libc/inet/resolv: fix per thread res_state access

- use the provided __res_state() method instead of direct access
  to struct __res_state pointer &_res/*__resp

- change the __UCLIBC_HAS_TLS__ protected __res_state() implementation
  to the one where the comment 'When threaded, _res may be a per-thread
  variable.' indicates this should be used with threads/TLS enabled

Fixes the following segfaults with buildroot raspberrypi3_64_defconfig
(uclibc, -Os, Note: runs fine using the raspberrypi3_defconfig):

  $ /usr/sbin/ntpd -n -d
  1970-01-01T00:01:49 ntpd[249]: INIT: ntpd ntpsec-1.2.0 2021-11-03T20:39:50Z: Starting
  1970-01-01T00:01:49 ntpd[249]: INIT: Command line: /usr/sbin/ntpd -n -d
  1970-01-01T00:01:49 ntpd[249]: INIT: precision = 7.240 usec (-17)
  1970-01-01T00:01:49 ntpd[249]: INIT: successfully locked into RAM
  1970-01-01T00:01:49 ntpd[249]: CONFIG: readconfig: parsing file: /etc/ntp.conf
  1970-01-01T00:01:49 ntpd[249]: CONFIG: restrict nopeer ignored
  1970-01-01T00:01:49 ntpd[249]: INIT: Using SO_TIMESTAMPNS
  1970-01-01T00:01:49 ntpd[249]: IO: Listen and drop on 0 v6wildcard [::]:123
  1970-01-01T00:01:49 ntpd[249]: IO: Listen and drop on 1 v4wildcard 0.0.0.0:123
  1970-01-01T00:01:49 ntpd[249]: IO: Listen normally on 2 lo 127.0.0.1:123
  1970-01-01T00:01:49 ntpd[249]: IO: Listen normally on 3 eth0 172.16.0.30:123
  1970-01-01T00:01:49 ntpd[249]: IO: Listen normally on 4 lo [::1]:123
  1970-01-01T00:01:49 ntpd[249]: IO: Listen normally on 5 eth0 [fe80::ba27:ebff:fea6:340%2]:123
  1970-01-01T00:01:49 ntpd[249]: IO: Listening on routing socket on fd #22 for interface updates
  1970-01-01T00:01:50 ntpd[249]: SYNC: Found 10 servers, suggest minsane at least 3
  1970-01-01T00:01:50 ntpd[249]: INIT: MRU 10922 entries, 13 hash bits, 65536 bytes
  1970-01-01T00:01:50 ntpd[249]: statistics directory /var/NTP/ does not exist or is unwriteable, error No such file or directory
  1970-01-01T00:01:51 ntpd[249]: DNS: dns_probe: 0.pool.ntp.org, cast_flags:8, flags:101
  Segmentation fault (core dumped)

