path: root/util/cbfstool/elfheaders.c

/*
 * elf header parsing.
 *
 * Copyright (C) 2013 Google, Inc.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; version 2 of the License.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA, 02110-1301 USA
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "elf.h"
#include "common.h"
#include "cbfs.h"

/*
 * Short form: this is complicated, but we've tried making it simple
 * and we keep hitting problems with our ELF parsing.
 *
 * The ELF parsing situation has always been a bit tricky.  In fact,
 * we (and most others) have been getting it wrong in small ways for
 * years. Recently this has caused real trouble for the ARM V8 build.
 * In this file we attempt to finally get it right for all variations
 * of endian-ness and word size and target architectures and
 * architectures we might get run on. Phew!. To do this we borrow a
 * page from the FreeBSD NFS xdr model (see elf_ehdr and elf_phdr),
 * the Plan 9 endianness functions (see xdr.c), and Go interfaces (see
 * how we use buffer structs in this file). This ends up being a bit
 * wordy at the lowest level, but greatly simplifies the elf parsing
 * code and removes a common source of bugs, namely, forgetting to
 * flip type endianness when referencing a struct member.
 *
 * ELF files can have four combinations of data layout: 32/64, and
 * big/little endian.  Further, to add to the fun, depending on the
 * word size, the size of the ELF structs varies. The coreboot SELF
 * format is simpler in theory: it's supposed to be always BE, and the
 * various struct members allow room for growth: the entry point is
 * always 64 bits, for example, so the size of a SELF struct is
 * constant, regardless of target architecture word size.  Hence, we
 * need to do some transformation of the ELF files.
 *
 * A given architecture, realistically, only supports one of the four
 * combinations at a time as the 'native' format. Hence, our code has
 * been sprinkled with every variation of [nh]to[hn][sll] over the
 * years. We've never quite gotten it all right, however, and a quick
 * pass over this code revealed another bug.  It's all worked because,
 * until now, all the working platforms that had CBFS were 32 LE. Even then,
 * however, bugs crept in: we recently realized that we're not
 * transforming the entry point to big format when we store into the
 * SELF image.
 *
 * The problem is essentially an XDR operation:
 * we have something in a foreign format and need to transform it.
 * It's most like XDR because:
 * 1) the byte order can be wrong
 * 2) the word size can be wrong
 * 3) the size of elements in the stream depends on the value
 *    of other elements in the stream
 * it's not like XDR because:
 * 1) the byte order can be right
 * 2) the word size can be right
 * 3) the struct members are all on a natural alignment
 *
 * Hence, this new approach.  To cover word size issues, we *always*
 * transform the two structs we care about, the file header and
 * program header, into a native struct in the 64 bit format:
 *
 * [32,little] -> [Elf64_Ehdr, Elf64_Phdr]
 * [64,little] -> [Elf64_Ehdr, Elf64_Phdr]
 * [32,big] -> [Elf64_Ehdr, Elf64_Phdr]
 * [64,big] -> [Elf64_Ehdr, Elf64_Phdr]
 * Then we just use those structs, and all the need for inline ntoh* goes away,
 * as well as all the chances for error.
 * This works because all the SELF structs have fields large enough for
 * the largest ELF 64 struct members, and all the Elf64 struct members
 * are at least large enough for all ELF 32 struct members.
 * We end up with one function to do all our ELF parsing, and two functions
 * to transform the headers. For the put case, we also have
 * XDR functions, and hopefully we'll never again spend 5 years with the
 * wrong endian-ness on an output value :-)
 * This should work for all word sizes and endianness we hope to target.
 * I *really* don't want to be here for 128 bit addresses.
 *
 * The parse functions are called with a pointer to an input buffer
 * struct. One might ask: are there enough bytes in the input buffer?
 * We know there need to be at *least* sizeof(Elf32_Ehdr) +
 * sizeof(Elf32_Phdr) bytes. Realistically, there has to be some data
 * too.  If we start to worry, though we have not in the past, we
 * might apply the simple test: the input buffer needs to be at least
 * sizeof(Elf64_Ehdr) + sizeof(Elf64_Phdr) bytes because, even if it's
 * ELF 32, there's got to be *some* data! This is not theoretically
 * accurate but it is actually good enough in practice. It allows the
 * header transformation code to ignore the possibility of underrun.
 *
 * We also must accomodate different ELF files, and hence formats,
 * in the same cbfs invocation. We might load a 64-bit payload
 * on a 32-bit machine; we might even have a mixed armv7/armv8
 * SOC or even a system with an x86/ARM!
 *
 * A possibly problematic (though unlikely to be so) assumption
 * is that we expect the BIOS to remain in the lowest 32 bits
 * of the physical address space. Since ARMV8 has standardized
 * on that, and x86_64 also has, this seems a safe assumption.
 *
 * To repeat, ELF structs are different sizes because ELF struct
 * members are different sizes, depending on values in the ELF file
 * header. For this we use the functions defined in xdr.c, which
 * consume bytes, convert the endianness, and advance the data pointer
 * in the buffer struct.
 */


static int iself(const void *input)
{
	const Elf32_Ehdr *ehdr = input;
	return !memcmp(ehdr->e_ident, ELFMAG, 4);
}

