From 8a168f2be5d90f972975abf0b40145a75c0231b7 Mon Sep 17 00:00:00 2001
From: dzwdz
Date: Wed, 21 Jul 2021 21:35:20 +0200
Subject: syscall parameter & return value passing

Sadly, sysenter on i386 limits me to only 4 arguments (so, 1 for the
syscall id + 3 real args). If that turns out to be an issue I'll either
just switch to interrupts, or switch to x64.
---
 src/kernel/arch/generic.h       |  2 +-
 src/kernel/arch/i386/sysenter.s |  9 +++++++++
 src/kernel/syscalls.c           | 11 ++++++++++-
 3 files changed, 20 insertions(+), 2 deletions(-)

(limited to 'src/kernel')

diff --git a/src/kernel/arch/generic.h b/src/kernel/arch/generic.h
index 08f26a7..0b005a2 100644
--- a/src/kernel/arch/generic.h
+++ b/src/kernel/arch/generic.h
@@ -17,7 +17,7 @@ void halt_cpu();
 // src/arch/i386/sysenter.s
 void sysexit(void (*fun)(), void *stack_top);
 void sysenter_setup();
-void syscall_handler();
+int syscall_handler(int, int, int, int);
 
 // all of those can allocate memory
 struct pagedir *pagedir_new();
diff --git a/src/kernel/arch/i386/sysenter.s b/src/kernel/arch/i386/sysenter.s
index 3fcd4c3..7375e06 100644
--- a/src/kernel/arch/i386/sysenter.s
+++ b/src/kernel/arch/i386/sysenter.s
@@ -49,6 +49,10 @@ sysenter_setup:
 
 sysenter_handler:
 	pushal
+	push %edi
+	push %esi
+	push %ebx
+	push %eax
 
 	mov %cr0, %eax
 	and $0x7FFFFFFF, %eax  // disable paging
@@ -56,9 +60,14 @@ sysenter_handler:
 
 	call syscall_handler
 
+	// save the return value
+	mov %eax, 44(%esp) // 16 [top of eflags] + 7*4 [skip until EAX]
+	mov %edx, 32(%esp) // 16                 + 4*4 [skip until EBX]
+
 	mov %cr0, %eax
 	or  $0x80000000, %eax  // enable paging
 	mov %eax, %cr0
 
+	add $16, %esp
 	popal
 	sysexit
diff --git a/src/kernel/syscalls.c b/src/kernel/syscalls.c
index 25b8eb5..a42dc44 100644
--- a/src/kernel/syscalls.c
+++ b/src/kernel/syscalls.c
@@ -1,6 +1,15 @@
 #include <kernel/arch/generic.h>
 #include <kernel/panic.h>
 
-void syscall_handler() {
+int syscall_handler(int a, int b, int c, int d) {
+	// verify that the parameters get passed correctly
+	if (a != 1) panic();
+	if (b != 2) panic();
+	if (c != 3) panic();
+	if (d != 4) panic();
+
 	log_const("in a syscall!");
+
+	// used to check if the return value gets passed correctly
+	return 0x4e;
 }
-- 
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