[UA-CH] Implement Sec-CH-UA-Bitness client hint header

Intent to Implement and Ship: https://groups.google.com/a/chromium.org/g/blink-dev/c/dafizBGwWMw WICG/ua-client-hints#105 To be honest, the only real point of interest is `content/common/user_agent.cc`, which is where `GetLowEntropyCpuBitness` is implemented. Everything else is various levels of plumbing to set up a new Client Hint header, populate it, test it, and add a new Permissions Policy to control it. Bug: 1199809 Change-Id: I0e7f36c8915720cb46e06452a85092a64bde8f06 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2840423 Commit-Queue: Aaron Tagliaboschi <[email protected]> Reviewed-by: Andrey Kosyakov <[email protected]> Reviewed-by: Alex Moshchuk <[email protected]> Reviewed-by: Yoav Weiss <[email protected]> Reviewed-by: Daniel Cheng <[email protected]> Reviewed-by: Colin Blundell <[email protected]> Cr-Commit-Position: refs/heads/master@{#894021}
blueboxd · Jun 18, 2021 · 1649e10 · 1649e10
1 parent a7194e8
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Showing 38 changed files with 128 additions and 49 deletions.
diff --git a/chrome/browser/client_hints/client_hints_browsertest.cc b/chrome/browser/client_hints/client_hints_browsertest.cc
@@ -61,7 +61,7 @@
 
 namespace {
 
-const unsigned expected_client_hints_number = 13u;
+const unsigned expected_client_hints_number = 14u;
 const int32_t uma_histogram_max_value = 1471228928;
 
 // An interceptor that records count of fetches and client hint headers for

diff --git a/chrome/browser/sessions/session_service_unittest.cc b/chrome/browser/sessions/session_service_unittest.cc
@@ -891,6 +891,8 @@ TEST_F(SessionServiceTest, PersistUserAgentOverrides) {
   client_hints_override.full_version = "18.0.1025.45";
   client_hints_override.platform = "Linux";
   client_hints_override.architecture = "x86_64";
+  // Doesn't have to match, just needs to be different than the default
+  client_hints_override.bitness = "8";
 
   SerializedNavigationEntry nav1 =
       ContentTestHelper::CreateNavigation("http://google.com", "abc");

diff --git a/chrome/browser/sessions/tab_restore_service_unittest.cc b/chrome/browser/sessions/tab_restore_service_unittest.cc
@@ -93,6 +93,7 @@ class TabRestoreServiceImplTest : public ChromeRenderViewHostTestHarness {
     user_agent_override_.ua_metadata_override->full_version = "18.0.1025.45";
     user_agent_override_.ua_metadata_override->platform = "Linux";
     user_agent_override_.ua_metadata_override->architecture = "x86_64";
+    user_agent_override_.ua_metadata_override->bitness = "32";
   }
 
