[DFSan] Add custom wrapper for sigaltstack.

[morehouse]

Summary

The wrapper clears shadow for old_ss.

compiler-rt/lib/dfsan/dfsan_custom.cpp

int __dfsw_sigaction(int signum, const struct sigaction *act,
   if (oldact) {
     dfsan_set_label(0, oldact, sizeof(struct sigaction));
   }
   *ret_label = 0;
   return ret;
 }
 
 SANITIZER_INTERFACE_ATTRIBUTE
+int __dfsw_sigaltstack(const stack_t *ss, stack_t *old_ss, dfsan_label ss_label,
+                       dfsan_label old_ss_label, dfsan_label *ret_label) {
+  int ret = sigaltstack(ss, old_ss);
+  if (ret != -1 && old_ss)
+    dfsan_set_label(0, old_ss, sizeof(*old_ss));
+  *ret_label = 0;
+  return ret;
+}
+
+SANITIZER_INTERFACE_ATTRIBUTE
 int __dfsw_gettimeofday(struct timeval *tv, struct timezone *tz,
                         dfsan_label tv_label, dfsan_label tz_label,
                         dfsan_label *ret_label) {
   int ret = gettimeofday(tv, tz);
   if (tv) {
     dfsan_set_label(0, tv, sizeof(struct timeval));
   }
   if (tz) {

compiler-rt/lib/dfsan/done_abilist.txt

 fun:getrlimit=custom
 fun:getrusage=custom
 fun:getsockname=custom
 fun:getsockopt=custom
 fun:nanosleep=custom
 fun:pread=custom
 fun:read=custom
 fun:recvmsg=custom
+fun:sigaltstack=custom
 fun:socketpair=custom
 fun:stat=custom
 fun:time=custom
 
 # Functions that produce an output that depend on the input (propagate the
 # shadow manually).
 fun:ctime_r=custom
 fun:inet_pton=custom

compiler-rt/test/dfsan/custom.cpp

 void test_sigaction() {
   struct sigaction oldact;
   dfsan_set_label(j_label, &oldact, 1);
   int ret = sigaction(SIGUSR1, NULL, &oldact);
   assert(ret == 0);
   ASSERT_READ_ZERO_LABEL(&oldact, sizeof(oldact));
 }
 
+void test_sigaltstack() {
+  stack_t old_altstack = {};
+  dfsan_set_label(j_label, &old_altstack, sizeof(old_altstack));
+  int ret = sigaltstack(NULL, &old_altstack);
+  assert(ret == 0);
+  ASSERT_ZERO_LABEL(ret);
+  ASSERT_READ_ZERO_LABEL(&old_altstack, sizeof(old_altstack));
+}
+
 void test_gettimeofday() {
   struct timeval tv;
   struct timezone tz;
   dfsan_set_label(i_label, &tv, sizeof(tv));
   dfsan_set_label(j_label, &tz, sizeof(tz));
   int ret = gettimeofday(&tv, &tz);
   assert(ret == 0);
   ASSERT_READ_ZERO_LABEL(&tv, sizeof(tv));
#endif
   test_poll();
   test_pread();
   test_pthread_create();
   test_read();
   test_recvmsg();
   test_sched_getaffinity();
   test_select();
   test_sigaction();
+  test_sigaltstack();
   test_sigemptyset();
   test_snprintf();
   test_socketpair();
   test_sprintf();
   test_stat();
   test_strcasecmp();
   test_strchr();
   test_strcmp();