linux, x86/x64: set fpregs to nullptr in CaptureContext()

uc_mcontext.fpregs is a pointer to the floating point context, but
CaptureContext() doesn't yet capture floating point context.

This error manages to slip by unit tests when run all together, but
fails when CrashpadClient.SimulateCrash is run by itself.

Bug: crashpad:30
Change-Id: I7adc30648642912d66a7ba8cf9973c9bc0fbd8bc
Reviewed-on: https://chromium-review.googlesource.com/1011504
Reviewed-by: Scott Graham <[email protected]>
Commit-Queue: Joshua Peraza <[email protected]>

client/crashpad_client_linux.cc

@@ -210,19 +210,12 @@ bool CrashpadClient::StartHandlerForClient(
 void CrashpadClient::DumpWithoutCrash(NativeCPUContext* context) {
   DCHECK(g_crash_handler);
 
-#if defined(ARCH_CPU_X86)
-  memset(&context->__fpregs_mem, 0, sizeof(context->__fpregs_mem));
-  context->__fpregs_mem.status = 0xffff0000;
-#elif defined(ARCH_CPU_X86_64)
-  memset(&context->__fpregs_mem, 0, sizeof(context->__fpregs_mem));
-#elif defined(ARCH_CPU_ARMEL)
+#if defined(ARCH_CPU_ARMEL)
   memset(context->uc_regspace, 0, sizeof(context->uc_regspace));
 #elif defined(ARCH_CPU_ARM64)
   memset(context->uc_mcontext.__reserved,
          0,
          sizeof(context->uc_mcontext.__reserved));
-#else
-#error Port.
 #endif
 
   siginfo_t siginfo;

util/misc/capture_context_linux.S

@@ -128,6 +128,8 @@ CAPTURECONTEXT_SYMBOL2:
   movl %ecx, 0x5c(%eax)  // context->uc_mcontext.xss
 
   // TODO(jperaza): save floating-point registers.
+  xorl %ecx, %ecx
+  movl %ecx, 0x60(%eax)  // context->uc_mcontext.fpregs
 
   // Clean up by restoring clobbered registers, even those considered volatile
   // by the ABI, so that the captured context represents the state at this
@@ -224,14 +226,15 @@ CAPTURECONTEXT_SYMBOL2:
   movq %r8, 0xd0(%rdi)  // context->uc_mcontext.oldmask
   movq %r8, 0xd8(%rdi)  // context->uc_mcontext.cr2
 
+  // TODO(jperaza): save floating-point registers.
+  movq %r8, 0xe0(%rdi)  // context->uc_mcontext.fpregs
+
   // Clean up by restoring clobbered registers, even those considered volatile
   // by the ABI, so that the captured context represents the state at this
   // function’s exit.
   movq 0x90(%rdi), %rax
   movq 0x28(%rdi), %r8
 
-  // TODO(jperaza): save floating-point registers.
-
   popfq
 
   popq %rbp

util/misc/capture_context_test_util_linux.cc

@@ -23,10 +23,13 @@ namespace test {
 
 void SanityCheckContext(const NativeCPUContext& context) {
 #if defined(ARCH_CPU_X86)
-  // Nothing to do here yet.
+  // TODO(jperaza): fpregs is nullptr until CaptureContext() supports capturing
+  // floating point context.
+  EXPECT_EQ(context.uc_mcontext.fpregs, nullptr);
 #elif defined(ARCH_CPU_X86_64)
   EXPECT_EQ(context.uc_mcontext.gregs[REG_RDI],
             FromPointerCast<intptr_t>(&context));
+  EXPECT_EQ(context.uc_mcontext.fpregs, nullptr);
 #elif defined(ARCH_CPU_ARMEL)
   EXPECT_EQ(context.uc_mcontext.arm_r0, FromPointerCast<uintptr_t>(&context));
 #elif defined(ARCH_CPU_ARM64)