[sanitizer] Use TASK_VM_INFO to get the maximum VM address on iOS/AArch64

[kubamracek]

Summary

We currently hardcode the maximum VM address on iOS/AArch64, which is not really correct and this value changes between device configurations. Let's use TASK_VM_INFO to retrieve the maximum VM address dynamically.

The patch manually declares some constants, which would normally be provided in a header file by the SDK, but since this API is only declared in a recent SDK and we want to continue supporting building against older SDKs, the patch declares the constants manually.

[kubamracek]

This depends on https://reviews.llvm.org/D35031.

[eugenis]

lib/sanitizer_common/sanitizer_mac.cc

813

Why use pointer cast, and not just load an integer_t and static_cast to uptr? Are they always the same width?

[filcab]

lib/sanitizer_common/sanitizer_mac.cc

809

Shouldn't this be a task_vm_info_data_t? Or is that not available?
As far as I can tell, integer_t are int, and we're not guaranteed to have this array properly aligned for that uptr load.

If it's not available, I'm ok with a stub somewhere, but I'd make this either a properly-sized and aligned struct, or a uptr array (proper alignment, AFAICT) and cast its pointer when calling task_info.

831

Why not static uptr max_vm = GetTaskInfoMaxAddress();?

[kubamracek]

lib/sanitizer_common/sanitizer_mac.cc

809

task_vm_info_data_t can't be used because we need to access a field that's only available in a new version of the SDK, and we still want to be able to build with the older SDKs.

You're right about the int/integer_t/uptr confusion. The new version should be more clear.

lib/sanitizer_common/sanitizer_mac.cc

void MaybeReexec() {
   if (new_env_pos == new_env + env_name_len + 1) new_env = NULL;
   LeakyResetEnv(kDyldInsertLibraries, new_env);
 }
 #endif  // SANITIZER_GO
 
 char **GetArgv() {
   return *_NSGetArgv();
 }
   
+#if defined(__aarch64__) && SANITIZER_IOS && !SANITIZER_IOSSIM
+// The task_vm_info struct is normally provided by the macOS SDK, but we need
+// fields only available in 10.12+. Declare the struct manually to be able to
+// build against older SDKs.
+struct __sanitizer_task_vm_info {
+  uptr _unused[(SANITIZER_WORDSIZE == 32) ? 20 : 19];
+  uptr min_address;
+  uptr max_address;
+};
+
+uptr GetTaskInfoMaxAddress() {
+  __sanitizer_task_vm_info vm_info = {{0}, 0, 0};
+  mach_msg_type_number_t count = sizeof(vm_info) / sizeof(int);
+  int err = task_info(mach_task_self(), TASK_VM_INFO, (int *)&vm_info, &count);
+  if (err == 0) {
+    return vm_info.max_address;
+  } else {
+    // xnu cannot provide vm address limit
+    return 0x200000000 - 1;
+  }
+}
+#endif
+
 uptr GetMaxVirtualAddress() {
 #if SANITIZER_WORDSIZE == 64
 # if defined(__aarch64__) && SANITIZER_IOS && !SANITIZER_IOSSIM
-  // Ideally, we would derive the upper bound from MACH_VM_MAX_ADDRESS. The
-  // upper bound can change depending on the device.
-  return 0x200000000 - 1;
+  // Get the maximum VM address
+  static uptr max_vm = GetTaskInfoMaxAddress();
+  CHECK(max_vm);
+  return max_vm;
 # else
   return (1ULL << 47) - 1;  // 0x00007fffffffffffUL;
 # endif
 #else  // SANITIZER_WORDSIZE == 32
   return (1ULL << 32) - 1;  // 0xffffffff;
 #endif  // SANITIZER_WORDSIZE
 }