4 files changed, 36 insertions, 1 deletions
diff --git a/pkg/sentry/arch/arch.go b/pkg/sentry/arch/arch.go
index 1d11cc472..a903d031c 100644
--- a/pkg/sentry/arch/arch.go
+++ b/pkg/sentry/arch/arch.go
@@ -88,6 +88,9 @@ type Context interface {
 	// SyscallNo returns the syscall number.
 	SyscallNo() uintptr
 
+	// SyscallSaveOrig save orignal register value.
+	SyscallSaveOrig()
+
 	// SyscallArgs returns the syscall arguments in an array.
 	SyscallArgs() SyscallArguments
 
diff --git a/pkg/sentry/arch/syscalls_amd64.go b/pkg/sentry/arch/syscalls_amd64.go
index 8b4f23007..3859f41ee 100644
--- a/pkg/sentry/arch/syscalls_amd64.go
+++ b/pkg/sentry/arch/syscalls_amd64.go
@@ -18,6 +18,13 @@ package arch
 
 const restartSyscallNr = uintptr(219)
 
+// SyscallSaveOrig save the value of the register which is clobbered in
+// syscall handler(doSyscall()).
+//
+// Noop on x86.
+func (c *context64) SyscallSaveOrig() {
+}
+
 // SyscallNo returns the syscall number according to the 64-bit convention.
 func (c *context64) SyscallNo() uintptr {
 	return uintptr(c.Regs.Orig_rax)
diff --git a/pkg/sentry/arch/syscalls_arm64.go b/pkg/sentry/arch/syscalls_arm64.go
index dc13b6124..92d062513 100755
--- a/pkg/sentry/arch/syscalls_arm64.go
+++ b/pkg/sentry/arch/syscalls_arm64.go
@@ -18,6 +18,17 @@ package arch
 
 const restartSyscallNr = uintptr(128)
 
+// SyscallSaveOrig save the value of the register R0 which is clobbered in
+// syscall handler(doSyscall()).
+//
+// In linux, at the entry of the syscall handler(el0_svc_common()), value of R0
+// is saved to the pt_regs.orig_x0 in kernel code. But currently, the orig_x0
+// was not accessible to the user space application, so we have to do the same
+// operation in the sentry code to save the R0 value into the App context.
+func (c *context64) SyscallSaveOrig() {
+	c.OrigR0 = c.Regs.Regs[0]
+}
+
 // SyscallNo returns the syscall number according to the 64-bit convention.
 func (c *context64) SyscallNo() uintptr {
 	return uintptr(c.Regs.Regs[8])
@@ -40,7 +51,7 @@ func (c *context64) SyscallNo() uintptr {
 // R30: the link register.
 func (c *context64) SyscallArgs() SyscallArguments {
 	return SyscallArguments{
-		SyscallArgument{Value: uintptr(c.Regs.Regs[0])},
+		SyscallArgument{Value: uintptr(c.OrigR0)},
 		SyscallArgument{Value: uintptr(c.Regs.Regs[1])},
 		SyscallArgument{Value: uintptr(c.Regs.Regs[2])},
 		SyscallArgument{Value: uintptr(c.Regs.Regs[3])},
diff --git a/pkg/sentry/kernel/task_syscall.go b/pkg/sentry/kernel/task_syscall.go
index d555d69a8..3d7a734ef 100644
--- a/pkg/sentry/kernel/task_syscall.go
+++ b/pkg/sentry/kernel/task_syscall.go
@@ -194,6 +194,19 @@ func (t *Task) executeSyscall(sysno uintptr, args arch.SyscallArguments) (rval u
 //
 // The syscall path is very hot; avoid defer.
 func (t *Task) doSyscall() taskRunState {
+	// Save value of the register which is clobbered in the following
+	// t.Arch().SetReturn(-ENOSYS) operation. This is dedicated to arm64.
+	//
+	// On x86, register rax was shared by syscall number and return
+	// value, and at the entry of the syscall handler, the rax was
+	// saved to regs.orig_rax which was exposed to user space.
+	// But on arm64, syscall number was passed through X8, and the X0
+	// was shared by the first syscall argument and return value. The
+	// X0 was saved to regs.orig_x0 which was not exposed to user space.
+	// So we have to do the same operation here to save the X0 value
+	// into the task context.
+	t.Arch().SyscallSaveOrig()
+
 	sysno := t.Arch().SyscallNo()
 	args := t.Arch().SyscallArgs()
 
@@ -269,6 +282,7 @@ func (*runSyscallAfterSyscallEnterStop) execute(t *Task) taskRunState {
 		return (*runSyscallExit)(nil)
 	}
 	args := t.Arch().SyscallArgs()
+
 	return t.doSyscallInvoke(sysno, args)
 }