/* * 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. */ #include <stdio.h> #include <stdlib.h> #include <string.h> #include "elfparsing.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'. " "%zu 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 int phdr_read(const struct buffer *in, struct parsed_elf *pelf, struct xdr *xdr, int bit64) { struct buffer b; Elf64_Phdr *phdr; Elf64_Ehdr *ehdr; int i; ehdr = &pelf->ehdr; /* 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 -1; /* gather up all the phdrs. * We do them all at once because there is more * than one loop over all the phdrs. */ phdr = calloc(ehdr->e_phnum, sizeof(*phdr)); for (i = 0; i < ehdr->e_phnum; i++) { DEBUG("Parsing segment %d\n", i); elf_phdr(&b, &phdr[i], ehdr->e_phentsize, xdr, bit64); /* Ensure the contents are valid within the elf file. */ if (check_size(in, phdr[i].p_offset, phdr[i].p_filesz, "segment contents")) { free(phdr); return -1; } } pelf->phdr = phdr; return 0; } static int shdr_read(const struct buffer *in, struct parsed_elf *pelf, struct xdr *xdr, int bit64) { struct buffer b; Elf64_Shdr *shdr; Elf64_Ehdr *ehdr; int i; ehdr = &pelf->ehdr; /* 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 -1; /* gather up all the shdrs. */ shdr = calloc(ehdr->e_shnum, sizeof(*shdr)); for (i = 0; i < ehdr->e_shnum; i++) { DEBUG("Parsing section %d\n", i); elf_shdr(&b, &shdr[i], ehdr->e_shentsize, xdr, bit64); } pelf->shdr = shdr; return 0; } static int reloc_read(const struct buffer *in, struct parsed_elf *pelf, struct xdr *xdr, int bit64) { struct buffer b; Elf64_Word i; Elf64_Ehdr *ehdr; ehdr = &pelf->ehdr; pelf->relocs = calloc(ehdr->e_shnum, sizeof(Elf64_Rela *)); /* Allocate array for each section that contains relocation entries. */ for (i = 0; i < ehdr->e_shnum; i++) { Elf64_Shdr *shdr; Elf64_Rela *rela; Elf64_Xword j; Elf64_Xword nrelocs; int is_rela; shdr = &pelf->shdr[i]; /* Only process REL and RELA sections. */ if (shdr->sh_type != SHT_REL && shdr->sh_type != SHT_RELA) continue; DEBUG("Checking relocation section %u\n", i); /* Ensure the section that relocations apply is a valid. */ if (shdr->sh_info >= ehdr->e_shnum || shdr->sh_info == SHN_UNDEF) { ERROR("Relocations apply to an invalid section: %u\n", shdr[i].sh_info); return -1; } is_rela = shdr->sh_type == SHT_RELA; /* Determine the number relocations in this section. */ nrelocs = shdr->sh_size / shdr->sh_entsize; pelf->relocs[i] = calloc(nrelocs, sizeof(Elf64_Rela)); buffer_splice(&b, in, shdr->sh_offset, shdr->sh_size); if (check_size(in, shdr->sh_offset, buffer_size(&b), "relocation section")) { ERROR("Relocation section %u failed.