363 lines
12 KiB
C
363 lines
12 KiB
C
/*
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* elf header parsing.
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*
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* Copyright (C) 2013 Google, Inc.
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; version 2 of the License.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA, 02110-1301 USA
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*/
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include "elf.h"
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#include "common.h"
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#include "cbfs.h"
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/*
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* Short form: this is complicated, but we've tried making it simple
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* and we keep hitting problems with our ELF parsing.
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*
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* The ELF parsing situation has always been a bit tricky. In fact,
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* we (and most others) have been getting it wrong in small ways for
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* years. Recently this has caused real trouble for the ARM V8 build.
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* In this file we attempt to finally get it right for all variations
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* of endian-ness and word size and target architectures and
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* architectures we might get run on. Phew!. To do this we borrow a
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* page from the FreeBSD NFS xdr model (see elf_ehdr and elf_phdr),
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* the Plan 9 endianness functions (see xdr.c), and Go interfaces (see
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* how we use buffer structs in this file). This ends up being a bit
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* wordy at the lowest level, but greatly simplifies the elf parsing
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* code and removes a common source of bugs, namely, forgetting to
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* flip type endianness when referencing a struct member.
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*
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* ELF files can have four combinations of data layout: 32/64, and
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* big/little endian. Further, to add to the fun, depending on the
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* word size, the size of the ELF structs varies. The coreboot SELF
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* format is simpler in theory: it's supposed to be always BE, and the
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* various struct members allow room for growth: the entry point is
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* always 64 bits, for example, so the size of a SELF struct is
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* constant, regardless of target architecture word size. Hence, we
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* need to do some transformation of the ELF files.
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*
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* A given architecture, realistically, only supports one of the four
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* combinations at a time as the 'native' format. Hence, our code has
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* been sprinkled with every variation of [nh]to[hn][sll] over the
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* years. We've never quite gotten it all right, however, and a quick
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* pass over this code revealed another bug. It's all worked because,
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* until now, all the working platforms that had CBFS were 32 LE. Even then,
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* however, bugs crept in: we recently realized that we're not
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* transforming the entry point to big format when we store into the
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* SELF image.
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*
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* The problem is essentially an XDR operation:
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* we have something in a foreign format and need to transform it.
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* It's most like XDR because:
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* 1) the byte order can be wrong
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* 2) the word size can be wrong
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* 3) the size of elements in the stream depends on the value
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* of other elements in the stream
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* it's not like XDR because:
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* 1) the byte order can be right
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* 2) the word size can be right
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* 3) the struct members are all on a natural alignment
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*
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* Hence, this new approach. To cover word size issues, we *always*
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* transform the two structs we care about, the file header and
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* program header, into a native struct in the 64 bit format:
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*
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* [32,little] -> [Elf64_Ehdr, Elf64_Phdr]
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* [64,little] -> [Elf64_Ehdr, Elf64_Phdr]
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* [32,big] -> [Elf64_Ehdr, Elf64_Phdr]
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* [64,big] -> [Elf64_Ehdr, Elf64_Phdr]
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* Then we just use those structs, and all the need for inline ntoh* goes away,
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* as well as all the chances for error.
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* This works because all the SELF structs have fields large enough for
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* the largest ELF 64 struct members, and all the Elf64 struct members
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* are at least large enough for all ELF 32 struct members.
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* We end up with one function to do all our ELF parsing, and two functions
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* to transform the headers. For the put case, we also have
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* XDR functions, and hopefully we'll never again spend 5 years with the
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* wrong endian-ness on an output value :-)
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* This should work for all word sizes and endianness we hope to target.
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* I *really* don't want to be here for 128 bit addresses.
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*
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* The parse functions are called with a pointer to an input buffer
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* struct. One might ask: are there enough bytes in the input buffer?
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* We know there need to be at *least* sizeof(Elf32_Ehdr) +
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* sizeof(Elf32_Phdr) bytes. Realistically, there has to be some data
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* too. If we start to worry, though we have not in the past, we
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* might apply the simple test: the input buffer needs to be at least
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* sizeof(Elf64_Ehdr) + sizeof(Elf64_Phdr) bytes because, even if it's
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* ELF 32, there's got to be *some* data! This is not theoretically
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* accurate but it is actually good enough in practice. It allows the
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* header transformation code to ignore the possibility of underrun.
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*
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* We also must accomodate different ELF files, and hence formats,
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* in the same cbfs invocation. We might load a 64-bit payload
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* on a 32-bit machine; we might even have a mixed armv7/armv8
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* SOC or even a system with an x86/ARM!
