[compiler-rt] Change GetMaxUserVirtualAddress to invoke syscall

LSan was recently refactored to call GetMaxUserVirtualAddress for
diagnostic purposes. This leads to failures for some of our downstream
tests which only run with lsan. This occurs because
GetMaxUserVirtualAddress depends on setting up shadow via a call to
__sanitizer_shadow_bounds, but shadow bounds aren't set for standalone
lsan because it doesn't use shadow. This updates the function to invoke
the same syscall used by __sanitizer_shadow_bounds calls for getting the
memory limit. Ideally this function would only be called once since we
only need to get the bounds once.

More context in https://fxbug.dev/437346226.

Author: PiJoules

compiler-rt/lib/asan/asan_fuchsia.cpp

@@ -32,11 +32,9 @@ void EarlySanitizerInit() {}
 
 namespace __asan {
 
-// The system already set up the shadow memory for us.
-// __sanitizer::GetMaxUserVirtualAddress has already been called by
-// AsanInitInternal->InitializeHighMemEnd (asan_rtl.cpp).
-// Just do some additional sanity checks here.
 void InitializeShadowMemory() {
+  InitShadowBounds();
+
   if (Verbosity())
     PrintAddressSpaceLayout();
 

compiler-rt/lib/sanitizer_common/sanitizer_fuchsia.cpp

@@ -119,9 +119,20 @@ sanitizer_shadow_bounds_t ShadowBounds;
 
 void InitShadowBounds() { ShadowBounds = __sanitizer_shadow_bounds(); }
 
+// TODO(leonardchan): It's not immediately clear from a user perspective if
+// `GetMaxUserVirtualAddress` should be called exatly once on runtime startup
+// or can be called multiple times. Currently it looks like most instances of
+// `GetMaxUserVirtualAddress` are meant to be called once, but if someone
+// decides to call this multiple times in the future, we should have a separate
+// function that's ok to call multiple times. Ideally we would just invoke this
+// syscall once. Also for Fuchsia, this syscall technically gets invoked twice
+// since `__sanitizer_shadow_bounds` also invokes this syscall under the hood.
 uptr GetMaxUserVirtualAddress() {
-  InitShadowBounds();
-  return ShadowBounds.memory_limit - 1;
+  zx_info_vmar_t info;
+  zx_status_t status = _zx_object_get_info(
+      _zircon_vmar_root_self(), ZX_INFO_VMAR, &info, sizeof(info), NULL, NULL);
+  CHECK_EQ(status, ZX_OK);
+  return info.base + info.len - 1;
 }
 
 uptr GetMaxVirtualAddress() { return GetMaxUserVirtualAddress(); }