\n", i); return -1; } rela = pelf->relocs[i]; for (j = 0; j < nrelocs; j++) { if (bit64) { rela->r_offset = xdr->get64(&b); rela->r_info = xdr->get64(&b); if (is_rela) rela->r_addend = xdr->get64(&b); } else { uint32_t r_info; rela->r_offset = xdr->get32(&b); r_info = xdr->get32(&b); rela->r_info = ELF64_R_INFO(ELF32_R_SYM(r_info), ELF32_R_TYPE(r_info)); if (is_rela) rela->r_addend = xdr->get32(&b); } rela++; } } return 0; } static int strtab_read(const struct buffer *in, struct parsed_elf *pelf) { Elf64_Ehdr *ehdr; Elf64_Word i; ehdr = &pelf->ehdr; if (ehdr->e_shstrndx >= ehdr->e_shnum) { ERROR("Section header string table index out of range: %d\n", ehdr->e_shstrndx); return -1; } /* For each section of type SHT_STRTAB create a symtab buffer. */ pelf->strtabs = calloc(ehdr->e_shnum, sizeof(struct buffer *)); for (i = 0; i < ehdr->e_shnum; i++) { struct buffer *b; Elf64_Shdr *shdr = &pelf->shdr[i]; if (shdr->sh_type != SHT_STRTAB) continue; b = calloc(1, sizeof(*b)); buffer_splice(b, in, shdr->sh_offset, shdr->sh_size); if (check_size(in, shdr->sh_offset, buffer_size(b), "strtab")) { ERROR("STRTAB section not within bounds: %d\n", i); free(b); return -1; } pelf->strtabs[i] = b; } return 0; } static int symtab_read(const struct buffer *in, struct parsed_elf *pelf, struct xdr *xdr, int bit64) { Elf64_Ehdr *ehdr; Elf64_Shdr *shdr; Elf64_Half i; Elf64_Xword nsyms; Elf64_Sym *sym; struct buffer b; ehdr = &pelf->ehdr; shdr = NULL; for (i = 0; i < ehdr->e_shnum; i++) { if (pelf->shdr[i].sh_type != SHT_SYMTAB) continue; if (shdr != NULL) { ERROR("Multiple symbol sections found. %u and %u\n", (unsigned int)(shdr - pelf->shdr), i); return -1; } shdr = &pelf->shdr[i]; } if (shdr == NULL) { ERROR("No symbol table found.\n"); return -1; } buffer_splice(&b, in, shdr->sh_offset, shdr->sh_size); if (check_size(in, shdr->sh_offset, buffer_size(&b), "symtab")) return -1; nsyms = shdr->sh_size / shdr->sh_entsize; pelf->syms = calloc(nsyms, sizeof(Elf64_Sym)); for (i = 0; i < nsyms; i++) { sym = &pelf->syms[i]; if (bit64) { sym->st_name = xdr->get32(&b); sym->st_info = xdr->get8(&b); sym->st_other = xdr->get8(&b); sym->st_shndx = xdr->get16(&b); sym->st_value = xdr->get64(&b); sym->st_size = xdr->get64(&b); } else { sym->st_name = xdr->get32(&b); sym->st_value = xdr->get32(&b); sym->st_size = xdr->get32(&b); sym->st_info = xdr->get8(&b); sym->st_other = xdr->get8(&b); sym->st_shndx = xdr->get16(&b); } } return 0; } int parse_elf(const struct buffer *pinput, struct parsed_elf *pelf, int flags) { struct xdr *xdr = &xdr_le; int bit64 = 0; struct buffer input; Elf64_Ehdr *ehdr; /* Zero out the parsed elf structure. */ memset(pelf, 0, sizeof(*pelf)); if (!iself(buffer_get(pinput))) { ERROR("The stage file is not in ELF format!\n"); return -1; } buffer_clone(&input, pinput); ehdr = &pelf->ehdr; 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); /* Relocation processing requires section header parsing. */ if (flags & ELF_PARSE_RELOC) flags |= ELF_PARSE_SHDR; /* String table processing requires section header parsing. */ if (flags & ELF_PARSE_STRTAB) flags |= ELF_PARSE_SHDR; /* Symbole table processing requires section header parsing. */ if (flags & ELF_PARSE_SYMTAB) flags |= ELF_PARSE_SHDR; if ((flags & ELF_PARSE_PHDR) && phdr_read(pinput, pelf, xdr, bit64)) goto