commit a42f6c233c879dee36aae58067d0bda7e08eb9d5
parent 1c68c48f6d838d901f45c2af43db5fbadaa8e7be
Author: Tom Ritter <tom@mozilla.com>
Date: Mon, 29 Dec 2025 20:29:00 +0000
Bug 2005866: Collect the FPU Control State r=timhuang
Differential Revision: https://phabricator.services.mozilla.com/D276715
Diffstat:
3 files changed, 158 insertions(+), 1 deletion(-)
diff --git a/toolkit/components/resistfingerprinting/metrics.yaml b/toolkit/components/resistfingerprinting/metrics.yaml
@@ -4009,6 +4009,30 @@ characteristics:
data_sensitivity:
- technical
+ fpu_control_state:
+ type: string
+ description: >
+ The current FPU (Floating-Point Unit) rounding mode and precision settings.
+ Format depends on architecture:
+ - x86/x86-64: "std:X;x87:Y;sse:Z;prec:P" where X,Y,Z are rounding modes (0-3)
+ and P is precision (single/double/extended)
+ - ARM: "std:X;arm:Y" where X,Y are rounding modes (0-3)
+ Rounding modes: 0=nearest, 1=down/negative, 2=up/positive, 3=toward-zero
+ This helps identify unusual FPU configurations that may cause math variations.
+ lifetime: application
+ send_in_pings:
+ - user-characteristics
+ notification_emails:
+ - tom@mozilla.com
+ bugs:
+ - https://bugzilla.mozilla.org/show_bug.cgi?id=1879151
+ - https://bugzilla.mozilla.org/show_bug.cgi?id=2005866
+ data_reviews:
+ - https://bugzilla.mozilla.org/show_bug.cgi?id=2005866
+ expires: never
+ data_sensitivity:
+ - technical
+
mathml1:
type: string
description: >
diff --git a/toolkit/components/resistfingerprinting/nsUserCharacteristics.cpp b/toolkit/components/resistfingerprinting/nsUserCharacteristics.cpp
@@ -72,6 +72,13 @@
# include "mozilla/widget/GSettings.h"
#endif
+// For FPU control state detection
+#include <cfenv>
+#if defined(_MSC_VER)
+# include <float.h>
+# include <intrin.h>
+#endif
+
using namespace mozilla;
static LazyLogModule gUserCharacteristicsLog("UserCharacteristics");
@@ -701,8 +708,126 @@ void PopulateProcessorCount() {
glean::characteristics::processor_count.Set(processorCount);
}
+/**
+ * Gets the complete FPU state including rounding mode and precision.
+ * Returns a string formatted as:
+ * - x86/x86-64: "std:X;x87:Y;sse:Z;prec:P"
+ * - ARM: "std:X;arm:Y"
+ * Where X,Y,Z are rounding mode values (0-3) and P is precision.
