x86/boot: Drop CRC-32 checksum and the build tool that generates it

Apart from some sanity checks on the size of setup.bin, the only
remaining task carried out by the arch/x86/boot/tools/build.c build tool
is generating the CRC-32 checksum of the bzImage. This feature was added
in commit

7d6e737c8d2698b6 ("x86: add a crc32 checksum to the kernel image.")

without any motivation (or any commit log text, for that matter). This
checksum is not verified by any known bootloader, and given that

a) the checksum of the entire bzImage is reported by most tools (zlib,
rhash) as 0xffffffff and not 0x0 as documented,

b) the checksum is corrupted when the image is signed for secure boot,
which means that no distro ships x86 images with valid CRCs,

it seems quite unlikely that this checksum is being used, so let's just
drop it, along with the tool that generates it.

Instead, use simple file concatenation and truncation to combine the two
pieces into bzImage, and replace the checks on the size of the setup
block with a couple of ASSERT()s in the linker script.

Signed-off-by: Ard Biesheuvel <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Cc: "H. Peter Anvin" <[e-mail]>
Cc: Ian Campbell <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/arch/x86/boot.rst

···
   This field contains maximal allowed type for setup_data and setup_indirect structs.
 
 
-The Image Checksum
-==================
-
-From boot protocol version 2.08 onwards the CRC-32 is calculated over
-the entire file using the characteristic polynomial 0x04C11DB7 and an
-initial remainder of 0xffffffff.  The checksum is appended to the
-file; therefore the CRC of the file up to the limit specified in the
-syssize field of the header is always 0.
-
-
 The Kernel Command Line
 =======================
 

arch/x86/boot/Makefile

···
 setup-y		+= video-bios.o
 
 targets		+= $(setup-y)
-hostprogs	:= tools/build
 hostprogs	+= mkcpustr
 
 HOST_EXTRACFLAGS += -I$(srctree)/tools/include \
···
 $(obj)/bzImage: asflags-y  := $(SVGA_MODE)
 
 quiet_cmd_image = BUILD   $@
-silent_redirect_image = >/dev/null
-cmd_image = $(obj)/tools/build $(obj)/setup.bin $(obj)/vmlinux.bin \
-			       $(obj)/zoffset.h $@ $($(quiet)redirect_image)
+      cmd_image = cp $< $@; truncate -s %4K $@; cat $(obj)/vmlinux.bin >>$@
 
-$(obj)/bzImage: $(obj)/setup.bin $(obj)/vmlinux.bin $(obj)/tools/build FORCE
+$(obj)/bzImage: $(obj)/setup.bin $(obj)/vmlinux.bin FORCE
 	$(call if_changed,image)
 	@$(kecho) 'Kernel: $@ is ready' ' (#'$(or $(KBUILD_BUILD_VERSION),`cat .version`)')'
 

arch/x86/boot/compressed/vmlinux.lds.S

···
 		*(.data)
 		*(.data.*)
 
-		/* Add 4 bytes of extra space for a CRC-32 checksum */
-		. = ALIGN(. + 4, 0x200);
+		. = ALIGN(0x200);
 		_edata = . ;
 	}
 	. = ALIGN(L1_CACHE_BYTES);

arch/x86/boot/setup.ld

···
 
 		setup_size = ALIGN(ABSOLUTE(.), 4096);
 		setup_sects = ABSOLUTE(setup_size / 512);
+		ASSERT(setup_sects >= 5, "The setup must be at least 5 sectors in size");
+		ASSERT(setup_sects <= 64, "The setup must be at most 64 sectors in size");
 	}
 
