Fix "-c dbg" build break

Remove allocation from vCPU.die() to save stack space.

Closes #131

PiperOrigin-RevId: 236238102
Change-Id: Iafca27a1a3a472d4cb11dcda9a2060e585139d11

2 files changed, 14 insertions, 7 deletions

diff --git a/pkg/sentry/platform/kvm/bluepill.go b/pkg/sentry/platform/kvm/bluepill.go
index d98ec8377..f24f1c662 100644
--- a/pkg/sentry/platform/kvm/bluepill.go
+++ b/pkg/sentry/platform/kvm/bluepill.go
@@ -49,7 +49,7 @@ var (
 //
 //go:nosplit
 func dieHandler(c *vCPU) {
-	throw(c.dieMessage)
+	throw(c.dieState.message)
 }
 
 // die is called to set the vCPU up to panic.
@@ -59,17 +59,16 @@ func dieHandler(c *vCPU) {
 //go:nosplit
 func (c *vCPU) die(context *arch.SignalContext64, msg string) {
 	// Save the death message, which will be thrown.
-	c.dieMessage = msg
+	c.dieState.message = msg
 
 	// Reload all registers to have an accurate stack trace when we return
 	// to host mode. This means that the stack should be unwound correctly.
-	var guestRegs userRegs
-	if errno := c.getUserRegisters(&guestRegs); errno != 0 {
+	if errno := c.getUserRegisters(&c.dieState.guestRegs); errno != 0 {
 		throw(msg)
 	}
 
 	// Setup the trampoline.
-	dieArchSetup(c, context, &guestRegs)
+	dieArchSetup(c, context, &c.dieState.guestRegs)
 }
 
 func init() {
diff --git a/pkg/sentry/platform/kvm/machine.go b/pkg/sentry/platform/kvm/machine.go
index deead1b5f..b8b3c9a4a 100644
--- a/pkg/sentry/platform/kvm/machine.go
+++ b/pkg/sentry/platform/kvm/machine.go
@@ -121,8 +121,16 @@ type vCPU struct {
 	// vCPUArchState is the architecture-specific state.
 	vCPUArchState
 
-	// dieMessage is thrown from die.
-	dieMessage string
+	dieState dieState
+}
+
+type dieState struct {
+	// message is thrown from die.
+	message string
+
+	// guestRegs is used to store register state during vCPU.die() to prevent
+	// allocation inside nosplit function.
+	guestRegs userRegs
 }
 
 // newVCPU creates a returns a new vCPU.