+ */
+static nsCString GetFPUControlState() {
+ nsCString result;
+
+ // Get standard C rounding mode (portable)
+ int std_mode = std::fegetround();
+ const char* mode_str = "unknown";
+ switch (std_mode) {
+ case FE_TONEAREST:
+ mode_str = "0";
+ break;
+ case FE_DOWNWARD:
+ mode_str = "1";
+ break;
+ case FE_UPWARD:
+ mode_str = "2";
+ break;
+ case FE_TOWARDZERO:
+ mode_str = "3";
+ break;
+ }
+ result.AppendLiteral("std:");
+ result.Append(mode_str);
+
+#if defined(__x86_64__) || defined(_M_X64) || defined(__i386__) || \
+ defined(_M_IX86)
+ // x86/x86-64: Read both x87 and SSE control registers
+
+ // Read x87 control word
+ uint16_t x87_cw = 0;
+# ifdef _MSC_VER
+ x87_cw = static_cast<uint16_t>(_control87(0, 0));
+# else
+ __asm__ __volatile__("fstcw %0" : "=m"(x87_cw));
+# endif
+
+ // Extract rounding mode (bits 10-11)
+ int x87_round = (x87_cw >> 10) & 0x3;
+ result.AppendLiteral(";x87:");
+ result.AppendInt(x87_round);
+
+ // Extract precision control (bits 8-9)
+ int precision = (x87_cw >> 8) & 0x3;
+ const char* prec_str = "unknown";
+ switch (precision) {
+ case 0:
+ prec_str = "single";
+ break;
+ case 1:
+ prec_str = "reserved";
+ break;
+ case 2:
+ prec_str = "double";
+ break;
+ case 3:
+ prec_str = "extended";
+ break;
+ }
+
+ // Read SSE MXCSR register (64-bit only, or 32-bit with SSE support)
+# if defined(__x86_64__) || defined(_M_X64) || defined(__SSE__)
+ uint32_t mxcsr = 0;
+# ifdef _MSC_VER
+ mxcsr = _mm_getcsr();
+# else
+ __asm__ __volatile__("stmxcsr %0" : "=m"(mxcsr));
+# endif
+
+ // Extract rounding mode (bits 13-14)
+ int sse_round = (mxcsr >> 13) & 0x3;
+ result.AppendLiteral(";sse:");
+ result.AppendInt(sse_round);
+# else
+ // 32-bit without SSE
+ result.AppendLiteral(";sse:na");
+# endif
+
+ result.AppendLiteral(";prec:");
+ result.Append(prec_str);
+
+#elif defined(__aarch64__)
+ // ARM64: Read FPCR register
+ uint64_t fpcr = 0;
+ __asm__ __volatile__("mrs %0, fpcr" : "=r"(fpcr));
+
+ // Extract rounding mode (bits 22-23)
+ int arm_round = (fpcr >> 22) & 0x3;
+ result.AppendLiteral(";arm:");
+ result.AppendInt(arm_round);
+
+#elif defined(__arm__)
+ // ARM32: Read FPSCR register
+ uint32_t fpscr = 0;
+ __asm__ __volatile__("vmrs %0, fpscr" : "=r"(fpscr));
+
+ // Extract rounding mode (bits 22-23)
+ int arm_round = (fpscr >> 22) & 0x3;
+ result.AppendLiteral(";arm:");
+ result.AppendInt(arm_round);
+
+#else
+ // Other architectures: only report standard mode
+ result.AppendLiteral(";arch:other");
+#endif
+
+ return result;
+}
+
void PopulateMisc(bool worksInGtest) {
if (worksInGtest) {
+ // Collect FPU control state
+ nsCString fpuState = GetFPUControlState();
+ glean::characteristics::fpu_control_state.Set(fpuState);
+
glean::characteristics::max_touch_points.Set(testing::MaxTouchPoints());
nsCOMPtr<nsIGfxInfo> gfxInfo = components::GfxInfo::Service();
if (gfxInfo) {
@@ -801,7 +926,7 @@ const RefPtr<PopulatePromise>& TimoutPromise(
// metric is set, this variable should be incremented. It'll be a lot. It's
// okay. We're going to need it to know (including during development) what is
// the source of the data we are looking at.
-const int kSubmissionSchema = 29;
+const int kSubmissionSchema = 30;
const auto* const kUUIDPref =
"toolkit.telemetry.user_characteristics_ping.uuid";
diff --git a/toolkit/components/resistfingerprinting/tests/gtest/test_usercharping.cpp b/toolkit/components/resistfingerprinting/tests/gtest/test_usercharping.cpp
@@ -60,6 +60,14 @@ TEST(ResistFingerprinting, UserCharacteristics_Complex)
mozilla::glean::characteristics::max_touch_points.TestGetValue()
.unwrap()
.ref());
+
+ // Test FPU control state metric to ensure code runs and doesn't crash
+ auto fpuState =
+ mozilla::glean::characteristics::fpu_control_state.TestGetValue()
+ .unwrap()
+ .value();
+ ASSERT_STRNE("", fpuState.get());
+ ASSERT_TRUE(strstr(fpuState.get(), "std:") != nullptr);
},
[]() { nsUserCharacteristics::SubmitPing(); }));
}