 	. = ALIGN(16);

arch/x86/boot/tools/.gitignore

···
-# SPDX-License-Identifier: GPL-2.0-only
-build

arch/x86/boot/tools/build.c

···
-// SPDX-License-Identifier: GPL-2.0
-/*
- *  Copyright (C) 1991, 1992  Linus Torvalds
- *  Copyright (C) 1997 Martin Mares
- *  Copyright (C) 2007 H. Peter Anvin
- */
-
-/*
- * This file builds a disk-image from three different files:
- *
- * - setup: 8086 machine code, sets up system parm
- * - system: 80386 code for actual system
- * - zoffset.h: header with ZO_* defines
- *
- * It does some checking that all files are of the correct type, and writes
- * the result to the specified destination, removing headers and padding to
- * the right amount. It also writes some system data to stdout.
- */
-
-/*
- * Changes by tytso to allow root device specification
- * High loaded stuff by Hans Lermen & Werner Almesberger, Feb. 1996
- * Cross compiling fixes by Gertjan van Wingerde, July 1996
- * Rewritten by Martin Mares, April 1997
- * Substantially overhauled by H. Peter Anvin, April 2007
- */
-
-#include <stdio.h>
-#include <string.h>
-#include <stdlib.h>
-#include <stdarg.h>
-#include <sys/types.h>
-#include <sys/stat.h>
-#include <unistd.h>
-#include <fcntl.h>
-#include <sys/mman.h>
-#include <tools/le_byteshift.h>
-
-typedef unsigned char  u8;
-typedef unsigned short u16;
-typedef unsigned int   u32;
-
-/* Minimal number of setup sectors */
-#define SETUP_SECT_MIN 5
-#define SETUP_SECT_MAX 64
-
-/* This must be large enough to hold the entire setup */
-u8 buf[SETUP_SECT_MAX*512];
-
-static unsigned long _edata;
-
-/*----------------------------------------------------------------------*/
-
-static const u32 crctab32[] = {
-	0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419,
-	0x706af48f, 0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4,
-	0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd, 0xe7b82d07,
-	0x90bf1d91, 0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de,
-	0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7, 0x136c9856,
-	0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9,
-	0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4,
-	0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
-	0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3,
-	0x45df5c75, 0xdcd60dcf, 0xabd13d59, 0x26d930ac, 0x51de003a,
-	0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423, 0xcfba9599,
-	0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
-	0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190,
-	0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f,
-	0x9fbfe4a5, 0xe8b8d433, 0x7807c9a2, 0x0f00f934, 0x9609a88e,
-	0xe10e9818, 0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,
-	0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed,
-	0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950,
-	0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3,
-	0xfbd44c65, 0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2,
-	0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a,
-	0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5,
-	0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa, 0xbe0b1010,
-	0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
-	0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17,
-	0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6,
-	0x03b6e20c, 0x74b1d29a, 0xead54739, 0x9dd277af, 0x04db2615,
-	0x73dc1683, 0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8,
-	0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1, 0xf00f9344,
-	0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb,
-	0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a,
-	0x67dd4acc, 0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
-	0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252, 0xd1bb67f1,
-	0xa6bc5767, 0x3fb506dd, 0x48b2364b, 0xd80d2bda, 0xaf0a1b4c,
-	0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55, 0x316e8eef,
-	0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
-	0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe,
-	0xb2bd0b28, 0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31,