   ~TabRestoreServiceImplTest() override {}

diff --git a/chrome/test/data/client_hints/accept_ch_with_lifetime.html.mock-http-headers b/chrome/test/data/client_hints/accept_ch_with_lifetime.html.mock-http-headers
@@ -1,3 +1,3 @@
 HTTP/1.1 200 OK
-Accept-CH: dpr,device-memory,viewport-width,rtt,downlink,ect,lang,sec-ch-ua-arch,sec-ch-ua-platform,sec-ch-ua-platform-version,sec-ch-ua-model,sec-ch-ua-full-version,sec-ch-prefers-color-scheme
+Accept-CH: dpr,device-memory,viewport-width,rtt,downlink,ect,lang,sec-ch-ua-arch,sec-ch-ua-platform,sec-ch-ua-platform-version,sec-ch-ua-model,sec-ch-ua-full-version,sec-ch-prefers-color-scheme,sec-ch-ua-bitness
 Accept-CH-Lifetime: 3600
diff --git a/chrome/test/data/client_hints/accept_ch_with_short_lifetime.html.mock-http-headers b/chrome/test/data/client_hints/accept_ch_with_short_lifetime.html.mock-http-headers
@@ -1,3 +1,3 @@
 HTTP/1.1 200 OK
-Accept-CH: dpr,device-memory,viewport-width,rtt,downlink,ect,lang,sec-ch-ua-arch,sec-ch-ua-platform,sec-ch-ua-platform-version,sec-ch-ua-model,sec-ch-ua-full-version,sec-ch-prefers-color-scheme
+Accept-CH: dpr,device-memory,viewport-width,rtt,downlink,ect,lang,sec-ch-ua-arch,sec-ch-ua-platform,sec-ch-ua-platform-version,sec-ch-ua-model,sec-ch-ua-full-version,sec-ch-prefers-color-scheme,sec-ch-ua-bitness
 Accept-CH-Lifetime: 1
diff --git a/chrome/test/data/client_hints/accept_ch_without_lifetime.html.mock-http-headers b/chrome/test/data/client_hints/accept_ch_without_lifetime.html.mock-http-headers
@@ -1,2 +1,2 @@
 HTTP/1.1 200 OK
-Accept-CH: dpr,device-memory,viewport-width,rtt,downlink,ect,lang,sec-ch-ua-arch,sec-ch-ua-platform,sec-ch-ua-platform-version,sec-ch-ua-model,sec-ch-ua-full-version,sec-ch-prefers-color-scheme
+Accept-CH: dpr,device-memory,viewport-width,rtt,downlink,ect,lang,sec-ch-ua-arch,sec-ch-ua-platform,sec-ch-ua-platform-version,sec-ch-ua-model,sec-ch-ua-full-version,sec-ch-prefers-color-scheme,sec-ch-ua-bitness
diff --git a/...me/test/data/client_hints/accept_ch_without_lifetime_img_localhost.html.mock-http-headers b/...me/test/data/client_hints/accept_ch_without_lifetime_img_localhost.html.mock-http-headers
@@ -1,2 +1,2 @@
 HTTP/1.1 200 OK
-Accept-CH: dpr,device-memory,viewport-width,rtt,downlink,ect,lang,sec-ch-ua-arch,sec-ch-ua-platform,sec-ch-ua-platform-version,sec-ch-ua-model,sec-ch-ua-full-version,sec-ch-prefers-color-scheme
+Accept-CH: dpr,device-memory,viewport-width,rtt,downlink,ect,lang,sec-ch-ua-arch,sec-ch-ua-platform,sec-ch-ua-platform-version,sec-ch-ua-model,sec-ch-ua-full-version,sec-ch-prefers-color-scheme,sec-ch-ua-bitness
diff --git a/chrome/test/data/client_hints/http_equiv_accept_ch_merge.html b/chrome/test/data/client_hints/http_equiv_accept_ch_merge.html
@@ -2,7 +2,7 @@
 <!-- This is split in two since data from header may round trip quickly
   enough from browser to be used, so this makes sure we actually merge -->
 <meta http-equiv="Accept-CH" content="lang,sec-ch-ua-arch,sec-ch-ua-platform">
-<meta http-equiv="Accept-CH" content="sec-ch-ua-platform-version,sec-ch-ua-model,sec-ch-ua-full-version,sec-ch-prefers-color-scheme">
+<meta http-equiv="Accept-CH" content="sec-ch-ua-platform-version,sec-ch-ua-model,sec-ch-ua-full-version,sec-ch-prefers-color-scheme,sec-ch-ua-bitness">
 <link rel="icon" href="data:;base64,=">
 <!-- A subresource! -->
 <img src="non-existing-image.jpg"></img>

