Add fpsimd support in sigreturn on Arm64

Signed-off-by: Bin Lu <bin.lu@arm.com>

pkg/sentry/arch/signal_arm64.go

@@ -19,6 +19,7 @@ import (
 	"syscall"
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/log"
 	"gvisor.dev/gvisor/pkg/usermem"
 )
 
@@ -134,6 +135,11 @@ func (c *context64) SignalSetup(st *Stack, act *SignalAct, info *SignalInfo, alt
 	c.Regs.Regs[1] = uint64(infoAddr)
 	c.Regs.Regs[2] = uint64(ucAddr)
 	c.Regs.Regs[30] = uint64(act.Restorer)
+
+	// Save the thread's floating point state.
+	c.sigFPState = append(c.sigFPState, c.aarch64FPState)
+	// Signal handler gets a clean floating point state.
+	c.aarch64FPState = newAarch64FPState()
 	return nil
 }
 
@@ -155,5 +161,21 @@ func (c *context64) SignalRestore(st *Stack, rt bool) (linux.SignalSet, SignalSt
 	c.Regs.Sp = uc.MContext.Sp
 	c.Regs.Pstate = uc.MContext.Pstate
 
+	// Restore floating point state.
+	l := len(c.sigFPState)
+	if l > 0 {
+		c.aarch64FPState = c.sigFPState[l-1]
+		// NOTE(cl/133042258): State save requires that any slice
+		// elements from '[len:cap]' to be zero value.
+		c.sigFPState[l-1] = nil
+		c.sigFPState = c.sigFPState[0 : l-1]
+	} else {
+		// This might happen if sigreturn(2) calls are unbalanced with
+		// respect to signal handler entries. This is not expected so
+		// don't bother to do anything fancy with the floating point
+		// state.
+		log.Warningf("sigreturn unable to restore application fpstate")
+	}
+
 	return uc.Sigset, uc.Stack, nil
 }