Merge pull request #615 from tkyymmt/patch3

Add UNWIND_HINT_IRET_REGS feature description

Initialization/linux-initialization-2.md

@@ -256,24 +256,29 @@ Okay, now we have filled and loaded `Interrupt Descriptor Table`, we know how th
 Early interrupts handlers
 --------------------------------------------------------------------------------
 
-As you can read above, we filled `IDT` with the address of the `early_idt_handler_array`. We can find it in the [arch/x86/kernel/head_64.S](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/head_64.S) assembly file:
+As you can read above, we filled `IDT` with the address of the `early_idt_handler_array`. We can find it in the [arch/x86/kernel/head_64.S](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/head_64.S) assembly file:
 
 ```assembly
-	.globl early_idt_handler_array
-early_idt_handlers:
+ENTRY(early_idt_handler_array)
 	i = 0
 	.rept NUM_EXCEPTION_VECTORS
-	.if (EXCEPTION_ERRCODE_MASK >> i) & 1
-	pushq $0
+	.if ((EXCEPTION_ERRCODE_MASK >> i) & 1) == 0
+		UNWIND_HINT_IRET_REGS
+		pushq $0	# Dummy error code, to make stack frame uniform
+	.else
+		UNWIND_HINT_IRET_REGS offset=8
 	.endif
-	pushq $i
+	pushq $i		# 72(%rsp) Vector number
 	jmp early_idt_handler_common
+	UNWIND_HINT_IRET_REGS
 	i = i + 1
 	.fill early_idt_handler_array + i*EARLY_IDT_HANDLER_SIZE - ., 1, 0xcc
 	.endr
+	UNWIND_HINT_IRET_REGS offset=16
+END(early_idt_handler_array)
 ```
 
-We can see here, interrupt handlers generation for the first `32` exceptions. We check here, if exception has an error code then we do nothing, if exception does not return error code, we push zero to the stack. We do it for that would stack was uniform. After that we push exception number on the stack and jump on the `early_idt_handler_array` which is generic interrupt handler for now. As we may see above, every nine bytes of the `early_idt_handler_array` array consists from optional push of an error code, push of `vector number` and jump instruction. We can see it in the output of the `objdump` util:
+Functions which tend to not be called directly by other functions, such as syscall and interrupt handlers, often do unusual non-C-function-type things with the stack pointer. Such code needs to be annotated by using `UNWIND_HINT_IRET_REGS` macro such that objtool can understand it. We can see here, interrupt handlers generation for the first `32` exceptions. We check here, if exception has an error code then we do nothing, if exception does not return error code, we push zero to the stack. We do it for that would stack was uniform. After that we push exception number on the stack and jump on the `early_idt_handler_array` which is generic interrupt handler for now. As we may see above, every nine bytes of the `early_idt_handler_array` array consists from optional push of an error code, push of `vector number` and jump instruction. We can see it in the output of the `objdump` util:
 
 ```
 $ objdump -D vmlinux