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.

5 files changed, 27 insertions, 5 deletions

diff --git a/src/init/main.c b/src/init/main.c
index cb3fa1d..6c4ccc3 100644
--- a/src/init/main.c
+++ b/src/init/main.c
@@ -1,15 +1,15 @@
 #include <stdint.h>
 
-void _syscall();
+int _syscall(int, int, int, int);
 
 int main() {
-	_syscall();
+	uint8_t color = _syscall(1, 2, 3, 4);
 
 	// change the colors of VGA text
 	// doesn't require a lot of code, but still shows that it's working
 	uint8_t *vga = (void*) 0xB8000;
 	for (int i = 0; i < 80 * 25; i++)
-		vga[(i << 1) + 1] = 0x4e;
+		vga[(i << 1) + 1] = color;
 
 	// try to mess with kernel memory
 	uint8_t *kernel = (void*) 0x100000;
diff --git a/src/init/syscall.s b/src/init/syscall.s
index ebd9d0e..86d3523 100644
--- a/src/init/syscall.s
+++ b/src/init/syscall.s
@@ -2,8 +2,12 @@
 .global _syscall
 .type _syscall, @function
 _syscall:
+	mov 4(%esp), %eax
+	mov 8(%esp), %ebx
 	mov %esp, %ecx
 	mov $_syscall_ret, %edx
+	mov 12(%esp), %esi
+	mov 16(%esp), %edi
 	sysenter
 _syscall_ret:
 	ret
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;
 }