-	0x2cd99e8b, 0x5bdeae1d, 0x9b64c2b0, 0xec63f226, 0x756aa39c,
-	0x026d930a, 0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
-	0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38, 0x92d28e9b,
-	0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242,
-	0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1,
-	0x18b74777, 0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c,
-	0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45, 0xa00ae278,
-	0xd70dd2ee, 0x4e048354, 0x3903b3c2, 0xa7672661, 0xd06016f7,
-	0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc, 0x40df0b66,
-	0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
-	0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605,
-	0xcdd70693, 0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8,
-	0x5d681b02, 0x2a6f2b94, 0xb40bbe37, 0xc30c8ea1, 0x5a05df1b,
-	0x2d02ef8d
-};
-
-static u32 partial_crc32_one(u8 c, u32 crc)
-{
-	return crctab32[(crc ^ c) & 0xff] ^ (crc >> 8);
-}
-
-static u32 partial_crc32(const u8 *s, int len, u32 crc)
-{
-	while (len--)
-		crc = partial_crc32_one(*s++, crc);
-	return crc;
-}
-
-static void die(const char * str, ...)
-{
-	va_list args;
-	va_start(args, str);
-	vfprintf(stderr, str, args);
-	va_end(args);
-	fputc('\n', stderr);
-	exit(1);
-}
-
-static void usage(void)
-{
-	die("Usage: build setup system zoffset.h image");
-}
-
-/*
- * Parse zoffset.h and find the entry points. We could just #include zoffset.h
- * but that would mean tools/build would have to be rebuilt every time. It's
- * not as if parsing it is hard...
- */
-#define PARSE_ZOFS(p, sym) do { \
-	if (!strncmp(p, "#define ZO_" #sym " ", 11+sizeof(#sym)))	\
-		sym = strtoul(p + 11 + sizeof(#sym), NULL, 16);		\
-} while (0)
-
-static void parse_zoffset(char *fname)
-{
-	FILE *file;
-	char *p;
-	int c;
-
-	file = fopen(fname, "r");
-	if (!file)
-		die("Unable to open `%s': %m", fname);
-	c = fread(buf, 1, sizeof(buf) - 1, file);
-	if (ferror(file))
-		die("read-error on `zoffset.h'");
-	fclose(file);
-	buf[c] = 0;
-
-	p = (char *)buf;
-
-	while (p && *p) {
-		PARSE_ZOFS(p, _edata);
-
-		p = strchr(p, '\n');
-		while (p && (*p == '\r' || *p == '\n'))
-			p++;
-	}
-}
-
-int main(int argc, char ** argv)
-{
-	unsigned int i, sz, setup_sectors;
-	int c;
-	struct stat sb;
-	FILE *file, *dest;
-	int fd;
-	void *kernel;
-	u32 crc = 0xffffffffUL;
-
-	if (argc != 5)
-		usage();
-	parse_zoffset(argv[3]);
-
-	dest = fopen(argv[4], "w");
-	if (!dest)
-		die("Unable to write `%s': %m", argv[4]);
-
-	/* Copy the setup code */
-	file = fopen(argv[1], "r");
-	if (!file)
-		die("Unable to open `%s': %m", argv[1]);
-	c = fread(buf, 1, sizeof(buf), file);
-	if (ferror(file))
-		die("read-error on `setup'");
-	if (c < 1024)
-		die("The setup must be at least 1024 bytes");
-	if (get_unaligned_le16(&buf[510]) != 0xAA55)
-		die("Boot block hasn't got boot flag (0xAA55)");
-	fclose(file);
-
-	/* Pad unused space with zeros */
-	setup_sectors = (c + 4095) / 4096;
-	setup_sectors *= 8;
-	if (setup_sectors < SETUP_SECT_MIN)
-		setup_sectors = SETUP_SECT_MIN;
-	i = setup_sectors*512;
-	memset(buf+c, 0, i-c);
-
-	/* Open and stat the kernel file */
-	fd = open(argv[2], O_RDONLY);
-	if (fd < 0)
-		die("Unable to open `%s': %m", argv[2]);
-	if (fstat(fd, &sb))
-		die("Unable to stat `%s': %m", argv[2]);
-	if (_edata != sb.st_size)
-		die("Unexpected file size `%s': %u != %u", argv[2], _edata,
-		    sb.st_size);
-	sz = _edata - 4;
-	kernel = mmap(NULL, sz, PROT_READ, MAP_SHARED, fd, 0);
-	if (kernel == MAP_FAILED)
-		die("Unable to mmap '%s': %m", argv[2]);
-
-	crc = partial_crc32(buf, i, crc);
-	if (fwrite(buf, 1, i, dest) != i)
-		die("Writing setup failed");
-
-	/* Copy the kernel code */
-	crc = partial_crc32(kernel, sz, crc);
-	if (fwrite(kernel, 1, sz, dest) != sz)
-		die("Writing kernel failed");
-
-	/* Write the CRC */
-	put_unaligned_le32(crc, buf);
-	if (fwrite(buf, 1, 4, dest) != 4)
-		die("Writing CRC failed");
-
-	/* Catch any delayed write failures */
-	if (fclose(dest))
-		die("Writing image failed");
-
-	close(fd);
-
-	/* Everything is OK */
-	return 0;
-}