/* Get the ident array, so we can figure out
 * endian-ness, word size, and in future other useful
 * parameters
 */
static void
elf_eident(struct buffer *input, Elf64_Ehdr *ehdr)
{
	bgets(input, ehdr->e_ident, sizeof(ehdr->e_ident));
}


static int
check_size(const struct buffer *b, size_t offset, size_t size, const char *desc)
{
	if (size == 0)
		return 0;

	if (offset >= buffer_size(b) || (offset + size) > buffer_size(b)) {
		ERROR("The file is not large enough for the '%s'. "
		      "%ld bytes @ offset %zu, input %zu bytes.\n",
		      desc, size, offset, buffer_size(b));
		return -1;
	}
	return 0;
}

static void
elf_ehdr(struct buffer *input, Elf64_Ehdr *ehdr, struct xdr *xdr, int bit64)
{
	ehdr->e_type = xdr->get16(input);
	ehdr->e_machine = xdr->get16(input);
	ehdr->e_version = xdr->get32(input);
	if (bit64){
		ehdr->e_entry = xdr->get64(input);
		ehdr->e_phoff = xdr->get64(input);
		ehdr->e_shoff = xdr->get64(input);
	} else {
		ehdr->e_entry = xdr->get32(input);
		ehdr->e_phoff = xdr->get32(input);
		ehdr->e_shoff = xdr->get32(input);
	}
	ehdr->e_flags = xdr->get32(input);
	ehdr->e_ehsize = xdr->get16(input);
	ehdr->e_phentsize = xdr->get16(input);
	ehdr->e_phnum = xdr->get16(input);
	ehdr->e_shentsize = xdr->get16(input);
	ehdr->e_shnum = xdr->get16(input);
	ehdr->e_shstrndx = xdr->get16(input);
}

static void
elf_phdr(struct buffer *pinput, Elf64_Phdr *phdr,
	 int entsize, struct xdr *xdr, int bit64)
{
	/*
	 * The entsize need not be sizeof(*phdr).
	 * Hence, it is easier to keep a copy of the input,
	 * as the xdr functions may not advance the input
	 * pointer the full entsize; rather than get tricky
	 * we just advance it below.
	 */
	struct buffer input;
	buffer_clone(&input, pinput);
	if (bit64){
		phdr->p_type = xdr->get32(&input);
		phdr->p_flags = xdr->get32(&input);
		phdr->p_offset = xdr->get64(&input);
		phdr->p_vaddr = xdr->get64(&input);
		phdr->p_paddr = xdr->get64(&input);
		phdr->p_filesz = xdr->get64(&input);
		phdr->p_memsz = xdr->get64(&input);
		phdr->p_align = xdr->get64(&input);
	} else {
		phdr->p_type = xdr->get32(&input);
		phdr->p_offset = xdr->get32(&input);
		phdr->p_vaddr = xdr->get32(&input);
		phdr->p_paddr = xdr->get32(&input);
		phdr->p_filesz = xdr->get32(&input);
		phdr->p_memsz = xdr->get32(&input);
		phdr->p_flags = xdr->get32(&input);
		phdr->p_align = xdr->get32(&input);
	}
	buffer_seek(pinput, entsize);
}

static void
elf_shdr(struct buffer *pinput, Elf64_Shdr *shdr,
	 int entsize, struct xdr *xdr, int bit64)
{
	/*
	 * The entsize need not be sizeof(*shdr).
	 * Hence, it is easier to keep a copy of the input,
	 * as the xdr functions may not advance the input
	 * pointer the full entsize; rather than get tricky
	 * we just advance it below.
	 */
	struct buffer input = *pinput;
	if (bit64){
		shdr->sh_name = xdr->get32(&input);
		shdr->sh_type = xdr->get32(&input);
		shdr->sh_flags = xdr->get64(&input);
		shdr->sh_addr = xdr->get64(&input);
		shdr->sh_offset = xdr->get64(&input);
		shdr->sh_size= xdr->get64(&input);
		shdr->sh_link = xdr->get32(&input);
		shdr->sh_info = xdr->get32(&input);
		shdr->sh_addralign = xdr->get64(&input);
		shdr->sh_entsize = xdr->get64(&input);
	} else {
		shdr->sh_name = xdr->get32(&input);
		shdr->sh_type = xdr->get32(&input);
		shdr->sh_flags = xdr->get32(&input);
		shdr->sh_addr = xdr->get32(&input);
		shdr->sh_offset = xdr->get32(&input);
		shdr->sh_size = xdr->get32(&input);
		shdr->sh_link = xdr->get32(&input);
		shdr->sh_info = xdr->get32(&input);
		shdr->sh_addralign = xdr->get32(&input);
		shdr->sh_entsize = xdr->get32(&input);
	}
	buffer_seek(pinput, entsize);
}