fail; if ((flags & ELF_PARSE_SHDR) && shdr_read(pinput, pelf, xdr, bit64)) goto fail; if ((flags & ELF_PARSE_RELOC) && reloc_read(pinput, pelf, xdr, bit64)) goto fail; if ((flags & ELF_PARSE_STRTAB) && strtab_read(pinput, pelf)) goto fail; if ((flags & ELF_PARSE_SYMTAB) && symtab_read(pinput, pelf, xdr, bit64)) goto fail; return 0; fail: parsed_elf_destroy(pelf); return -1; } void parsed_elf_destroy(struct parsed_elf *pelf) { Elf64_Half i; free(pelf->phdr); free(pelf->shdr); if (pelf->relocs != NULL) { for (i = 0; i < pelf->ehdr.e_shnum; i++) free(pelf->relocs[i]); } free(pelf->relocs); if (pelf->strtabs != NULL) { for (i = 0; i < pelf->ehdr.e_shnum; i++) free(pelf->strtabs[i]); } free(pelf->strtabs); free(pelf->syms); } /* 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, Elf64_Ehdr *ehdr, Elf64_Phdr **pphdr, Elf64_Shdr **pshdr) { struct parsed_elf pelf; int flags; flags = ELF_PARSE_PHDR; if (pshdr != NULL) flags |= ELF_PARSE_SHDR; if (parse_elf(pinput, &pelf, flags)) return -1; /* Copy out the parsed elf header. */ memcpy(ehdr, &pelf.ehdr, sizeof(*ehdr)); *pphdr = calloc(ehdr->e_phnum, sizeof(Elf64_Phdr)); memcpy(*pphdr, pelf.phdr, ehdr->e_phnum * sizeof(Elf64_Phdr)); if (pshdr != NULL) { *pshdr = calloc(ehdr->e_shnum, sizeof(Elf64_Shdr)); memcpy(*pshdr, pelf.shdr, ehdr->e_shnum * sizeof(Elf64_Shdr)); } parsed_elf_destroy(&pelf); return 0; } /* ELF Writing Support * * The ELF file is written according to the following layout: * +------------------+ * | ELF Header | * +------------------+ * | Section Headers | * +------------------+ * | Program Headers | * +------------------+ * | String table | * +------------------+ <- 4KiB Aligned * | Code/Data | * +------------------+ */ void elf_init_eheader(Elf64_Ehdr *ehdr, int machine, int nbits, int endian) { memset(ehdr, 0, sizeof(*ehdr)); ehdr->e_ident[EI_MAG0] = ELFMAG0; ehdr->e_ident[EI_MAG1] = ELFMAG1; ehdr->e_ident[EI_MAG2] = ELFMAG2; ehdr->e_ident[EI_MAG3] = ELFMAG3; ehdr->e_ident[EI_CLASS] = nbits; ehdr->e_ident[EI_DATA] = endian; ehdr->e_ident[EI_VERSION] = EV_CURRENT; ehdr->e_type = ET_EXEC; ehdr->e_machine = machine; ehdr->e_version = EV_CURRENT; if (nbits == ELFCLASS64) { ehdr->e_ehsize = sizeof(Elf64_Ehdr); ehdr->e_phentsize = sizeof(Elf64_Phdr); ehdr->e_shentsize = sizeof(Elf64_Shdr); } else { ehdr->e_ehsize = sizeof(Elf32_Ehdr); ehdr->e_phentsize = sizeof(Elf32_Phdr); ehdr->e_shentsize = sizeof(Elf32_Shdr); } } /* Arbitray maximum number of sections. */ #define MAX_SECTIONS 16 struct elf_writer_section { Elf64_Shdr shdr; struct buffer content; const char *name; }; struct elf_writer_string_table { size_t next_offset; size_t max_size; char *buffer; }; struct elf_writer_sym_table { size_t max_entries; size_t num_entries; Elf64_Sym *syms; }; #define MAX_REL_NAME 32 struct elf_writer_rel { size_t num_entries; size_t max_entries; Elf64_Rel *rels; struct elf_writer_section *sec; char name[MAX_REL_NAME]; }; struct elf_writer { Elf64_Ehdr ehdr; struct