Avoid using X11 SHM for remote connections

For remote connections, SHM doesn't work, but the MIT-SHM extension may
still be available, so we need to do an explicit check before trying to
use SHM.

BUG=1035803

Change-Id: I189ab94b82922572e52b717dc2043b8404745dec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1980971
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728564}

ui/base/x/BUILD.gn

@@ -64,6 +64,7 @@ jumbo_component("x") {
   deps = [
     "//base",
     "//base:i18n",
+    "//net",
     "//skia",
     "//ui/base:hit_test",
     "//ui/base/clipboard:clipboard_types",

ui/base/x/x11_shm_image_pool_base.cc

@@ -18,6 +18,7 @@
 #include "base/strings/string_util.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "build/build_config.h"
+#include "net/base/url_util.h"
 #include "ui/events/platform/platform_event_dispatcher.h"
 #include "ui/events/platform/platform_event_source.h"
 #include "ui/gfx/geometry/rect.h"
@@ -54,8 +55,35 @@ std::size_t MaxShmSegmentSize() {
   return max_size;
 }
 
+bool IsRemoteHost(const std::string& name) {
+  if (name.empty())
+    return false;
+
+  char hostname[256];
+  if (!gethostname(hostname, sizeof(hostname)) && name == hostname)
+    return false;
+
+  return !net::HostStringIsLocalhost(name);
+}
+
+bool ShouldUseMitShm(XDisplay* display) {
+  // MIT-SHM may be available on remote connetions, but it will be unusable.  Do
+  // a best-effort check to see if the host is remote to disable the SHM
+  // codepath.  It may be possible in contrived cases for there to be a
+  // false-positive, but in that case we'll just fallback to the non-SHM
+  // codepath.
+  char* display_string = DisplayString(display);
+  char* host = nullptr;
+  int display_id = 0;
+  int screen = 0;
+  if (xcb_parse_display(display_string, &host, &display_id, &screen)) {
+    std::string name = host;
+    free(host);
+    if (IsRemoteHost(name))
+      return false;
+  }
+
 #if !defined(OS_CHROMEOS)
-bool ShouldUseMitShm() {
   std::unique_ptr<base::Environment> env = base::Environment::Create();
 
   // Used by QT.
@@ -73,10 +101,10 @@ bool ShouldUseMitShm() {
   // Used by GTK.
   if (base::CommandLine::ForCurrentProcess()->HasSwitch(switches::kNoXshm))
     return false;
+#endif
 
   return true;
 }
-#endif
 
 }  // namespace
 
@@ -119,10 +147,8 @@ bool XShmImagePoolBase::Resize(const gfx::Size& pixel_size) {
   if (!event_task_runner_)
     return false;
 
-#if !defined(OS_CHROMEOS)
-  if (!ShouldUseMitShm())
+  if (!ShouldUseMitShm(display_))
     return false;
-#endif
 
   if (!ui::QueryShmSupport())
     return false;