cross-compile a Linux kernel on Darwin using Nix

linux.nix

let cross = self: super: {

	# https://github.com/NixOS/nixpkgs/pull/113225
	buildLinux = attrs: self.callPackage "${self.fetchFromGitHub {
		owner = "mroi";
		repo = "nixpkgs";
		rev = "patch-linux";
		sha256 = "07i6wwf4vh5ahnkp3wvyzdiaqhrxakjjxbdrlwdviis777p9gl5v";
	}}/pkgs/os-specific/linux/kernel/generic.nix" attrs;

	# apply patch from https://lkml.org/lkml/2020/12/25/97
	# the Linux kernel assumes a glibc build environment:
	# fix compilation of build-time tools on Darwin, provide extra headers
	# remove nettools from inputs, it requires an outdated openssl and is not needed
	linux = super.linux.overrideAttrs (attrs: {
		patches = attrs.patches ++ [(self.writeText "posix-regex.patch" ''
			--- a/arch/x86/tools/relocs.c
			+++ b/arch/x86/tools/relocs.c
			@@ -57,12 +57,12 @@
				[S_REL] =
				"^(__init_(begin|end)|"
				"__x86_cpu_dev_(start|end)|"
			-	"(__parainstructions|__alt_instructions)(|_end)|"
			-	"(__iommu_table|__apicdrivers|__smp_locks)(|_end)|"
			+	"(__parainstructions|__alt_instructions)(()|_end)|"
			+	"(__iommu_table|__apicdrivers|__smp_locks)(()|_end)|"
				"__(start|end)_pci_.*|"
				"__(start|end)_builtin_fw|"
			-	"__(start|stop)___ksymtab(|_gpl|_unused|_unused_gpl|_gpl_future)|"
			-	"__(start|stop)___kcrctab(|_gpl|_unused|_unused_gpl|_gpl_future)|"
			+	"__(start|stop)___ksymtab(()|_gpl|_unused|_unused_gpl|_gpl_future)|"
			+	"__(start|stop)___kcrctab(()|_gpl|_unused|_unused_gpl|_gpl_future)|"
				"__(start|stop)___param|"
				"__(start|stop)___modver|"
				"__(start|stop)___bug_table|"
		'') (self.writeText "darwin-build-fixes.patch" ''
			--- a/scripts/mod/file2alias.c
			+++ b/scripts/mod/file2alias.c
			@@ -44,7 +44,8 @@
			 } uuid_le;
			 typedef struct {
				__u8 b[16];
			-} uuid_t;
			+} uuid_be;
			+#define uuid_t uuid_be
			 #define	UUID_STRING_LEN		36
			 
			 /* Big exception to the "don't include kernel headers into userspace, which
			--- a/tools/scripts/Makefile.include
			+++ b/tools/scripts/Makefile.include
			@@ -39,7 +39,7 @@
			 EXTRA_WARNINGS += -Wwrite-strings
			 EXTRA_WARNINGS += -Wformat
			 
			-CC_NO_CLANG := $(shell $(CC) -dM -E -x c /dev/null | grep -Fq "__clang__"; echo $$?)
			+CC_NO_CLANG := $(shell $(HOSTCC) -dM -E -x c /dev/null | grep -Fq "__clang__"; echo $$?)
			 
			 # Makefiles suck: This macro sets a default value of $(2) for the
			 # variable named by $(1), unless the variable has been set by
			--- a/tools/include/asm-generic/bitops/fls.h
			+++ b/tools/include/asm-generic/bitops/fls.h
			@@ -1,5 +1,6 @@
			 /* SPDX-License-Identifier: GPL-2.0 */
			-#ifndef _ASM_GENERIC_BITOPS_FLS_H_
			+#include <string.h>
			+#if 0
			 #define _ASM_GENERIC_BITOPS_FLS_H_
			 
			 /**
			--- a/tools/objtool/elf.h
			+++ b/tools/objtool/elf.h
			@@ -11,6 +11,8 @@
			 #include <linux/list.h>
			 #include <linux/hashtable.h>
			 
			+#define R_X86_64_PC32 2
			+
			 #ifdef LIBELF_USE_DEPRECATED
			 # define elf_getshdrnum    elf_getshnum
			 # define elf_getshdrstrndx elf_getshstrndx
			--- a/tools/lib/string.c
			+++ a/tools/lib/string.c
			@@ -20,6 +20,8 @@
			 #include <linux/ctype.h>
			 #include <linux/compiler.h>
			 
			+#undef strlcpy
			+
			 /**
			  * memdup - duplicate region of memory
			  *
		'')];
		nativeBuildInputs = (self.lib.remove self.nettools attrs.nativeBuildInputs) ++ [(self.runCommand "glibc-headers" {} ''
			mkdir -p $out/include/asm
			sed 's/^#include <features.h>/#include <stdint.h>/' < ${self.glibcCross.dev}/include/elf.h > $out/include/elf.h
			cat > $out/include/endian.h <<- EOF
				#define __LITTLE_ENDIAN 1234
				#define __BIG_ENDIAN 4321
				#define __BYTE_ORDER __LITTLE_ENDIAN
			EOF
			cat > $out/include/asm/types.h <<- EOF
				typedef char __s8;
				typedef unsigned char __u8;
				typedef short __s16;
				typedef unsigned short __u16;
				typedef int __s32;
				typedef unsigned int __u32;
				typedef long long __s64;
				typedef unsigned long long __u64;
				#define BITS_PER_LONG (__CHAR_BIT__ * __SIZEOF_LONG__)
			EOF
			touch $out/include/byteswap.h $out/include/asm/posix_types.h
		'')];
	});
};
in (import <nixpkgs> {
	overlays = [ cross ];
	crossSystem = { config = "x86_64-linux"; };
}).linux

My ultimate goal is to enable HyperKit as a qemu replacement within Nix’s vmTools subsystem. This would allow using ephemeral VMs as wrappers to run things on Linux. My current state is that I can build HyperKit, VPNKit, the Linux kernel (see above), and an initrd on macOS. The resulting VM boots fine. I am now working on two things:

• update to a recent Linux kernel (the kernel build system now contains even more Linux-isms)
• get OCaml-9p running to share the host file system with the guest (I can build and run it, but it does not mount in the guest)

Once this is all working, I will post another gist. Along the way I will probably do pull requests to NixPkgs.

Of all this stuff, is there anything you think might be useful for you right now? For now, I do everything in a single file that is not even version-controlled, but I’d have no problem sharing the whole darn thing.

I updated the gist. It cross-compiles a recent Linux kernel on Darwin now. I issued pull requests for the indicated changes.

See also NixOS/nixpkgs#108984

Thanks for the pointer. Getting ephemeral Linux VMs on macOS is my ultimate goal. I want to use Nix’ vmTools on macOS.