static Elf64_Phdr *
phdr_read(const struct buffer *in, Elf64_Ehdr *ehdr, struct xdr *xdr, int bit64)
{
	struct buffer b;
	Elf64_Phdr *phdr;
	int i;

	/* cons up an input buffer for the headers.
	 * Note that the program headers can be anywhere,
	 * per the ELF spec, You'd be surprised how many ELF
	 * readers miss this little detail.
	 */
	buffer_splice(&b, in, ehdr->e_phoff, ehdr->e_phentsize * ehdr->e_phnum);
	if (check_size(in, ehdr->e_phoff, buffer_size(&b), "program headers"))
		return NULL;

	/* gather up all the phdrs.
	 * We do them all at once because there is more
	 * than one loop over all the phdrs.
	 */
	phdr = calloc(sizeof(*phdr), ehdr->e_phnum);
	for (i = 0; i < ehdr->e_phnum; i++)
		elf_phdr(&b, &phdr[i], ehdr->e_phentsize, xdr, bit64);

	return phdr;
}

static Elf64_Shdr *
shdr_read(const struct buffer *in, Elf64_Ehdr *ehdr, struct xdr *xdr, int bit64)
{
	struct buffer b;
	Elf64_Shdr *shdr;
	int i;

	/* cons up an input buffer for the section headers.
	 * Note that the section headers can be anywhere,
	 * per the ELF spec, You'd be surprised how many ELF
	 * readers miss this little detail.
	 */
	buffer_splice(&b, in, ehdr->e_shoff, ehdr->e_shentsize * ehdr->e_shnum);
	if (check_size(in, ehdr->e_shoff, buffer_size(&b), "section headers"))
		return NULL;

	/* gather up all the shdrs. */
	shdr = calloc(sizeof(*shdr), ehdr->e_shnum);
	for (i = 0; i < ehdr->e_shnum; i++)
		elf_shdr(&b, &shdr[i], ehdr->e_shentsize, xdr, bit64);

	return shdr;
}

/* Get the headers from the buffer.
 * Return -1 in the event of an error.
 * The section headers are optional; if NULL
 * is passed in for pshdr they won't be parsed.
 * We don't (yet) make payload parsing optional
 * because we've never seen a use case.
 */
int
elf_headers(const struct buffer *pinput,
	    uint32_t arch,
	    Elf64_Ehdr *ehdr,
	    Elf64_Phdr **pphdr,
	    Elf64_Shdr **pshdr)
{
	struct xdr *xdr = &xdr_le;
	int bit64 = 0;
	struct buffer input;

	buffer_clone(&input, pinput);

	if (!iself(buffer_get(pinput))) {
		ERROR("The stage file is not in ELF format!\n");
		return -1;
	}

	elf_eident(&input, ehdr);
	bit64 = ehdr->e_ident[EI_CLASS] == ELFCLASS64;
	/* Assume LE unless we are sure otherwise.
	 * We're not going to take on the task of
	 * fully validating the ELF file. That way
	 * lies madness.
	 */
	if (ehdr->e_ident[EI_DATA] == ELFDATA2MSB)
		xdr = &xdr_be;

	elf_ehdr(&input, ehdr, xdr, bit64);

	// The tool may work in architecture-independent way.
	if (arch != CBFS_ARCHITECTURE_UNKNOWN &&
	    !((ehdr->e_machine == EM_ARM) && (arch == CBFS_ARCHITECTURE_ARMV7)) &&
	    !((ehdr->e_machine == EM_386) && (arch == CBFS_ARCHITECTURE_X86))) {
		ERROR("The stage file has the wrong architecture\n");
		return -1;
	}

	*pphdr = phdr_read(pinput, ehdr, xdr, bit64);
	if (*pphdr == NULL)
		return -1;

	if (!pshdr)
		return 0;

	*pshdr = shdr_read(pinput, ehdr, xdr, bit64);
	if (*pshdr == NULL)
		return -1;

	return 0;
}