X86-64: Difference between revisions

== Linking a 64-bit ELF64 kernel against a 32-bit ELF32 bootstrap (for use with Multiboot) ==

<source lang="bash">

# I assume that you've already compiled and linked your 64-bit kernel

# your 64-bit kernel

kernel64="build/kernel64.bin"

# its base address -- make sure it doesn't overlap with the kernel32 sections!

kernel64_baseaddress="0x00140000"

# the symbols from it you want available in your 32-bit loader

kernel64export32_symbols="k_GDT k_GDTptr k_PML4 k_PML3 k_PML2 k_PML1_first8M kinit64 k_multibootdata k64_sbss k64_ebss k_pages k_npages"

kernel64export32_ldscript=`mktemp`

kernel64_section=`mktemp`

# generate .kernel64 section's data

readelf -SW "$kernel64" | python -c "`cat <<EOF

import re, sys

regex=r"\[\s*\d+\]\s*(?!NULL)(\S+)\s+(PROGBITS|NOBITS)\s+([0-9a-fA-F]+)\s+([0-9a-fA-F]+)\s+([0-9a-fA-F]+)"

baseaddress=$kernel64_baseaddress

k64f = open("$kernel64", "r")

f = open("$kernel64_section", "w")

for line in sys.stdin:

r = re.search(regex, line)

if not r:

continue

section, stype, LMA, offset, size = r.groups()

LMA, offset, size = map(lambda s: int(s, 16), (LMA, offset, size))

if LMA < baseaddress:

raise ValueError("section ('%s' offset=0x%08x size=0x%08x ) at address < 0x%x" % (section, offset, size, baseaddress))

k64f.seek(offset)

if stype == "PROGBITS":

f.seek(LMA-baseaddress)

f.write(k64f.read(size))

f.close()

EOF`" || fail

# export wanted symbols from kernel64

readelf -sW "$kernel64" | python -c "`cat <<EOF

import re, sys

regex=r"\s*\:\s+([0-9a-fA-F]+)\s+\d+\s+\S+\s+\S+\s+\S+\s+\S+\s+(\S+)"

syms=filter(len,"$kernel64export32_symbols".split(" "))

f = open("$kernel64export32_ldscript", "w")

for line in sys.stdin:

r = re.search(regex, line)

if not r:

continue

val, sym = r.groups()

if not sym in syms:

continue

val = int(val, 16)

if val > 0xffffffff:

raise ValueError("symbol value must be below 0xffffffff limit")

f.write('"%s" = %s;\n' % (sym, hex(val)))

f.close()

EOF`" || fail

# now we have to build the 32-bit loader

# your actual lines may vary, change to your needs

gcc -m32 -Wall -Wextra -nostdlib -nostartfiles -nodefaultlibs -Isrc -O5 -fno-strict-aliasing -c \

src/start64/start64.c -o build/start64/start64.o

nasm -f elf32 \

src/start64/loader32.asm -o build/start64/loader32.o

# add the contents of $kernel64_section as a section in one of the 32-bit object files

objcopy --add-section .kernel64="$kernel64_section" \

--set-section-flag .kernel64=alloc,data,load,contents \

build/start64/loader32.o || fail

# link the previous two files, taking care to include (-T) the autogenerated syms file

ld -melf_i386 -T src/start64/linker.ld -T "$kernel64export32_ldscript" -o kernel.bin \

build/start64/start64.o build/start64/loader32.o || fail

</source>