fit into the
+-			   user-provided buffer.  Maybe the second
+-			   answer will.  */
+-			*anssizp2 = orig_anssizp;
+-			*ansp2 = *ansp;
+-		}
+-
+ 		thisanssizp = anssizp2;
+ 		thisansp = ansp2;
+ 		thisresplenp = resplen2;
+@@ -843,10 +905,14 @@ send_vc(res_state statp,
+ 	anhp = (HEADER *) *thisansp;
+ 
+ 	*thisresplenp = rlen;
+-	if (rlen > *thisanssizp) {
+-		/* Yes, we test ANSCP here.  If we have two buffers
+-		   both will be allocatable.  */
+-		if (__glibc_likely (anscp != NULL))       {
++	/* Is the answer buffer too small?  */
++	if (*thisanssizp < rlen) {
++		/* If the current buffer is non-NULL and it's not
++		   pointing at the static user-supplied buffer then
++		   we can reallocate it.  */
++		if (thisansp != NULL && thisansp != ansp) {
++			/* Always allocate MAXPACKET, callers expect
++			   this specific size.  */
+ 			u_char *newp = malloc (MAXPACKET);
+ 			if (newp == NULL) {
+ 				*terrno = ENOMEM;
+@@ -858,6 +924,9 @@ send_vc(res_state statp,
+ 			if (thisansp == ansp2)
+ 			  *ansp2_malloced = 1;
+ 			anhp = (HEADER *) newp;
++			/* A uint16_t can't be larger than MAXPACKET
++			   thus it's safe to allocate MAXPACKET but
++			   read RLEN bytes instead.  */
+ 			len = rlen;
+ 		} else {
+ 			Dprint(statp->options & RES_DEBUG,
+@@ -1021,6 +1090,66 @@ reopen (res_state statp, int *terrno, in
+ 	return 1;
+ }
+ 
++/* The send_dg function is responsible for sending a DNS query over UDP
++   to the nameserver numbered NS from the res_state STATP i.e.
++   EXT(statp).nssocks[ns].  The function supports IPv4 and IPv6 queries
++   along with the ability to send the query in parallel for both stacks
++   (default) or serially (RES_SINGLKUP).  It also supports serial lookup
++   with a close and reopen of the socket used to talk to the server
++   (RES_SNGLKUPREOP) to work around broken name servers.
++
++   The query stored in BUF of BUFLEN length is sent first followed by
++   the query stored in BUF2 of BUFLEN2 length.  Queries are sent
++   in parallel (default) or serially (RES_SINGLKUP or RES_SNGLKUPREOP).
++
++   Answers to the query are stored firstly in *ANSP up to a max of
++   *ANSSIZP bytes.  If more than *ANSSIZP bytes are needed and ANSCP
++   is non-NULL (to indicate that modifying the answer buffer is allowed)
++   then malloc is used to allocate a new response buffer and ANSCP and
++   ANSP will both point to the new buffer.  If more than *ANSSIZP bytes
++   are needed but ANSCP is NULL, then as much of the response as
++   possible is read into the buffer, but the results will be truncated.
++   When truncation happens because of a small answer buffer the DNS
++   packets header feild TC will bet set to 1, indicating a truncated
++   message, while the rest of the UDP packet is discarded.
++
++   Answers to the query are stored secondly in *ANSP2 up to a max of
++   *ANSSIZP2 bytes, with the actual response length stored in
++   *RESPLEN2.  If more than *ANSSIZP bytes are needed and ANSP2
++   is non-NULL (required for a second query) then malloc is used to
++   allocate a new response buffer, *ANSSIZP2 is set to the new buffer
++   size and *ANSP2_MALLOCED is set to 1.
++
++   The ANSP2_MALLOCED argument will eventually be removed as the
++   change in buffer pointer can be used to detect the buffer has
++   changed and that the caller should use free on the new buffer.
++
++   Note that the answers may arrive in any order from the server and
++   therefore the first and second answer buffers may not correspond to
++   the first and second queries.
++
++   It is not supported to call this function with a non-NULL ANSP2
++   but a NULL ANSCP.  Put another way, you can call send_vc with a
++   single unmodifiable buffer or two modifiable buffers, but no other
++   combination is supported.
++
++   It is the caller's responsibility to free the malloc allocated
++   buffers by detecting that the pointers have changed from their
++   original values i.e. *ANSCP or *ANSP2 has changed.
++
++   If an answer is truncated because of UDP datagram DNS limits then
++   *V_CIRCUIT is set to 1 and the return value non-zero to indicate to
++   the caller to retry with TCP.  The value *GOTSOMEWHERE is set to 1
++   if any progress was made reading a response from the nameserver and
++   is used by the caller to distinguish between ECONNREFUSED and
++   ETIMEDOUT (the latter if *GOTSOMEWHERE is 1).
