4 files changed, 60 insertions, 9 deletions
diff --git a/src/kernel/arch/generic.h b/src/kernel/arch/generic.h
index 0b005a2..6200320 100644
--- a/src/kernel/arch/generic.h
+++ b/src/kernel/arch/generic.h
@@ -24,3 +24,7 @@ struct pagedir *pagedir_new();
 void pagedir_map(struct pagedir *dir, void *virt, void *phys,
                  bool user, bool writeable);
 void pagedir_switch(struct pagedir *);
+
+// return 0 on failure
+void *pagedir_virt2phys(struct pagedir *dir, const void *virt,
+                        bool user, bool writeable);
diff --git a/src/kernel/arch/i386/pagedir.c b/src/kernel/arch/i386/pagedir.c
index 8ab406d..02361e6 100644
--- a/src/kernel/arch/i386/pagedir.c
+++ b/src/kernel/arch/i386/pagedir.c
@@ -91,3 +91,30 @@ void pagedir_map(struct pagedir *dir, void *virt, void *phys,
 void pagedir_switch(struct pagedir *dir) {
 	asm volatile("mov %0, %%cr3;" : : "r" (dir) : "memory");
 }
+
+void *pagedir_virt2phys(struct pagedir *dir, const void *virt,
+                        bool user, bool writeable)
+{
+	uintptr_t virt_casted = (uintptr_t) virt;
+	uintptr_t phys;
+	uint32_t pd_idx = virt_casted >> 22;
+	uint32_t pt_idx = virt_casted >> 12 & 0x03FF;
+	struct pagetable_entry *pagetable, page;
+
+	/* DOESN'T CHECK PERMISSIONS ON PAGE DIRS, TODO
+	 * while i don't currently see a reason to set permissions
+	 * directly on page dirs, i might see one in the future.
+	 * leaving this as-is would be a security bug */
+	if (!dir->e[pd_idx].present) return 0;
+
+	pagetable = (void*)(dir->e[pd_idx].address << 11);
+	page      = pagetable[pt_idx];
+
+	if (!page.present)                return 0;
+	if (user      && !page.user)      return 0;
+	if (writeable && !page.writeable) return 0;
+
+	phys  = page.address << 11;
+	phys |= (uintptr_t)virt & 0xFFF;
+	return (void*)phys;
+}
diff --git a/src/kernel/syscalls.c b/src/kernel/syscalls.c
index a42dc44..2af8cf9 100644
--- a/src/kernel/syscalls.c
+++ b/src/kernel/syscalls.c
@@ -1,15 +1,29 @@
 #include <kernel/arch/generic.h>
 #include <kernel/panic.h>
+#include <kernel/proc.h>
+#include <kernel/syscalls.h>
+#include <stdint.h>
 
-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();
+int sc_debuglog(const char *msg, size_t len) {
+	struct pagedir *pages = process_current->pages;
+	void *phys = pagedir_virt2phys(pages, msg, true, false);
 
-	log_const("in a syscall!");
+	// page overrun check
+	if (((uintptr_t)msg & PAGE_MASK) + len > PAGE_SIZE)
+		len = PAGE_SIZE - ((uintptr_t)msg & PAGE_MASK);
+	if (((uintptr_t)msg & PAGE_MASK) + len > PAGE_SIZE)
+		panic(); // just in case I made an off by 1 error
 
-	// used to check if the return value gets passed correctly
-	return 0x4e;
+	log_write(phys, len);
+	return len;
+}
+
+int syscall_handler(int num, int a, int b, int c) {
+	switch (num) {
+		case SC_DEBUGLOG:
+			return sc_debuglog((void*)a, b);
+		default:
+			log_const("unknown syscall ");
+			panic();
+	}
 }
diff --git a/src/kernel/syscalls.h b/src/kernel/syscalls.h
new file mode 100644
index 0000000..39db6a9
--- /dev/null
+++ b/src/kernel/syscalls.h
@@ -0,0 +1,6 @@
+#pragma once
+
+// not caring about stable syscall numbers just yet
+enum {
+	SC_DEBUGLOG
+};