xdr *xdr; size_t num_secs; struct elf_writer_section sections[MAX_SECTIONS]; struct elf_writer_rel rel_sections[MAX_SECTIONS]; Elf64_Phdr *phdrs; struct elf_writer_section *shstrtab_sec; struct elf_writer_section *strtab_sec; struct elf_writer_section *symtab_sec; struct elf_writer_string_table strtab; struct elf_writer_sym_table symtab; int bit64; }; static size_t section_index(struct elf_writer *ew, struct elf_writer_section *sec) { return sec - &ew->sections[0]; } static struct elf_writer_section *last_section(struct elf_writer *ew) { return &ew->sections[ew->num_secs - 1]; } static void strtab_init(struct elf_writer *ew, size_t size) { struct buffer b; Elf64_Shdr shdr; /* Start adding strings after the initial NUL entry. */ ew->strtab.next_offset = 1; ew->strtab.max_size = size; ew->strtab.buffer = calloc(1, ew->strtab.max_size); buffer_init(&b, NULL, ew->strtab.buffer, ew->strtab.max_size); memset(&shdr, 0, sizeof(shdr)); shdr.sh_type = SHT_STRTAB; shdr.sh_addralign = 1; shdr.sh_size = ew->strtab.max_size; elf_writer_add_section(ew, &shdr, &b, ".strtab"); ew->strtab_sec = last_section(ew); } static void symtab_init(struct elf_writer *ew, size_t max_entries) { struct buffer b; Elf64_Shdr shdr; memset(&shdr, 0, sizeof(shdr)); shdr.sh_type = SHT_SYMTAB; if (ew->bit64) { shdr.sh_entsize = sizeof(Elf64_Sym); shdr.sh_addralign = sizeof(Elf64_Addr); } else { shdr.sh_entsize = sizeof(Elf32_Sym); shdr.sh_addralign = sizeof(Elf32_Addr); } shdr.sh_size = shdr.sh_entsize * max_entries; ew->symtab.syms = calloc(max_entries, sizeof(Elf64_Sym)); ew->symtab.num_entries = 1; ew->symtab.max_entries = max_entries; buffer_init(&b, NULL, ew->symtab.syms, shdr.sh_size); elf_writer_add_section(ew, &shdr, &b, ".symtab"); ew->symtab_sec = last_section(ew); } struct elf_writer *elf_writer_init(const Elf64_Ehdr *ehdr) { struct elf_writer *ew; Elf64_Shdr shdr; struct buffer empty_buffer; if (!iself(ehdr)) return NULL; ew = calloc(1, sizeof(*ew)); memcpy(&ew->ehdr, ehdr, sizeof(ew->ehdr)); ew->bit64 = ew->ehdr.e_ident[EI_CLASS] == ELFCLASS64; /* Set the endinan ops. */ if (ew->ehdr.e_ident[EI_DATA] == ELFDATA2MSB) ew->xdr = &xdr_be; else ew->xdr = &xdr_le; /* Reset count and offsets */ ew->ehdr.e_phoff = 0; ew->ehdr.e_shoff = 0; ew->ehdr.e_shnum = 0; ew->ehdr.e_phnum = 0; memset(&empty_buffer, 0, sizeof(empty_buffer)); memset(&shdr, 0, sizeof(shdr)); /* Add SHT_NULL section header. */ shdr.sh_type = SHT_NULL; elf_writer_add_section(ew, &shdr, &empty_buffer, NULL); /* Add section header string table and maintain reference to it. */ shdr.sh_type = SHT_STRTAB; elf_writer_add_section(ew, &shdr, &empty_buffer, ".shstrtab"); ew->shstrtab_sec = last_section(ew); ew->ehdr.e_shstrndx = section_index(ew, ew->shstrtab_sec); /* Add a small string table and symbol table. */ strtab_init(ew, 4096); symtab_init(ew, 100); return ew; } /* * Clean up any internal state represented by ew. Aftewards the elf_writer * is invalid. */ void