++
++   If errors are encountered then *TERRNO is set to an appropriate
++   errno value and a zero result is returned for a recoverable error,
++   and a less-than zero result is returned for a non-recoverable error.
++
++   If no errors are encountered then *TERRNO is left unmodified and
++   a the length of the first response in bytes is returned.  */
+ static int
+ send_dg(res_state statp,
+ 	const u_char *buf, int buflen, const u_char *buf2, int buflen2,
+@@ -1030,8 +1159,6 @@ send_dg(res_state statp,
+ {
+ 	const HEADER *hp = (HEADER *) buf;
+ 	const HEADER *hp2 = (HEADER *) buf2;
+-	u_char *ans = *ansp;
+-	int orig_anssizp = *anssizp;
+ 	struct timespec now, timeout, finish;
+ 	struct pollfd pfd[1];
+ 	int ptimeout;
+@@ -1064,6 +1191,8 @@ send_dg(res_state statp,
+ 	int need_recompute = 0;
+ 	int nwritten = 0;
+ 	int recvresp1 = 0;
++	/* Skip the second response if there is no second query.
++           To do that we mark the second response as received.  */
+ 	int recvresp2 = buf2 == NULL;
+ 	pfd[0].fd = EXT(statp).nssocks[ns];
+ 	pfd[0].events = POLLOUT;
+@@ -1227,55 +1356,56 @@ send_dg(res_state statp,
+ 		int *thisresplenp;
+ 
+ 		if ((recvresp1 | recvresp2) == 0 || buf2 == NULL) {
++			/* We have not received any responses
++			   yet or we only have one response to
++			   receive.  */
+ 			thisanssizp = anssizp;
+ 			thisansp = anscp ?: ansp;
+ 			assert (anscp != NULL || ansp2 == NULL);
+ 			thisresplenp = &resplen;
+ 		} else {
+-			if (*anssizp != MAXPACKET) {
+-				/* No buffer allocated for the first
+-				   reply.  We can try to use the rest
+-				   of the user-provided buffer.  */
+-#if _STRING_ARCH_unaligned
+-				*anssizp2 = orig_anssizp - resplen;
+-				*ansp2 = *ansp + resplen;
+-#else
+-				int aligned_resplen
+-				  = ((resplen + __alignof__ (HEADER) - 1)
+-				     & ~(__alignof__ (HEADER) - 1));
+-				*anssizp2 = orig_anssizp - aligned_resplen;
+-				*ansp2 = *ansp + aligned_resplen;
+-#endif
+-			} else {
+-				/* The first reply did not fit into the
+-				   user-provided buffer.  Maybe the second
+-				   answer will.  */
+-				*anssizp2 = orig_anssizp;
+-				*ansp2 = *ansp;
+-			}
+-
+ 			thisanssizp = anssizp2;
+ 			thisansp = ansp2;
+ 			thisresplenp = resplen2;
+ 		}
+ 
+ 		if (*thisanssizp < MAXPACKET
+-		    /* Yes, we test ANSCP here.  If we have two buffers
+-		       both will be allocatable.  */
+-		    && anscp
++		    /* If the current buffer is non-NULL and it's not
++		       pointing at the static user-supplied buffer then
++		       we can reallocate it.  */
++		    && (thisansp != NULL && thisansp != ansp)
+ #ifdef FIONREAD
++		    /* Is the size too small?  */
+ 		    && (ioctl (pfd[0].fd, FIONREAD, thisresplenp) < 0
+ 			|| *thisanssizp < *thisresplenp)
+ #endif
+                     ) {
++			/* Always allocate MAXPACKET, callers expect
++			   this specific size.  */
+ 			u_char *newp = malloc (MAXPACKET);
+ 			if (newp != NULL) {
+-				*anssizp = MAXPACKET;
+-				*thisansp = ans = newp;
++				*thisanssizp = MAXPACKET;
++				*thisansp = newp;
+ 				if (thisansp == ansp2)
+ 				  *ansp2_malloced = 1;
+ 			}
+ 		}
++		/* We could end up with truncation if anscp was NULL
++		   (not allowed to change caller's buffer) and the
++		   response buffer size is too small.  This isn't a
++		   reliable way to detect truncation because the ioctl
++		   may be an inaccurate report of the UDP message size.
++		   Therefore we use this only to issue debug output.
++		   To do truncation accurately with UDP we need
++		   MSG_TRUNC which is only available on Linux.  We
++		   can abstract out the Linux-specific feature in the
++		   future to detect truncation.  */
++		if (__glibc_unlikely (*thisanssizp < *thisresplenp)) {
++			Dprint(statp->options & RES_DEBUG,
++			       (stdout, ";; response may be truncated (UDP)\n")
++			);
++		}
++
+ 		HEADER *anhp = (HEADER *) *thisansp;
+ 		socklen_t fromlen = sizeof(struct sockaddr_in6);
+ 		assert (sizeof(from) <= fromlen);