build: keep elf64 headers in memory image

We need the headers so we can provide the symbols to loaded libraries.
This means we cannot use objcopy to munge the binary format to elf32-i386,
as the loader doesn't understand this format.

Instead, add a separate 32-bit only stage (in elf32-i386 format) that loads
the 64-bit image into memory as its payload.  This keeps multiboot happy and
leave our headers alone.

Makefile

@@ -6,8 +6,11 @@ autodepend = -MD $(@.o=.d) -MT $@
 
 all: loader.bin
 
-loader.bin: loader.elf
-	objcopy -O elf32-i386 $^ $@
+loader.bin: arch/x64/boot32.o arch/x64/loader32.ld
+	$(LD) -nostartfiles -static -nodefaultlibs -o $@ \
+	    $(filter-out %.bin, $(^:%.ld=-T %.ld))
+
+arch/x64/boot32.o: loader.elf
 
 fs = fs/fs.o fs/bootfs.o bootfs.o
 

arch/x64/boot.s

@@ -2,14 +2,6 @@
 .text
 .code32
 
-mb_magic = 0x1BADB002
-mb_flags = 0x00000001    # flags: 4k module alignment
-
-multiboot:
-    .long mb_magic
-    .long mb_flags
-    .long 0-mb_magic-mb_flags
-
 .data
 .align 4096
 ident_pt_l4:

arch/x64/boot32.s

+.text
+.code32
+
+mb_magic = 0x1BADB002
+mb_flags = 0x00000001    # flags: 4k module alignment
+
+multiboot:
+    .long mb_magic
+    .long mb_flags
+    .long 0-mb_magic-mb_flags
+
+.global start32
+start32:
+	jmp *24+elf64_header
+
+.align 4096
+
+elf64_header:
+.incbin "loader.elf"

arch/x64/loader32.ld

+OUTPUT_FORMAT(elf32-i386)
+
+SECTIONS
+{
+    .text 0x1ff000 : { *(.text*) }
+}
+ENTRY(start32);