boot: Add menu option to disable experimental kaslr

boot/config.c

@@ -105,6 +105,10 @@ int kmain() {
 			"Enables write-combining PAT configuration for",
 			"framebuffers. Toggle if graphics are slow.");
 
+	BOOT_OPTION(_kaslr,       1, "KASLR (experimental)",
+			"Enables rudimentary randomization of the kernel",
+			"load address within a small range.");
+
 	while (1) {
 		/* Loop over rendering the menu */
 		show_menu();
@@ -171,6 +175,9 @@ int kmain() {
 			strcat(cmdline, "lfbwc ");
 		}
 
+		extern int disable_kaslr;
+		disable_kaslr = !_kaslr;
+
 		if (!boot_edit) break;
 		if (boot_editor()) break;
 

boot/multiboot.c

@@ -59,6 +59,7 @@ static uintptr_t ramdisk_off = 0;
 static uintptr_t ramdisk_len = 0;
 uintptr_t final_offset = 0;
 uintptr_t _xmain = 0;
+int disable_kaslr = 0;
 
 static inline uint64_t read_tsc(void) {
 	uint32_t lo, hi;
@@ -70,8 +71,12 @@ static int load_aout(uint32_t * hdr) {
 	uintptr_t base_offset = (uintptr_t)hdr - (uintptr_t)kernel_load_start;
 	uintptr_t hdr_offset  = hdr[3] - base_offset;
 	uint32_t  rando = 0;
-	asm volatile ( "rdtsc" : "=a"(rando), "=d"((uint32_t){0}) );
-	size_t    xtra = (rando & 0xFF) << 12;
+	size_t    xtra = 0;
+
+	if (!disable_kaslr) {
+		asm volatile ( "rdtsc" : "=a"(rando), "=d"((uint32_t){0}) );
+		xtra = (rando & 0xFF) << 12;
+	}
 
 	memcpy((void*)(uintptr_t)hdr[4] + xtra, kernel_load_start + (hdr[4] - hdr_offset), (hdr[5] - hdr[4]));
 	memset((void*)(uintptr_t)hdr[5] + xtra, 0, (hdr[6] - hdr[5]));