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*
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* A possibly problematic (though unlikely to be so) assumption
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* is that we expect the BIOS to remain in the lowest 32 bits
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* of the physical address space. Since ARMV8 has standardized
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* on that, and x86_64 also has, this seems a safe assumption.
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*
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* To repeat, ELF structs are different sizes because ELF struct
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* members are different sizes, depending on values in the ELF file
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* header. For this we use the functions defined in xdr.c, which
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* consume bytes, convert the endianness, and advance the data pointer
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* in the buffer struct.
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*/
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static int iself(const void *input)
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{
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const Elf32_Ehdr *ehdr = input;
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return !memcmp(ehdr->e_ident, ELFMAG, 4);
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}
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/* Get the ident array, so we can figure out
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* endian-ness, word size, and in future other useful
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* parameters
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*/
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static void
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elf_eident(struct buffer *input, Elf64_Ehdr *ehdr)
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{
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bgets(input, ehdr->e_ident, sizeof(ehdr->e_ident));
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}
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static int
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check_size(const struct buffer *b, size_t offset, size_t size, const char *desc)
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{
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if (size == 0)
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return 0;
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if (offset >= buffer_size(b) || (offset + size) > buffer_size(b)) {
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ERROR("The file is not large enough for the '%s'. "
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"%ld bytes @ offset %zu, input %zu bytes.\n",
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desc, size, offset, buffer_size(b));
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return -1;
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}
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return 0;
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}
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static void
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elf_ehdr(struct buffer *input, Elf64_Ehdr *ehdr, struct xdr *xdr, int bit64)
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{
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ehdr->e_type = xdr->get16(input);
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ehdr->e_machine = xdr->get16(input);
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ehdr->e_version = xdr->get32(input);
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if (bit64){
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ehdr->e_entry = xdr->get64(input);
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ehdr->e_phoff = xdr->get64(input);
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ehdr->e_shoff = xdr->get64(input);
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} else {
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ehdr->e_entry = xdr->get32(input);
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ehdr->e_phoff = xdr->get32(input);
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ehdr->e_shoff = xdr->get32(input);
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}
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ehdr->e_flags = xdr->get32(input);
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ehdr->e_ehsize = xdr->get16(input);
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ehdr->e_phentsize = xdr->get16(input);
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ehdr->e_phnum = xdr->get16(input);
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ehdr->e_shentsize = xdr->get16(input);
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ehdr->e_shnum = xdr->get16(input);
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ehdr->e_shstrndx = xdr->get16(input);
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}
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static void
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elf_phdr(struct buffer *pinput, Elf64_Phdr *phdr,
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int entsize, struct xdr *xdr, int bit64)
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{
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/*
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* The entsize need not be sizeof(*phdr).
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* Hence, it is easier to keep a copy of the input,
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* as the xdr functions may not advance the input
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* pointer the full entsize; rather than get tricky
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* we just advance it below.
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*/
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struct buffer input;
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buffer_clone(&input, pinput);
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if (bit64){
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phdr->p_type = xdr->get32(&input);
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phdr->p_flags = xdr->get32(&input);
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phdr->p_offset = xdr->get64(&input);
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phdr->p_vaddr = xdr->get64(&input);
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phdr->p_paddr = xdr->get64(&input);
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phdr->p_filesz = xdr->get64(&input);
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phdr->p_memsz = xdr->get64(&input);
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phdr->p_align = xdr->get64(&input);
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} else {
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phdr->p_type = xdr->get32(&input);
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phdr->p_offset = xdr->get32(&input);
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phdr->p_vaddr = xdr->get32(&input);
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phdr->p_paddr = xdr->get32(&input);
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phdr->p_filesz = xdr->get32(&input);
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phdr->p_memsz = xdr->get32(&input);
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phdr->p_flags = xdr->get32(&input);
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phdr->p_align = xdr->get32(&input);
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}
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buffer_seek(pinput, entsize);
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}
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static void
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elf_shdr(struct buffer *pinput, Elf64_Shdr *shdr,
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int entsize, struct xdr *xdr, int bit64)
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{
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/*
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* The entsize need not be sizeof(*shdr).
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* Hence, it is easier to keep a copy of the input,
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* as the xdr functions may not advance the input
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* pointer the full entsize; rather than get tricky
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* we just advance it below.