diff --git a/chrome/test/data/client_hints/http_equiv_accept_ch_with_lifetime.html b/chrome/test/data/client_hints/http_equiv_accept_ch_with_lifetime.html
@@ -1,5 +1,5 @@
 <html>
-<meta http-equiv="Accept-CH" content="dpr,device-memory,viewport-width,rtt,downlink,ect,lang,sec-ch-ua-arch,sec-ch-ua-platform,sec-ch-ua-model,sec-ch-ua-full-version">
+<meta http-equiv="Accept-CH" content="dpr,device-memory,viewport-width,rtt,downlink,ect,lang,sec-ch-ua-arch,sec-ch-ua-platform,sec-ch-ua-model,sec-ch-ua-full-version,sec-ch-ua-bitness">
 <meta http-equiv="Accept-CH-Lifetime" content="3600">
 <link rel="icon" href="data:;base64,=">
 </html>
diff --git a/chrome/test/data/client_hints/http_equiv_accept_ch_without_lifetime.html b/chrome/test/data/client_hints/http_equiv_accept_ch_without_lifetime.html
@@ -1,5 +1,5 @@
 <html>
-<meta http-equiv="Accept-CH" content="dpr,device-memory,viewport-width,rtt,downlink,ect,lang,sec-ch-ua-arch,sec-ch-ua-platform,sec-ch-ua-model,sec-ch-ua-full-version">
+<meta http-equiv="Accept-CH" content="dpr,device-memory,viewport-width,rtt,downlink,ect,lang,sec-ch-ua-arch,sec-ch-ua-platform,sec-ch-ua-model,sec-ch-ua-full-version,sec-ch-ua-bitness">
 <link rel="icon" href="data:;base64,=">
 <head></head>
 Empty file which uses link-rel to disable favicon fetches. The corresponding

diff --git a/chrome/test/data/client_hints/http_equiv_accept_ch_without_lifetime_img_localhost.html b/chrome/test/data/client_hints/http_equiv_accept_ch_without_lifetime_img_localhost.html
@@ -1,5 +1,5 @@
 <html>
-<meta http-equiv="Accept-CH" content="dpr,device-memory,viewport-width,rtt,downlink,ect,lang,sec-ch-ua-arch,sec-ch-ua-platform,sec-ch-ua-platform-version,sec-ch-ua-model,sec-ch-ua-full-version,sec-ch-prefers-color-scheme">
+<meta http-equiv="Accept-CH" content="dpr,device-memory,viewport-width,rtt,downlink,ect,lang,sec-ch-ua-arch,sec-ch-ua-platform,sec-ch-ua-platform-version,sec-ch-ua-model,sec-ch-ua-full-version,sec-ch-prefers-color-scheme,sec-ch-ua-bitness">
 <link rel="icon" href="data:;base64,=">
 <head></head>
 Empty file which uses link-rel to disable favicon fetches. The corresponding

diff --git a/components/embedder_support/user_agent_utils.cc b/components/embedder_support/user_agent_utils.cc
@@ -142,7 +142,6 @@ blink::UserAgentMetadata GetUserAgentMetadata() {
   metadata.platform = GetPlatformForUAMetadata();
   metadata.architecture = content::GetLowEntropyCpuArchitecture();
   metadata.model = content::BuildModelInfo();
-
   metadata.mobile = false;
 #if defined(OS_ANDROID)
   metadata.mobile = base::CommandLine::ForCurrentProcess()->HasSwitch(
@@ -153,6 +152,11 @@ blink::UserAgentMetadata GetUserAgentMetadata() {
   base::SysInfo::OperatingSystemVersionNumbers(&major, &minor, &bugfix);
   metadata.platform_version =
       base::StringPrintf("%d.%d.%d", major, minor, bugfix);
+  // These methods use the same information as the User-Agent string, but are
+  // "low entropy" in that they reduce the number of options for output to a
+  // set number. For more information, see the respective headers.
+  metadata.architecture = content::GetLowEntropyCpuArchitecture();
+  metadata.bitness = content::GetLowEntropyCpuBitness();
 
