kernel: skip return address of <autogenerated> function wrapper in tr…

…apret before IRET instruction

NOTE: this commit will ONLY work for Go 1.17, it is not backwards
compatible with Go 1.16. We should probably find a way to handle
both (e.g. build tags or a general approach).

To give context, an <autogenerated> function wrapper is generated
in Go 1.17 for kernel.trapret.

	Dump of assembler code for function github.com/icexin/eggos/kernel.trapret<autogenerated>:
	=> 0x00000000002bbf60 <+0>:   call   0x2b9e40 <github.com/icexin/eggos/kernel.trapret>
		0x00000000002bbf65 <+5>:   xorps  %xmm15,%xmm15
		0x00000000002bbf69 <+9>:   mov    %fs:0xfffffffffffffff8,%r14
		0x00000000002bbf72 <+18>:  ret

In this autogenerated function, a call to the real kernel.trapret
is made, which places a return address on the stack. However, the
trapret function will use IRET to return, so to handle this case,
we skip the return address pushed onto the stack by the autogenerated
function by adding 8 (ptr size) to rsp.

We should definitely try to find a better way to handle this situation
as it seems very fragile and may break in the future if Go changes
their autogenerated functions.

So, once more, this is mostly to get intuition into the problem domain,
it is not meant as a solution that should be merged into eggos.

kernel/trap.s

@@ -57,5 +57,13 @@ TEXT ·trapret(SB), NOSPLIT, $0
 
 	ADDQ $16, SP // skip trapno and errcode
 
+	ADDQ $8, SP // NOTE: skip return address of <autogenerated> function (added in Go 1.17).
+
+	// Dump of assembler code for function github.com/icexin/eggos/kernel.trapret<autogenerated>:
+	// => 0x00000000002bbf60 <+0>:   call   0x2b9e40 <github.com/icexin/eggos/kernel.trapret>
+	//    0x00000000002bbf65 <+5>:   xorps  %xmm15,%xmm15
+	//    0x00000000002bbf69 <+9>:   mov    %fs:0xfffffffffffffff8,%r14
+	//    0x00000000002bbf72 <+18>:  ret
+
 	IRETQ