[compiler-rt][Fuchsia] Change GetMaxUserVirtualAddress to invoke syscall

compiler-rt/lib/asan/asan_fuchsia.cpp

@@ -32,11 +32,11 @@ 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() {
+  // Explicitly setup shadow here right beforer any of the ShadowBounds members
+  // are used.
+  InitShadowBounds();
+
   if (Verbosity())
     PrintAddressSpaceLayout();
 

compiler-rt/lib/sanitizer_common/sanitizer_fuchsia.cpp

@@ -14,6 +14,7 @@
 #include "sanitizer_fuchsia.h"
 #if SANITIZER_FUCHSIA
 
+#  include <limits.h>
 #  include <pthread.h>
 #  include <stdlib.h>
 #  include <unistd.h>
@@ -117,11 +118,37 @@ uptr GetMmapGranularity() { return _zx_system_get_page_size(); }
 
 sanitizer_shadow_bounds_t ShadowBounds;
 
+// Any sanitizer that utilizes shadow should explicitly call whenever it's
+// appropriate for that sanitizer to reference shadow bounds. For ASan, this is
+// done in `InitializeShadowMemory` and for HWASan, this is done in
+// `InitShadow`.
 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(_zx_vmar_root_self(), ZX_INFO_VMAR,
+                                           &info, sizeof(info), NULL, NULL);
+  CHECK_EQ(status, ZX_OK);
+
+  // Find the top of the accessible address space.
+  uintptr_t top = info.base + info.len;
+
+  // Round it up to a power-of-two size.  There may be some pages at
+  // the top that can't actually be mapped, but for purposes of the
+  // the shadow, we'll pretend they could be.
+  int bit = (sizeof(uintptr_t) * CHAR_BIT) - __builtin_clzl(top);
+  if (top != (uintptr_t)1 << bit)
+    top = (uintptr_t)1 << (bit + 1);
+
+  return top - 1;
 }
 
 uptr GetMaxVirtualAddress() { return GetMaxUserVirtualAddress(); }