   return metadata;
 }
@@ -171,9 +175,12 @@ void SetDesktopUserAgentOverride(content::WebContents* web_contents,
   spoofed_ua.ua_metadata_override->platform = "Linux";
   spoofed_ua.ua_metadata_override->platform_version =
       std::string();  // match content::GetOSVersion(false) on Linux
-  spoofed_ua.ua_metadata_override->architecture = "x86";
   spoofed_ua.ua_metadata_override->model = std::string();
   spoofed_ua.ua_metadata_override->mobile = false;
+  // Match the above "CpuInfo" string, which is also the most common Linux
+  // CPU architecture and bitness.`
+  spoofed_ua.ua_metadata_override->architecture = "x86";
+  spoofed_ua.ua_metadata_override->bitness = "64";
 
   web_contents->SetUserAgentOverride(spoofed_ua, override_in_new_tabs);
 }

diff --git a/components/embedder_support/user_agent_utils_unittest.cc b/components/embedder_support/user_agent_utils_unittest.cc
@@ -305,6 +305,7 @@ TEST(UserAgentUtilsTest, UserAgentMetadata) {
 #endif
   EXPECT_EQ(metadata.architecture, content::GetLowEntropyCpuArchitecture());
   EXPECT_EQ(metadata.model, content::BuildModelInfo());
+  EXPECT_EQ(metadata.bitness, content::GetLowEntropyCpuBitness());
 }
 
 TEST(UserAgentUtilsTest, GenerateBrandVersionList) {

diff --git a/content/browser/client_hints/client_hints.cc b/content/browser/client_hints/client_hints.cc
@@ -556,6 +556,11 @@ void UpdateNavigationRequestClientUaHeadersImpl(
       AddUAHeader(headers, network::mojom::WebClientHintsType::kUAModel,
                   SerializeHeaderString(ua_metadata->model));
     }
+    if (ShouldAddClientHint(data,
+                            network::mojom::WebClientHintsType::kUABitness)) {
+      AddUAHeader(headers, network::mojom::WebClientHintsType::kUABitness,
+                  SerializeHeaderString(ua_metadata->bitness));
+    }
   } else if (call_type == ClientUaHeaderCallType::kAfterCreated) {
     RemoveClientHintHeader(network::mojom::WebClientHintsType::kUA, headers);
     RemoveClientHintHeader(network::mojom::WebClientHintsType::kUAMobile,
@@ -570,6 +575,8 @@ void UpdateNavigationRequestClientUaHeadersImpl(
         network::mojom::WebClientHintsType::kUAPlatformVersion, headers);
     RemoveClientHintHeader(network::mojom::WebClientHintsType::kUAModel,
                            headers);
+    RemoveClientHintHeader(network::mojom::WebClientHintsType::kUABitness,
+                           headers);
   }
 }
 
@@ -678,7 +685,7 @@ void AddRequestClientHintsHeaders(
   // If possible, logic should be added above so that the request headers for
   // the newly added client hint can be added to the request.
   static_assert(
-      network::mojom::WebClientHintsType::kPrefersColorScheme ==
+      network::mojom::WebClientHintsType::kUABitness ==
           network::mojom::WebClientHintsType::kMaxValue,
       "Consider adding client hint request headers from the browser process");
 

diff --git a/content/common/user_agent.cc b/content/common/user_agent.cc
@@ -6,6 +6,7 @@
 
 #include <stdint.h>
 
+#include "base/containers/contains.h"
 #include "base/strings/strcat.h"
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
@@ -125,11 +126,7 @@ std::string GetLowEntropyCpuArchitecture() {
     return "arm";
   }
 #elif defined(OS_POSIX) && !defined(OS_ANDROID)
-  // This extra cpu_info_str variable is required to make sure the compiler
-  // doesn't optimize the copy away and have the StringPiece point at the
-  // internal std::string, resulting in a memory violation.
-  std::string cpu_info_str = BuildCpuInfo();
-  base::StringPiece cpu_info = cpu_info_str;
+  std::string cpu_info = BuildCpuInfo();
   if (base::StartsWith(cpu_info, "arm") ||
       base::StartsWith(cpu_info, "aarch")) {
     return "arm";
@@ -142,6 +139,21 @@ std::string GetLowEntropyCpuArchitecture() {
   return std::string();
 }
 
