[sanitizer] When setting up shadow memory on iOS, fix handling the return value of task_info on older OS versions

[kubamracek]

Summary

task_vm_info is a "revisioned" structure, new OS versions add fields to the end, and compatibility is based on the reported size. On older OS versions, min_address/max_address is not filled back. Let's handle that case. Unfortunately, we can't really write a test (as the failure only happens when on a specific OS version).

[delcypher]

lib/sanitizer_common/sanitizer_mac.cc

893 ↗

(On Diff #159102)

It looks like you're relying on task_vm_info being initialised to zero because presumably on older platforms task_info(...) will use a different task_vm_info layout that probably has less fields.
Given that task_info doesn't take the size of task_vm_info there's no way it could to set the other fields.

It looks like that's what = {}; does but it wasn't immediately obvious to me that this was calling a synthesised constructor that initialized everything to zero. Could you add a comment about zero initialization?

Other than that LGTM.

compiler-rt/trunk/lib/sanitizer_common/sanitizer_mac.cc

struct __sanitizer_task_vm_info {
   mach_vm_size_t phys_footprint;
   mach_vm_address_t min_address;
   mach_vm_address_t max_address;
 };
 #define __SANITIZER_TASK_VM_INFO_COUNT ((mach_msg_type_number_t) \
     (sizeof(__sanitizer_task_vm_info) / sizeof(natural_t)))
 
 uptr GetTaskInfoMaxAddress() {
-  __sanitizer_task_vm_info vm_info = {};
+  __sanitizer_task_vm_info vm_info = {} /* zero initialize */;
   mach_msg_type_number_t count = __SANITIZER_TASK_VM_INFO_COUNT;
   int err = task_info(mach_task_self(), TASK_VM_INFO, (int *)&vm_info, &count);
-  if (err == 0) {
+  if (err == 0 && vm_info.max_address != 0) {
     return vm_info.max_address - 1;
   } else {
     // xnu cannot provide vm address limit
     return 0x200000000 - 1;
   }
 }
 #endif