  $ ./host/bin/aarch64-buildroot-linux-uclibc-gdb ./build/ntpsec-1_2_0/build/main/ntpd/ntpd core
  Core was generated by `/usr/sbin/ntpd -n -d'.
  Program terminated with signal SIGSEGV, Segmentation fault.
  (gdb) where
  #0  0x0000007f8ff1f150 in res_sync_func () at libc/inet/resolv.c:3356
  #1  0x0000007f8ff1c468 in __open_nameservers () at libc/inet/resolv.c:949
  #2  0x0000007f8ff1b498 in __dns_lookup (name=0x55943c67f0 "0.pool.ntp.org",
      type=1, outpacket=0x7f8fe91c48, a=0x7f8fe91c08) at libc/inet/resolv.c:1134
  #3  0x0000007f8ff1d744 in __GI_gethostbyname_r (
      name=0x55943c67f0 "0.pool.ntp.org", result_buf=0x7f8fe92628,
      buf=0x7f8fe91d90 "", buflen=992, result=0x7f8fe92670,
      h_errnop=0x7f8fe92668) at libc/inet/resolv.c:1966
  #4  0x0000007f8ff1d9a0 in __GI_gethostbyname2_r (
      name=0x55943c67f0 "0.pool.ntp.org", family=2, result_buf=0x7f8fe92628,
      buf=0x7f8fe91d70 "0.pool.ntp.org", buflen=1024, result=0x7f8fe92670,
      h_errnop=0x7f8fe92668) at libc/inet/resolv.c:2065
  #5  0x0000007f8ff16924 in gaih_inet (name=0x55943c67f0 "0.pool.ntp.org",
      service=0x7f8fe92828, req=0x7f8fe92890, pai=0x7f8fe92838)
      at libc/inet/getaddrinfo.c:596
  #6  0x0000007f8ff17624 in __GI_getaddrinfo (
      name=0x55943c67f0 "0.pool.ntp.org",
      service=0x5582eb8acd "\377H\213D$\bL\211\367H\213\260\270",
      hints=0x7f8fe92890, pai=0x5582ee1bf8) at libc/inet/getaddrinfo.c:957
  #7  0x0000005582ea60f4 in _start ()
  (gdb) p _res
  $1 = {options = 0, nsaddr_list = {{sin_family = 0, sin_port = 0, sin_addr = {
          s_addr = 0}, sin_zero = "\000\000\000\000\000\000\000"}, {
        sin_family = 0, sin_port = 0, sin_addr = {s_addr = 0},
        sin_zero = "\000\000\000\000\000\000\000"}, {sin_family = 0,
        sin_port = 0, sin_addr = {s_addr = 0},
        sin_zero = "\000\000\000\000\000\000\000"}}, dnsrch = {0x0, 0x0, 0x0,
      0x0, 0x0, 0x0, 0x0}, nscount = 0 '\000', ndots = 0 '\000',
    retrans = 0 '\000', retry = 0 '\000', defdname = '\000' <repeats 255 times>,
    nsort = 0 '\000', pfcode = 0, id = 0, res_h_errno = 0, sort_list = {{addr = {
          s_addr = 0}, mask = 0}, {addr = {s_addr = 0}, mask = 0}, {addr = {
          s_addr = 0}, mask = 0}, {addr = {s_addr = 0}, mask = 0}, {addr = {
          s_addr = 0}, mask = 0}, {addr = {s_addr = 0}, mask = 0}, {addr = {
          s_addr = 0}, mask = 0}, {addr = {s_addr = 0}, mask = 0}, {addr = {
          s_addr = 0}, mask = 0}, {addr = {s_addr = 0}, mask = 0}}, _u = {
      _ext = {nsaddrs = {0x0, 0x0, 0x0}, nscount = 0 '\000', nstimes = {0, 0,
          0}, nssocks = {0, 0, 0}, nscount6 = 0, nsinit = 0}}}
  (gdb) p &_res
  $2 = (struct __res_state *) 0x7f8ff8fd98 <_res>
  (gdb) p rp
  $3 = (struct __res_state *) 0x7fffffffff

  And the following uclibc code at libc/inet/resolv.c:3356:

  3345 static void res_sync_func(void)
  3346 {
  3347         struct __res_state *rp = &(_res);
  3348         int n;
  3349
  3350         /* If we didn't get malloc failure earlier... */
  3351         if (__nameserver != (void*) &__local_nameserver) {
  3352                 /* TODO:
  3353                  * if (__nameservers < rp->nscount) - try to grow __nameserver[]?
  3354                  */
  3355 #ifdef __UCLIBC_HAS_IPV6__
  3356                 if (__nameservers > rp->_u._ext.nscount)
  3357                         __nameservers = rp->_u._ext.nscount;
  3358                 n = __nameservers;

  The special thing about ntpsec is the DNS lookup in an extra thread
  and/or the call to res_init(), see ntpsec-1_2_0/ntpd/ntp_dns.c:

