[kernel] Update kernel binary's header structure

The kernel's file header has not been verified for a long time. This
change returns file verification to the bootloader to make sure the ELF
loaded in position 0 is actually the kernel.

src/boot/loader.cpp

@@ -113,5 +113,29 @@ load_program(
 	program.entrypoint = header->entrypoint;
 }
 
+void
+verify_kernel_header(
+	init::program &program,
+	uefi::boot_services *bs)
+{
+	status_line status(L"Verifying kernel header");
+
+    const init::header *header =
+        reinterpret_cast<const init::header *>(program.sections[0].phys_addr);
+
+    if (header->magic != init::header_magic)
+        error::raise(uefi::status::load_error, L"Bad kernel magic number");
+
+    if (header->length < sizeof(init::header))
+        error::raise(uefi::status::load_error, L"Bad kernel header length");
+
+    if (header->version < init::min_header_version)
+        error::raise(uefi::status::unsupported, L"Kernel header version not supported");
+
+	console::print(L"    Loaded kernel vserion: %d.%d.%d %lx\r\n",
+            header->version_major, header->version_minor, header->version_patch,
+            header->version_gitsha);
+}
+
 } // namespace loader
 } // namespace boot

src/boot/loader.h

@@ -38,5 +38,13 @@ load_program(
 	buffer data,
 	uefi::boot_services *bs);
 
+/// Verify that a loaded ELF has the j6 kernel header
+/// \arg program  The program to check for a header
+/// \arg bs       Boot services
+void
+verify_kernel_header(
+	kernel::init::program &program,
+	uefi::boot_services *bs);
+
 } // namespace loader
 } // namespace boot

src/boot/main.cpp

@@ -134,8 +134,8 @@ uefi_preboot(uefi::handle image, uefi::system_table *st)
 	init::args *args =
 		allocate_args_structure(bs, max_modules, max_programs);
 
-	args->magic = init::magic;
-	args->version = init::version;
+	args->magic = init::args_magic;
+	args->version = init::args_version;
 	args->runtime_services = rs;
 	args->acpi_table = hw::find_acpi_table(st);
 	paging::allocate_tables(args, bs);
@@ -154,6 +154,9 @@ uefi_preboot(uefi::handle image, uefi::system_table *st)
 		loader::load_program(program, desc.name, buf, bs);
 	}
 
+    // First program *must* be the kernel
+    loader::verify_kernel_header(args->programs[0], bs);
+
 	return args;
 }
 
@@ -196,8 +199,8 @@ efi_main(uefi::handle image, uefi::system_table *st)
 
 	memory::fix_frame_blocks(args);
 
-	kernel::entrypoint kentry =
-		reinterpret_cast<kernel::entrypoint>(kernel.entrypoint);
+	init::entrypoint kentry =
+		reinterpret_cast<init::entrypoint>(kernel.entrypoint);
 	status.next();
 
 	hw::setup_control_regs();

src/include/kernel_args.h

@@ -7,8 +7,12 @@
 namespace kernel {
 namespace init {
 
-constexpr uint32_t magic = 0x600dda7a;
-constexpr uint16_t version = 1;
+constexpr uint32_t args_magic = 'j6ia'; // "jsix init args"
+constexpr uint16_t args_version = 1;
+
+constexpr uint64_t header_magic = 0x4c454e52454b366aull; // 'j6KERNEL'
+constexpr uint16_t header_version = 2;
+constexpr uint16_t min_header_version = 2;
 
 enum class mod_type : uint32_t {
 	symbol_table
@@ -92,7 +96,8 @@ enum class boot_flags : uint16_t {
 	debug = 0x0001
 };
 
-struct args {
+struct args
+{
 	uint32_t magic;
 	uint16_t version;
 	boot_flags flags;
@@ -122,8 +127,24 @@ struct args {
 }
 __attribute__((aligned(alignof(max_align_t))));
 
-} // namespace init
+struct header
+{
+    uint64_t magic;
+
+    uint16_t length;
+    uint16_t version;
+
+    uint16_t version_major;
+    uint16_t version_minor;
+    uint16_t version_patch;
+    uint16_t reserved;
+
+    uint32_t version_gitsha;
+
+    uint64_t flags;
+};
 
 using entrypoint = __attribute__((sysv_abi)) void (*)(init::args *);
 
+} // namespace init
 } // namespace kernel

src/kernel/boot.s

@@ -1,16 +1,18 @@
-MAGIC	equ  0x600db007			; jsix OS header magic number
+MAGIC	equ  'j6KERNEL'	; jsix kernel header magic number
 
 section .header
-align 4
-global _header
-_header:
-	dd MAGIC			; Kernel header magic
-	dw 1				; Header version 1
-	dw 16				; Kernel header length
-	db VERSION_MAJOR	; Kernel version
-	db VERSION_MINOR
+align 8
+global _kernel_header
+_kernel_header:
+	dq MAGIC            ; Kernel header magic
+	dw 32               ; Kernel header length
+	dw 2                ; Header version 2
+	dw VERSION_MAJOR    ; Kernel version
+	dw VERSION_MINOR
 	dw VERSION_PATCH
+	dw 0                ; reserved
 	dd VERSION_GITSHA
+	dq 0                ; Flags
 
 section .text
 align 16

src/kernel/main.cpp

@@ -92,6 +92,9 @@ kernel_main(init::args *args)
 
 	cpu_validate();
 
+	kassert(args->magic == init::args_magic,
+            "Bad kernel args magic number");
+
 	log::debug(logs::boot, "jsix init args are at: %016lx", args);
 	log::debug(logs::boot, "     Memory map is at: %016lx", args->mem_map);
 	log::debug(logs::boot, "ACPI root table is at: %016lx", args->acpi_table);