+std::string GetLowEntropyCpuBitness() {
+#if defined(OS_WIN)
+  return (base::win::OSInfo::GetInstance()->GetArchitecture() ==
+          base::win::OSInfo::X86_ARCHITECTURE)
+             ? "32"
+             : "64";
+#elif defined(OS_MAC)
+  return "64";
+#elif defined(OS_POSIX) && !defined(OS_ANDROID)
+  return base::Contains(BuildCpuInfo(), "64") ? "64" : "32";
+#else
+  return std::string();
+#endif
+}
+
 std::string GetOSVersion(IncludeAndroidBuildNumber include_android_build_number,
                          IncludeAndroidModel include_android_model) {
   std::string os_version;

diff --git a/content/public/common/common_param_traits_macros.h b/content/public/common/common_param_traits_macros.h
@@ -88,6 +88,7 @@ IPC_STRUCT_TRAITS_BEGIN(blink::UserAgentMetadata)
   IPC_STRUCT_TRAITS_MEMBER(architecture)
   IPC_STRUCT_TRAITS_MEMBER(model)
   IPC_STRUCT_TRAITS_MEMBER(mobile)
+  IPC_STRUCT_TRAITS_MEMBER(bitness)
 IPC_STRUCT_TRAITS_END()
 
 IPC_STRUCT_TRAITS_BEGIN(blink::UserAgentOverride)

diff --git a/content/public/common/user_agent.h b/content/public/common/user_agent.h
@@ -47,6 +47,10 @@ CONTENT_EXPORT std::string BuildCpuInfo();
 // most common cases.
 CONTENT_EXPORT std::string GetLowEntropyCpuArchitecture();
 
+// Takes the cpu info (see BuildCpuInfo()) and extracts the CPU bitness for
+// most common cases.
+CONTENT_EXPORT std::string GetLowEntropyCpuBitness();
+
 // Builds a User-agent compatible string that describes the OS and CPU type.
 // On Android, the string will only include the build number and model if
 // relevant enums indicate they should be included.

diff --git a/content/shell/browser/shell_content_browser_client.cc b/content/shell/browser/shell_content_browser_client.cc
@@ -220,6 +220,8 @@ blink::UserAgentMetadata GetShellUserAgentMetadata() {
   metadata.architecture = BuildCpuInfo();
   metadata.model = BuildModelInfo();
 
+  metadata.bitness = GetLowEntropyCpuBitness();
+
   return metadata;
 }
 

diff --git a/services/network/public/cpp/client_hints.cc b/services/network/public/cpp/client_hints.cc
@@ -33,7 +33,8 @@ const char* const kClientHintsNameMapping[] = {"device-memory",
                                                "sec-ch-ua-mobile",
                                                "sec-ch-ua-full-version",
                                                "sec-ch-ua-platform-version",
-                                               "sec-ch-prefers-color-scheme"};
+                                               "sec-ch-prefers-color-scheme",
+                                               "sec-ch-ua-bitness"};
 
 const size_t kClientHintsMappingsCount = base::size(kClientHintsNameMapping);
 

diff --git a/services/network/public/cpp/cors/cors.cc b/services/network/public/cpp/cors/cors.cc
@@ -466,6 +466,7 @@ bool IsCorsSafelistedHeader(const std::string& name, const std::string& value) {
       //
       // https://tomayac.github.io/user-preference-media-features-headers/#sec-ch-prefers-color-scheme
       "sec-ch-prefers-color-scheme",
+      "sec-ch-ua-bitness",
   };
   if (std::find(std::begin(safe_names), std::end(safe_names), lower_name) ==
       std::end(safe_names))

diff --git a/services/network/public/mojom/web_client_hints_types.mojom b/services/network/public/mojom/web_client_hints_types.mojom
@@ -32,6 +32,7 @@ enum WebClientHintsType {
   kUAFullVersion = 13,
   kUAPlatformVersion = 14,
   kPrefersColorScheme = 15,
+  kUABitness = 16,
 