   69         msyslog(LOG_INFO, "DNS: dns_probe: %s, cast_flags:%x, flags:%x%s",
   70                 hostname, pp->cast_flags, pp->cfg.flags, busy);
   71         if (NULL != active)     /* normally redundant */
   72                 return false;
   73
   74         active = pp;
   75
   76         sigfillset(&block_mask);
   77         pthread_sigmask(SIG_BLOCK, &block_mask, &saved_sig_mask);
   78         rc = pthread_create(&worker, NULL, dns_lookup, pp);

  and

  165 static void* dns_lookup(void* arg)
  166 {
  167         struct peer *pp = (struct peer *) arg;
  168         struct addrinfo hints;
  169
  170 #ifdef HAVE_SECCOMP_H
  171         setup_SIGSYS_trap();      /* enable trap for this thread */
  172 #endif
  173
  174 #ifdef HAVE_RES_INIT
  175         /* Reload DNS servers from /etc/resolv.conf in case DHCP has updated it.
  176          * We only need to do this occasionally, but it's not expensive
  177          * and simpler to do it every time than it is to figure out when
  178          * to do it.
  179          * This res_init() covers NTS too.
  180          */
  181         res_init();
  182 #endif
  183
  184         if (pp->cfg.flags & FLAG_NTS) {
  185 #ifndef DISABLE_NTS
  186                 nts_probe(pp);
  187 #endif
  188         } else {
  189                 ZERO(hints);
  190                 hints.ai_protocol = IPPROTO_UDP;
  191                 hints.ai_socktype = SOCK_DGRAM;
  192                 hints.ai_family = AF(&pp->srcadr);
  193                 gai_rc = getaddrinfo(pp->hostname, NTP_PORTA, &hints, &answer);
  194         }

  $ /usr/lib/uclibc-ng-test/test/inet/tst-res
  Segmentation fault (core dumped)

  $ ./host/bin/aarch64-buildroot-linux-uclibc-gdb ./build/uclibc-ng-test-0844445e7358eb10e716155b55b0fb23e88d644a/test/inet/tst-res core
  Core was generated by `/usr/lib/uclibc-ng-test/test/inet/tst-res'.
  Program terminated with signal SIGSEGV, Segmentation fault.
  (gdb) where
  #0  __GI___res_init () at libc/inet/resolv.c:3514
  #1  0x0000005591e507e4 in main (argc=<optimized out>, argv=<optimized out>)
      at tst-res.c:20

First reported here:
https://lore.kernel.org/buildroot/20211028230131.5f50d6e7@gmx.net/
https://www.mail-archive.com/devel@uclibc-ng.org/msg01085.html

Signed-off-by: Peter Seiderer <ps.report@gmx.net>

2 files changed, 3 insertions, 3 deletions

diff --git a/include/resolv.h b/include/resolv.h
index 485b8db98..2a82641a4 100644
--- a/include/resolv.h
+++ b/include/resolv.h
@@ -457,7 +457,7 @@ __END_DECLS
 #    ifndef NOT_IN_libc
 #     define __resp __libc_resp
 #    endif
-#    define _res (*__resp)
+#    define _res (*__res_state())
 extern __thread struct __res_state *__resp attribute_tls_model_ie;
 #   endif
 #  else
diff --git a/libc/inet/resolv.c b/libc/inet/resolv.c
index 8bbd7c7cd..cf170fba6 100644
--- a/libc/inet/resolv.c
+++ b/libc/inet/resolv.c
@@ -3344,7 +3344,7 @@ libc_hidden_def(dn_skipname)
 /* Will be called under __resolv_lock. */
 static void res_sync_func(void)
 {
-	struct __res_state *rp = &(_res);
+	struct __res_state *rp = __res_state();
 	int n;
 
 	/* If we didn't get malloc failure earlier... */
@@ -3896,7 +3896,7 @@ res_ninit(res_state statp)
 #endif /* L_res_init */
 
 #ifdef L_res_state
-# if defined __UCLIBC_HAS_TLS__
+# if !defined __UCLIBC_HAS_TLS__
 struct __res_state *
 __res_state (void)
 {