[Fuchsia] Map VMOs with ZX_VM_REQUIRE_NON_RESIZABLE

It's not safe to map resizeable VMOs because they can be resized
while being mapped and Zircon faults the thread that tries to access
mapped memory outside of the VMO bounds. This CL adds
ZX_VM_REQUIRE_NON_RESIZABLE to zx_vmar_map() calls, which ensures that
mapped VMOs are always non-resizable.

Change-Id: I5b40347400f839c2e4d879054d222950ba0d3d31
Reviewed-on: https://chromium-review.googlesource.com/c/1281051
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599745}

base/memory/platform_shared_memory_region_fuchsia.cc

@@ -111,12 +111,12 @@ bool PlatformSharedMemoryRegion::MapAtInternal(off_t offset,
                                                size_t size,
                                                void** memory,
                                                size_t* mapped_size) const {
-  bool write_allowed = mode_ != Mode::kReadOnly;
   uintptr_t addr;
+  zx_vm_option_t options = ZX_VM_REQUIRE_NON_RESIZABLE | ZX_VM_FLAG_PERM_READ;
+  if (mode_ != Mode::kReadOnly)
+    options |= ZX_VM_FLAG_PERM_WRITE;
   zx_status_t status = zx::vmar::root_self()->map(
-      0, handle_, offset, size,
-      ZX_VM_FLAG_PERM_READ | (write_allowed ? ZX_VM_FLAG_PERM_WRITE : 0),
-      &addr);
+      /*vmar_offset=*/0, handle_, offset, size, options, &addr);
   if (status != ZX_OK) {
     ZX_DLOG(ERROR, status) << "zx_vmar_map";
     return false;

base/memory/shared_memory_fuchsia.cc

@@ -85,12 +85,13 @@ bool SharedMemory::MapAt(off_t offset, size_t bytes) {
   if (memory_)
     return false;
 
-  int flags = ZX_VM_FLAG_PERM_READ;
+  zx_vm_option_t options = ZX_VM_REQUIRE_NON_RESIZABLE | ZX_VM_FLAG_PERM_READ;
   if (!read_only_)
-    flags |= ZX_VM_FLAG_PERM_WRITE;
+    options |= ZX_VM_FLAG_PERM_WRITE;
   uintptr_t addr;
   zx_status_t status = zx::vmar::root_self()->map(
-      0, *zx::unowned_vmo(shm_.GetHandle()), offset, bytes, flags, &addr);
+      /*vmar_offset=*/0, *zx::unowned_vmo(shm_.GetHandle()), offset, bytes,
+      options, &addr);
   if (status != ZX_OK) {
     ZX_DLOG(ERROR, status) << "zx_vmar_map";
     return false;