   // Warning: Before adding a new client hint, read the warning at the top.
 };
diff --git a/third_party/blink/common/client_hints/client_hints.cc b/third_party/blink/common/client_hints/client_hints.cc
@@ -19,24 +19,23 @@
 
 namespace blink {
 
-const char* const kClientHintsHeaderMapping[] = {
-    "device-memory",
-    "dpr",
-    "width",
-    "viewport-width",
-    "rtt",
-    "downlink",
-    "ect",
-    "sec-ch-lang",
-    "sec-ch-ua",
-    "sec-ch-ua-arch",
-    "sec-ch-ua-platform",
-    "sec-ch-ua-model",
-    "sec-ch-ua-mobile",
-    "sec-ch-ua-full-version",
-    "sec-ch-ua-platform-version",
-    "sec-ch-prefers-color-scheme",
-};
+const char* const kClientHintsHeaderMapping[] = {"device-memory",
+                                                 "dpr",
+                                                 "width",
+                                                 "viewport-width",
+                                                 "rtt",
+                                                 "downlink",
+                                                 "ect",
+                                                 "sec-ch-lang",
+                                                 "sec-ch-ua",
+                                                 "sec-ch-ua-arch",
+                                                 "sec-ch-ua-platform",
+                                                 "sec-ch-ua-model",
+                                                 "sec-ch-ua-mobile",
+                                                 "sec-ch-ua-full-version",
+                                                 "sec-ch-ua-platform-version",
+                                                 "sec-ch-prefers-color-scheme",
+                                                 "sec-ch-ua-bitness"};
 
 const unsigned kClientHintsNumberOfLegacyHints = 4;
 
@@ -60,6 +59,7 @@ const mojom::PermissionsPolicyFeature kClientHintsPermissionsPolicyMapping[] = {
     mojom::PermissionsPolicyFeature::kClientHintUAFullVersion,
     mojom::PermissionsPolicyFeature::kClientHintUAPlatformVersion,
     mojom::PermissionsPolicyFeature::kClientHintPrefersColorScheme,
+    mojom::PermissionsPolicyFeature::kClientHintUABitness,
 };
 
 const size_t kClientHintsMappingsCount = base::size(kClientHintsHeaderMapping);
@@ -117,6 +117,7 @@ absl::optional<std::vector<network::mojom::WebClientHintsType>> FilterAcceptCH(
       case network::mojom::WebClientHintsType::kUAModel:
       case network::mojom::WebClientHintsType::kUAMobile:
       case network::mojom::WebClientHintsType::kUAFullVersion:
+      case network::mojom::WebClientHintsType::kUABitness:
         if (permit_ua_hints)
           result.push_back(hint);
         break;

diff --git a/third_party/blink/common/client_hints/client_hints_unittest.cc b/third_party/blink/common/client_hints/client_hints_unittest.cc
@@ -127,11 +127,11 @@ TEST(ClientHintsTest, FindClientHintsToRemoveLegacy) {
   std::vector<std::string> removed_headers;
   FindClientHintsToRemove(nullptr, GURL(), &removed_headers);
   EXPECT_THAT(removed_headers,
-              UnorderedElementsAre("rtt", "downlink", "ect", "sec-ch-lang",
-                                   "sec-ch-ua-arch", "sec-ch-ua-platform",
-                                   "sec-ch-ua-model", "sec-ch-ua-full-version",
-                                   "sec-ch-ua-platform-version",
-                                   "sec-ch-prefers-color-scheme"));
+              UnorderedElementsAre(
+                  "rtt", "downlink", "ect", "sec-ch-lang", "sec-ch-ua-arch",
+                  "sec-ch-ua-platform", "sec-ch-ua-model",
+                  "sec-ch-ua-full-version", "sec-ch-ua-platform-version",
+                  "sec-ch-prefers-color-scheme", "sec-ch-ua-bitness"));
 }
 
