resolv: Test case for accepting mismatching, corrupted packets

The test skeleton was auto-generated.  I think this is fine because
the harness is so specific to glibc.

Assisted-by: LLM
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

resolv/Makefile

@@ -123,6 +123,7 @@ tests += \
   tst-resolv-noaaaa \
   tst-resolv-noaaaa-vc \
   tst-resolv-nondecimal \
+  tst-resolv-querymatch-short \
   tst-resolv-res_init-failure \
   tst-resolv-res_init-multi \
   tst-resolv-search \
@@ -326,6 +327,8 @@ $(objpfx)tst-resolv-no-search: $(objpfx)libresolv.so $(shared-thread-library)
 $(objpfx)tst-resolv-noaaaa: $(objpfx)libresolv.so $(shared-thread-library)
 $(objpfx)tst-resolv-noaaaa-vc: $(objpfx)libresolv.so $(shared-thread-library)
 $(objpfx)tst-resolv-nondecimal: $(objpfx)libresolv.so $(shared-thread-library)
+$(objpfx)tst-resolv-querymatch-short: $(objpfx)libresolv.so \
+  $(shared-thread-library)
 $(objpfx)tst-resolv-qtypes: $(objpfx)libresolv.so $(shared-thread-library)
 $(objpfx)tst-resolv-rotate: $(objpfx)libresolv.so $(shared-thread-library)
 $(objpfx)tst-resolv-search: $(objpfx)libresolv.so $(shared-thread-library)

resolv/tst-resolv-querymatch-short.c

@@ -0,0 +1,109 @@
+/* Test res_queriesmatch buffer handling (bug 34345, bug 34346).
+   Copyright (C) 2026 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
+   <https://www.gnu.org/licenses/>.  */
+
+#include <netdb.h>
+#include <resolv.h>
+#include <support/check.h>
+#include <support/check_nss.h>
+#include <support/resolv_test.h>
+
+static void
+response (const struct resolv_response_context *ctx,
+          struct resolv_response_builder *b,
+          const char *qname, uint16_t qclass, uint16_t qtype)
+{
+  switch (ctx->server_index)
+    {
+    case 0:
+      {
+        struct resolv_response_flags flags = { .rcode = 3 }; /* NXDOMAIN.  */
+        resolv_response_init (b, flags);
+        resolv_response_add_question (b, qname, qclass, qtype);
+
+        /* Cause a mismatch in the transaction ID.  */
+        *resolv_response_buffer (b) ^= 1;
+      }
+      break;
+
+    case 1:
+      {
+        struct resolv_response_flags flags = { .rcode = 3 }; /* NXDOMAIN.  */
+        resolv_response_init (b, flags);
+        resolv_response_add_question (b, qname, qclass, qtype);
+
+        /* Truncate the packet.  If bug 34346 is present, this
+           response will be accepted because the final byte (which is
+           overread) has the expected value, carried over from the
+           previous response.  With bug 34345, the response is
+           accepted (as NXDOMAIN) because the packet is corrupt.  */
+        resolv_response_set_buffer (b,
+                                  resolv_response_buffer (b),
+                                  resolv_response_length (b) - 1);
+
+      }
+      break;
+
+    case 2:
+      {
+        /* Finally, provide a valid response.  With either bug
+           present, this is never reached because the stub resolver
+           uses the response from the second server above.  */
+        resolv_response_init (b, (struct resolv_response_flags) {});
+        resolv_response_add_question (b, qname, qclass, qtype);
+        resolv_response_section (b, ns_s_an);
+        resolv_response_open_record (b, qname, qclass, qtype, 0);
+        char ipv4[4] = { 192, 0, 2, 17 };
+        resolv_response_add_data (b, &ipv4, sizeof (ipv4));
+        resolv_response_close_record (b);
+      }
+      break;
+    }
+}
+
+static int
+do_test (void)
+{
+  struct resolv_test *aux = resolv_test_start
+    ((struct resolv_redirect_config)
+     {
+       .response_callback = response,
+     });
+
+  /* Reduce test run time.  The test hits multiple timeouts as it
+     switches between name server.  */
+  _res.retrans = 1;
+  _res.retry = 1;
+
+  struct addrinfo hints =
+    {
+      .ai_family = AF_INET,
+      .ai_socktype = SOCK_STREAM,
+    };
+  struct addrinfo *ai;
+  int ret = getaddrinfo ("www.example", "80", &hints, &ai);
+  check_addrinfo ("www.example", ai, ret,
+                  "address: STREAM/TCP 192.0.2.17 80\n");
+  if (ret == 0)
+    freeaddrinfo (ai);
+
+  resolv_test_end (aux);
+
+  return 0;
+}
+
+#include <support/test-driver.c>

support/resolv_test.c

@@ -441,8 +441,14 @@ resolv_response_set_buffer (struct resolv_response_builder *b,
   if (length > max_response_length)
     FAIL_EXIT1 ("resolv_response_set_buffer: length %zu exceeds maximum %d",
                 length, max_response_length);
+  if (b->current_rdata_offset != 0)
+    FAIL_EXIT1 ("resolv_response_set_buffer: called with pending RDATA");
   memmove (b->buffer, data, length);
   b->offset = length;
+
+  /* The cached compression offsets are likely invalid now.  */
+  tdestroy (b->compression_offsets, free);
+  b->compression_offsets = NULL;
 }
 
 struct resolv_response_builder *