[compiler-rt] XFAIL test on iOS

[yln]

Summary

On iOS, when we longjmp() out of the signal handler, a subsequent call
to sigaltstack() still reports that we are executing on the signal
handler stack.
Tracking rdar://66789814

[robot]

Interesting. So iOS' C library doesn't support jumping out of signal handlers? From what I understand, that'd be a bug in iOS' C library.

LGTM but I should not have any power here (?)

[yln]

In D85677#2209856, @robot wrote:

Interesting. So iOS' C library doesn't support jumping out of signal handlers? From what I understand, that'd be a bug in iOS' C library.

Yes, I reported it as a bug, but it might be a while before it is fixed.

Just making sure you are good with my changes to the test. Thanks for the quick review! :)

compiler-rt/test/asan/TestCases/Posix/unpoison-alternate-stack.cpp

 // Tests that __asan_handle_no_return properly unpoisons the signal alternate
 // stack.
 
 // Don't optimize, otherwise the variables which create redzones might be
 // dropped.
 // RUN: %clangxx_asan -std=c++20 -fexceptions -O0 %s -o %t -pthread
 // RUN: %run %t
 
+// XFAIL: ios && !iossim
+
 #include <cassert>
 #include <cerrno>
 #include <csetjmp>
 #include <cstdint>
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
 
void testOnCurrentStack() {
 
   if (0 == setjmp(c.JmpBuf))
     poisonStackAndJump(c, [&] { longjmp(c.JmpBuf, 1); });
 
   assert(0 == __asan_region_is_poisoned(c.LeftRedzone,
                                         c.RightRedzone - c.LeftRedzone));
 }
 
-void signalHandler(int, siginfo_t *, void *) {
-  {
-    stack_t Stack;
-    sigaltstack(nullptr, &Stack);
-    assert(Stack.ss_flags == SS_ONSTACK);
-  }
+bool isOnSignalStack() {
+  return Stack.ss_flags == SS_ONSTACK;
 
+void signalHandler(int, siginfo_t *, void *) {
+  assert(isOnSignalStack());
+
   // test on signal alternate stack
   testOnCurrentStack();
 
   // test unpoisoning when jumping between stacks
   poisonStackAndJump(signalStack, [] { longjmp(defaultStack.JmpBuf, 1); });
 }
 
 void setSignalAlternateStack(void *AltStack) {
void *threadFun(void *AltStack) {
   // test on default stack again, but now the signal alternate stack is set
   testOnCurrentStack();
 
   // set up jump to test unpoisoning when jumping between stacks
   if (0 == setjmp(defaultStack.JmpBuf))
     // Test on signal alternate stack, via signalHandler
     poisonStackAndJump(defaultStack, [] { raise(SIGUSR1); });
 
+  assert(!isOnSignalStack());
+
   assert(0 == __asan_region_is_poisoned(
                   defaultStack.LeftRedzone,
                   defaultStack.RightRedzone - defaultStack.LeftRedzone));
 
   assert(0 == __asan_region_is_poisoned(
                   signalStack.LeftRedzone,
                   signalStack.RightRedzone - signalStack.LeftRedzone));