[kernel] Add fake preludes to isr handler to trick GDB

By adding more debug information to the symbols and adding function
frame preludes to the isr handler assembly functions, GDB sees them as
valid locations for stack frames, and can display backtraces through
interrupts.

src/kernel/interrupts.s

@@ -1,8 +1,14 @@
 %include "push_all.inc"
 
+section .text
+
 extern isr_handler
-global isr_handler_prelude
+global isr_handler_prelude:function (isr_handler_prelude.end - isr_handler_prelude)
 isr_handler_prelude:
+	push rbp     ; Never executed, fake function prelude
+	mov rbp, rsp ; to calm down gdb
+
+.real:
 	push_all
 	check_swap_gs
 
@@ -10,10 +16,15 @@ isr_handler_prelude:
 	mov rsi, rsp
 	call isr_handler
 	jmp isr_handler_return
+.end:
 
 extern irq_handler
-global irq_handler_prelude
+global irq_handler_prelude:function (irq_handler_prelude.end - irq_handler_prelude)
 irq_handler_prelude:
+	push rbp     ; Never executed, fake function prelude
+	mov rbp, rsp ; to calm down gdb
+
+.real:
 	push_all
 	check_swap_gs
 
@@ -21,36 +32,41 @@ irq_handler_prelude:
 	mov rsi, rsp
 	call irq_handler
 	; fall through to isr_handler_return
+.end:
 
-global isr_handler_return
+global isr_handler_return:function (isr_handler_return.end - isr_handler_return)
 isr_handler_return:
 	check_swap_gs
 	pop_all
 
 	add rsp, 16		; because the ISRs added err/num
 	iretq
+.end:
 
 %macro EMIT_ISR 2
-	global %1
+	global %1:function (%1.end - %1)
 	%1:
 		push 0
 		push %2
-		jmp isr_handler_prelude
+		jmp isr_handler_prelude.real
+	.end:
 %endmacro
 
 %macro EMIT_EISR 2
-	global %1
+	global %1:function (%1.end - %1)
 	%1:
 		push %2
-		jmp isr_handler_prelude
+		jmp isr_handler_prelude.real
+	.end:
 %endmacro
 
 %macro EMIT_IRQ 2
-	global %1
+	global %1:function (%1.end - %1)
 	%1:
 		push 0
 		push %2
-		jmp irq_handler_prelude
+		jmp irq_handler_prelude.real
+	.end:
 %endmacro
 
 %define EISR(i, s, name)     EMIT_EISR name, i   ; ISR with error code