Commit 5bbf89fc authored by Rusty Russell's avatar Rusty Russell
Browse files

Loading bzImage directly. 



Now arch/i386/boot/compressed/head.S understands the hardware_platform field,
we can directly execute bzImages.  No more horrific unpacking code.

Signed-off-by: default avatarRusty Russell <rusty@rustcorp.com.au>
parent 814a0e5c

Documentation/lguest/lguest.c

+29 −64
Original line number Diff line number Diff line
@@ -326,74 +326,39 @@ static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr) 
	return ehdr->e_entry;

}



/*L:160 Unfortunately the entire ELF image isn't compressed: the segments

 * which need loading are extracted and compressed raw.  This denies us the

 * information we need to make a fully-general loader. */

static unsigned long unpack_bzimage(int fd)

{

	gzFile f;

	int ret, len = 0;

	/* A bzImage always gets loaded at physical address 1M.  This is

	 * actually configurable as CONFIG_PHYSICAL_START, but as the comment

	 * there says, "Don't change this unless you know what you are doing".

	 * Indeed. */

	void *img = from_guest_phys(0x100000);



	/* gzdopen takes our file descriptor (carefully placed at the start of

	 * the GZIP header we found) and returns a gzFile. */

	f = gzdopen(fd, "rb");

	/* We read it into memory in 64k chunks until we hit the end. */

	while ((ret = gzread(f, img + len, 65536)) > 0)

		len += ret;

	if (ret < 0)

		err(1, "reading image from bzImage");



	verbose("Unpacked size %i addr %p\n", len, img);



	/* The entry point for a bzImage is always the first byte */

	return (unsigned long)img;

}



/*L:150 A bzImage, unlike an ELF file, is not meant to be loaded.  You're

 * supposed to jump into it and it will unpack itself.  We can't do that

 * because the Guest can't run the unpacking code, and adding features to

 * lguest kills puppies, so we don't want to.

 * supposed to jump into it and it will unpack itself.  We used to have to

 * perform some hairy magic because the unpacking code scared me.

 *

 * The bzImage is formed by putting the decompressing code in front of the

 * compressed kernel code.  So we can simple scan through it looking for the

 * first "gzip" header, and start decompressing from there. */

 * Fortunately, Jeremy Fitzhardinge convinced me it wasn't that hard and wrote

 * a small patch to jump over the tricky bits in the Guest, so now we just read

 * the funky header so we know where in the file to load, and away we go! */

static unsigned long load_bzimage(int fd)

{

	unsigned char c;

	int state = 0;



	/* GZIP header is 0x1F 0x8B <method> <flags>... <compressed-by>. */

	while (read(fd, &c, 1) == 1) {

		switch (state) {

		case 0:

			if (c == 0x1F)

				state++;

			break;

		case 1:

			if (c == 0x8B)

				state++;

			else

				state = 0;

			break;

		case 2 ... 8:

			state++;

			break;

		case 9:

			/* Seek back to the start of the gzip header. */

			lseek(fd, -10, SEEK_CUR);

			/* One final check: "compressed under UNIX". */

			if (c != 0x03)

				state = -1;

			else

				return unpack_bzimage(fd);

		}

	}

	errx(1, "Could not find kernel in bzImage");

	u8 hdr[1024];

	int r;

	/* Modern bzImages get loaded at 1M. */

	void *p = from_guest_phys(0x100000);



	/* Go back to the start of the file and read the header.  It should be

	 * a Linux boot header (see Documentation/i386/boot.txt) */

	lseek(fd, 0, SEEK_SET);

	read(fd, hdr, sizeof(hdr));



	/* At offset 0x202, we expect the magic "HdrS" */

	if (memcmp(hdr + 0x202, "HdrS", 4) != 0)

		errx(1, "This doesn't look like a bzImage to me");



	/* The byte at 0x1F1 tells us how many extra sectors of

	 * header: skip over them all. */

	lseek(fd, (unsigned long)(hdr[0x1F1]+1) * 512, SEEK_SET);



	/* Now read everything into memory. in nice big chunks. */

	while ((r = read(fd, p, 65536)) > 0)

		p += r;



	/* Finally, 0x214 tells us where to start the kernel. */

	return *(unsigned long *)&hdr[0x214];

}



/*L:140 Loading the kernel is easy when it's a "vmlinux", but most kernels