Arm64 signal#2: signal support in arch module

SA_RESTORER is always used on Intel platform.
But this flag is optional on other platforms.

The vdso is enabled, so we can use the sigreturn trampolines
the vdso provides instead on Arm platform.

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

pkg/sentry/arch/signal_arm64.go

@@ -83,9 +83,12 @@ func (c *context64) SignalSetup(st *Stack, act *SignalAct, info *SignalInfo, alt
 	if ucSize < 0 {
 		panic("can't get size of UContext64")
 	}
-	// st.Arch.Width() is for the restorer address. sizeof(siginfo) == 128.
-	frameSize := int(st.Arch.Width()) + ucSize + 128
-	frameBottom := (sp-usermem.Addr(frameSize)) & ^usermem.Addr(15) - 8
+
+	// frameSize = ucSize + sizeof(siginfo).
+	// sizeof(siginfo) == 128.
+	// R30 stores the restorer address.
+	frameSize := ucSize + 128
+	frameBottom := (sp - usermem.Addr(frameSize)) & ^usermem.Addr(15)
 	sp = frameBottom + usermem.Addr(frameSize)
 	st.Bottom = sp
 
@@ -115,12 +118,27 @@ func (c *context64) SignalSetup(st *Stack, act *SignalAct, info *SignalInfo, alt
 	c.Regs.Regs[0] = uint64(info.Signo)
 	c.Regs.Regs[1] = uint64(infoAddr)
 	c.Regs.Regs[2] = uint64(ucAddr)
-
+	c.Regs.Regs[30] = uint64(act.Restorer)
 	return nil
 }
 
 // SignalRestore implements Context.SignalRestore.
-// Only used on intel.
 func (c *context64) SignalRestore(st *Stack, rt bool) (linux.SignalSet, SignalStack, error) {
-	return 0, SignalStack{}, nil
+	// Copy out the stack frame.
+	var uc UContext64
+	if _, err := st.Pop(&uc); err != nil {
+		return 0, SignalStack{}, err
+	}
+	var info SignalInfo
+	if _, err := st.Pop(&info); err != nil {
+		return 0, SignalStack{}, err
+	}
+
+	// Restore registers.
+	c.Regs.Regs = uc.MContext.Regs
+	c.Regs.Pc = uc.MContext.Pc
+	c.Regs.Sp = uc.MContext.Sp
+	c.Regs.Pstate = uc.MContext.Pstate
+
+	return uc.Sigset, uc.Stack, nil
 }

pkg/sentry/kernel/task_signals.go

@@ -263,6 +263,19 @@ func (t *Task) deliverSignalToHandler(info *arch.SignalInfo, act arch.SignalAct)
 	if t.haveSavedSignalMask {
 		mask = t.savedSignalMask
 	}
+
+	// Set up the restorer.
+	// x86-64 should always uses SA_RESTORER, but this flag is optional on other platforms.
+	// Please see the linux code as reference:
+	// linux/arch/x86/kernel/signal.c:__setup_rt_frame()
+	// If SA_RESTORER is not configured, we can use the sigreturn trampolines
+	// the vdso provides instead.
+	// Please see the linux code as reference:
+	// linux/arch/arm64/kernel/signal.c:setup_return()
+	if act.Flags&linux.SA_RESTORER == 0 {
+		act.Restorer = t.MemoryManager().VDSOSigReturn()
+	}
+
 	if err := t.Arch().SignalSetup(st, &act, info, &alt, mask); err != nil {
 		return err
 	}