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*/
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struct buffer input = *pinput;
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if (bit64){
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shdr->sh_name = xdr->get32(&input);
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shdr->sh_type = xdr->get32(&input);
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shdr->sh_flags = xdr->get64(&input);
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shdr->sh_addr = xdr->get64(&input);
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shdr->sh_offset = xdr->get64(&input);
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shdr->sh_size= xdr->get64(&input);
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shdr->sh_link = xdr->get32(&input);
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shdr->sh_info = xdr->get32(&input);
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shdr->sh_addralign = xdr->get64(&input);
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shdr->sh_entsize = xdr->get64(&input);
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} else {
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shdr->sh_name = xdr->get32(&input);
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shdr->sh_type = xdr->get32(&input);
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shdr->sh_flags = xdr->get32(&input);
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shdr->sh_addr = xdr->get32(&input);
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shdr->sh_offset = xdr->get32(&input);
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shdr->sh_size = xdr->get32(&input);
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shdr->sh_link = xdr->get32(&input);
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shdr->sh_info = xdr->get32(&input);
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shdr->sh_addralign = xdr->get32(&input);
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shdr->sh_entsize = xdr->get32(&input);
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}
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buffer_seek(pinput, entsize);
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}
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static Elf64_Phdr *
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phdr_read(const struct buffer *in, Elf64_Ehdr *ehdr, struct xdr *xdr, int bit64)
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{
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struct buffer b;
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Elf64_Phdr *phdr;
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int i;
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/* cons up an input buffer for the headers.
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* Note that the program headers can be anywhere,
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* per the ELF spec, You'd be surprised how many ELF
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* readers miss this little detail.
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*/
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buffer_splice(&b, in, ehdr->e_phoff, ehdr->e_phentsize * ehdr->e_phnum);
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if (check_size(in, ehdr->e_phoff, buffer_size(&b), "program headers"))
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return NULL;
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/* gather up all the phdrs.
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* We do them all at once because there is more
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* than one loop over all the phdrs.
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*/
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phdr = calloc(sizeof(*phdr), ehdr->e_phnum);
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for (i = 0; i < ehdr->e_phnum; i++)
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elf_phdr(&b, &phdr[i], ehdr->e_phentsize, xdr, bit64);
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return phdr;
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}
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static Elf64_Shdr *
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shdr_read(const struct buffer *in, Elf64_Ehdr *ehdr, struct xdr *xdr, int bit64)
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{
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struct buffer b;
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Elf64_Shdr *shdr;
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int i;
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/* cons up an input buffer for the section headers.
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* Note that the section headers can be anywhere,
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* per the ELF spec, You'd be surprised how many ELF
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* readers miss this little detail.
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*/
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buffer_splice(&b, in, ehdr->e_shoff, ehdr->e_shentsize * ehdr->e_shnum);
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if (check_size(in, ehdr->e_shoff, buffer_size(&b), "section headers"))
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return NULL;
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/* gather up all the shdrs. */
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shdr = calloc(sizeof(*shdr), ehdr->e_shnum);
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for (i = 0; i < ehdr->e_shnum; i++)
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elf_shdr(&b, &shdr[i], ehdr->e_shentsize, xdr, bit64);
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return shdr;
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}
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/* Get the headers from the buffer.
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* Return -1 in the event of an error.
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* The section headers are optional; if NULL
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* is passed in for pshdr they won't be parsed.
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* We don't (yet) make payload parsing optional
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* because we've never seen a use case.
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*/
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int
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elf_headers(const struct buffer *pinput,
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uint32_t arch,
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Elf64_Ehdr *ehdr,
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Elf64_Phdr **pphdr,
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Elf64_Shdr **pshdr)
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{
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struct xdr *xdr = &xdr_le;
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int bit64 = 0;
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struct buffer input;
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buffer_clone(&input, pinput);
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if (!iself(buffer_get(pinput))) {
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ERROR("The stage file is not in ELF format!\n");
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return -1;
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}
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elf_eident(&input, ehdr);
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bit64 = ehdr->e_ident[EI_CLASS] == ELFCLASS64;
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/* Assume LE unless we are sure otherwise.
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* We're not going to take on the task of
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* fully validating the ELF file. That way
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* lies madness.
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*/
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if (ehdr->e_ident[EI_DATA] == ELFDATA2MSB)
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xdr = &xdr_be;
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elf_ehdr(&input, ehdr, xdr, bit64);
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// The tool may work in architecture-independent way.
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if (arch != CBFS_ARCHITECTURE_UNKNOWN &&
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!((ehdr->e_machine == EM_ARM) && (arch == CBFS_ARCHITECTURE_ARMV7)) &&
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!((ehdr->e_machine == EM_386) && (arch == CBFS_ARCHITECTURE_X86))) {
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ERROR("The stage file has the wrong architecture\n");
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return -1;
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}
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*pphdr = phdr_read(pinput, ehdr, xdr, bit64);
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if (*pphdr == NULL)
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return -1;
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if (!pshdr)
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return 0;
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*pshdr = shdr_read(pinput, ehdr, xdr, bit64);
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if (*pshdr == NULL)
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return -1;
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return 0;
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}
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