 // Checks that the removed header list includes legacy headers but not the
@@ -142,12 +142,12 @@ TEST(ClientHintsTest, FindClientHintsToRemoveNoLegacy) {
       features::kAllowClientHintsToThirdParty);
   std::vector<std::string> removed_headers;
   FindClientHintsToRemove(nullptr, GURL(), &removed_headers);
-  EXPECT_THAT(
-      removed_headers,
-      UnorderedElementsAre(
-          "device-memory", "dpr", "width", "viewport-width", "rtt", "downlink",
-          "ect", "sec-ch-lang", "sec-ch-ua-arch", "sec-ch-ua-platform",
-          "sec-ch-ua-model", "sec-ch-ua-full-version",
-          "sec-ch-ua-platform-version", "sec-ch-prefers-color-scheme"));
+  EXPECT_THAT(removed_headers,
+              UnorderedElementsAre(
+                  "device-memory", "dpr", "width", "viewport-width", "rtt",
+                  "downlink", "ect", "sec-ch-lang", "sec-ch-ua-arch",
+                  "sec-ch-ua-platform", "sec-ch-ua-model",
+                  "sec-ch-ua-full-version", "sec-ch-ua-platform-version",
+                  "sec-ch-prefers-color-scheme", "sec-ch-ua-bitness"));
 }
 }  // namespace blink
diff --git a/third_party/blink/common/user_agent/user_agent_metadata.cc b/third_party/blink/common/user_agent/user_agent_metadata.cc
@@ -61,6 +61,7 @@ absl::optional<std::string> UserAgentMetadata::Marshal(
   out.WriteString(in->architecture);
   out.WriteString(in->model);
   out.WriteBool(in->mobile);
+  out.WriteString(in->bitness);
   return std::string(reinterpret_cast<const char*>(out.data()), out.size());
 }
 
@@ -102,6 +103,8 @@ absl::optional<UserAgentMetadata> UserAgentMetadata::Demarshal(
     return absl::nullopt;
   if (!in.ReadBool(&out.mobile))
     return absl::nullopt;
+  if (!in.ReadString(&out.bitness))
+    return absl::nullopt;
   return absl::make_optional(std::move(out));
 }
 
@@ -114,7 +117,7 @@ bool operator==(const UserAgentMetadata& a, const UserAgentMetadata& b) {
          a.full_version == b.full_version && a.platform == b.platform &&
          a.platform_version == b.platform_version &&
          a.architecture == b.architecture && a.model == b.model &&
-         a.mobile == b.mobile;
+         a.mobile == b.mobile && a.bitness == b.bitness;
 }
 
 bool operator==(const UserAgentOverride& a, const UserAgentOverride& b) {

diff --git a/third_party/blink/common/user_agent/user_agent_metadata_unittest.cc b/third_party/blink/common/user_agent/user_agent_metadata_unittest.cc
@@ -24,6 +24,7 @@ blink::UserAgentMetadata MakeToEncode() {
   to_encode.architecture = "Z80";
   to_encode.model = "unofficial";
   to_encode.mobile = false;
+  to_encode.bitness = "8";
   return to_encode;
 }
 

diff --git a/third_party/blink/common/user_agent/user_agent_mojom_traits.cc b/third_party/blink/common/user_agent/user_agent_mojom_traits.cc
@@ -53,6 +53,10 @@ bool StructTraits<blink::mojom::UserAgentMetadataDataView,
   out->model = string;
   out->mobile = data.mobile();
 
+  if (!data.ReadBitness(&string))
+    return false;
+  out->bitness = string;
+
   return true;
 }
 

diff --git a/third_party/blink/public/common/user_agent/user_agent_metadata.h b/third_party/blink/public/common/user_agent/user_agent_metadata.h
@@ -47,6 +47,7 @@ struct BLINK_COMMON_EXPORT UserAgentMetadata {
   std::string architecture;
   std::string model;
   bool mobile = false;
+  std::string bitness;
 };
 
 // Used when customizing the sent User-Agent and Sec-CH-UA-* for