elf_writer_destroy(struct elf_writer *ew) { int i; if (ew->phdrs != NULL) free(ew->phdrs); free(ew->strtab.buffer); free(ew->symtab.syms); for (i = 0; i < MAX_SECTIONS; i++) free(ew->rel_sections[i].rels); free(ew); } /* * Add a section to the ELF file. Section type, flags, and memsize are * maintained from the passed in Elf64_Shdr. The buffer represents the * content of the section while the name is the name of section itself. * Returns < 0 on error, 0 on success. */ int elf_writer_add_section(struct elf_writer *ew, const Elf64_Shdr *shdr, struct buffer *contents, const char *name) { struct elf_writer_section *newsh; if (ew->num_secs == MAX_SECTIONS) return -1; newsh = &ew->sections[ew->num_secs]; ew->num_secs++; memcpy(&newsh->shdr, shdr, sizeof(newsh->shdr)); newsh->shdr.sh_offset = 0; newsh->name = name; if (contents != NULL) buffer_clone(&newsh->content, contents); return 0; } static void ehdr_write(struct elf_writer *ew, struct buffer *m) { int i; for (i = 0; i < EI_NIDENT; i++) ew->xdr->put8(m, ew->ehdr.e_ident[i]); ew->xdr->put16(m, ew->ehdr.e_type); ew->xdr->put16(m, ew->ehdr.e_machine); ew->xdr->put32(m, ew->ehdr.e_version); if (ew->bit64) { ew->xdr->put64(m, ew->ehdr.e_entry); ew->xdr->put64(m, ew->ehdr.e_phoff); ew->xdr->put64(m, ew->ehdr.e_shoff); } else { ew->xdr->put32(m, ew->ehdr.e_entry); ew->xdr->put32(m, ew->ehdr.e_phoff); ew->xdr->put32(m, ew->ehdr.e_shoff); } ew->xdr->put32(m, ew->ehdr.e_flags); ew->xdr->put16(m, ew->ehdr.e_ehsize); ew->xdr->put16(m, ew->ehdr.e_phentsize); ew->xdr->put16(m, ew->ehdr.e_phnum); ew->xdr->put16(m, ew->ehdr.e_shentsize); ew->xdr->put16(m, ew->ehdr.e_shnum); ew->xdr->put16(m, ew->ehdr.e_shstrndx); } static void shdr_write(struct elf_writer *ew, size_t n, struct buffer *m) { struct xdr *xdr = ew->xdr; int bit64 = ew->bit64; struct elf_writer_section *sec = &ew->sections[n]; Elf64_Shdr *shdr = &sec->shdr; xdr->put32(m, shdr->sh_name); xdr->put32(m, shdr->sh_type); if (bit64) { xdr->put64(m, shdr->sh_flags); xdr->put64(m, shdr->sh_addr); xdr->put64(m, shdr->sh_offset); xdr->put64(m, shdr->sh_size); xdr->put32(m, shdr->sh_link); xdr->put32(m, shdr->sh_info); xdr->put64(m, shdr->sh_addralign); xdr->put64(m, shdr->sh_entsize); } else { xdr->put32(m, shdr->sh_flags); xdr->put32(m, shdr->sh_addr); xdr->put32(m, shdr->sh_offset); xdr->put32(m, shdr->sh_size); xdr->put32(m, shdr->sh_link); xdr->put32(m, shdr->sh_info); xdr->put32(m, shdr->sh_addralign); xdr->put32(m, shdr->sh_entsize); } } static void phdr_write(struct elf_writer *ew, struct buffer *m, Elf64_Phdr *phdr) { if (ew->bit64) { ew->xdr->put32(m, phdr->p_type); ew->xdr->put32(m, phdr->p_flags); ew->xdr->put64(m, phdr->p_offset); ew->xdr->put64(m, phdr->p_vaddr); ew->xdr->put64(m, phdr->p_paddr); ew->xdr->put64(m, phdr->p_filesz); ew->xdr->put64(m, phdr->p_memsz); ew->xdr->put64(m, phdr->p_align); } else { ew->xdr->put32(m, phdr->p_type); ew->xdr->put32(m, phdr->p_offset); ew->xdr->put32(m, phdr->p_vaddr); ew->xdr->put32(m, phdr->p_paddr); ew->xdr->put32(m, phdr->p_filesz); ew->xdr->put32(m, phdr->p_memsz); ew->xdr->put32(m, phdr->p_flags); ew->xdr->put32(m, phdr->p_align); } } static int section_consecutive(struct elf_writer *ew, Elf64_Half secidx) { Elf64_Half i; struct elf_writer_section *prev_alloc = NULL; if (secidx == 0) return 0; for (i = 0; i < secidx; i++) { if (ew->sections[i].shdr.sh_flags & SHF_ALLOC) prev_alloc = &ew->sections[i]; } if (prev_alloc == NULL) return 0; if (prev_alloc->shdr.sh_addr + prev_alloc->shdr.sh_size == ew->sections[secidx].shdr.sh_addr) return 1; return 0; } static void write_phdrs(struct elf_writer *ew, struct buffer *phdrs) { Elf64_Half i; Elf64_Phdr phdr; size_t num_written = 0; for (i = 0; i < ew->num_secs; i++) { struct elf_writer_section *sec = &ew->sections[i]; if (!(sec->shdr.sh_flags & SHF_ALLOC)) continue; if (!section_consecutive(ew, i)) { /* Write out previously set phdr. */ if (num_written != 0) { phdr_write(ew, phdrs, &phdr); num_written++; } phdr.p_type = PT_LOAD; phdr.p_offset = sec->shdr.sh_offset; phdr.p_vaddr = sec->shdr.sh_addr; phdr.p_paddr = sec->shdr.sh_addr; phdr.p_filesz = buffer_size(&sec->content); phdr.p_memsz = sec->shdr.sh_size; phdr.p_flags = 0; if (sec->shdr.sh_flags & SHF_EXECINSTR) phdr.p_flags |= PF_X | PF_R; if (sec->shdr.sh_flags & SHF_WRITE) phdr.p_flags |= PF_W; phdr.p_align = sec->shdr.sh_addralign; } else { /* Accumulate file size and memsize. The assumption * is that each section is either NOBITS or full * (sh_size == file size). This is standard in that * an ELF section doesn't have a file size component. */ if (sec->shdr.sh_flags & SHF_EXECINSTR) phdr.p_flags |= PF_X | PF_R; if (sec->shdr.sh_flags & SHF_WRITE) phdr.p_flags |= PF_W; phdr.p_filesz += buffer_size(&sec->content); phdr.p_memsz += sec->shdr.sh_size; } } /* Write out the last phdr. */ if (num_written != ew->ehdr.e_phnum) phdr_write(ew, phdrs, &phdr); } static void fixup_symbol_table(struct elf_writer *ew) { struct elf_writer_section *sec = ew->symtab_sec; /* If there is only the NULL section, mark section as inactive. */ if (ew->symtab.num_entries == 1) { sec->shdr.sh_type = SHT_NULL; sec->shdr.sh_size = 0; } else { size_t i; struct buffer wr; buffer_clone(&wr, &sec->content); /* To appease xdr. */ buffer_set_size(&wr, 0); for (i = 0; i < ew->symtab.num_entries; i++) { /* Create local copy as were over-writing backing * store of the symbol. */ Elf64_Sym sym = ew->symtab.syms[i]; if (ew->bit64) { ew->xdr->put32(&wr, sym.st_name); ew->xdr->put8(&wr, sym.st_info); ew->xdr->put8(&wr, sym.st_other); ew->xdr->put16(&wr, sym.st_shndx); ew->xdr->put64(&wr, sym.st_value); ew->xdr->put64(&wr, sym.st_size); } else { ew->xdr->put32(&wr, sym.st_name); ew->xdr->put32(&wr, sym.st_value); ew->xdr->put32(&wr, sym.st_size); ew->xdr->put8(&wr, sym.st_info); ew->xdr->put8(&wr, sym.st_other); ew->xdr->put16(&wr, sym.st_shndx); } } /* Update section size. */ sec->shdr.sh_size = sec->shdr.sh_entsize; sec->shdr.sh_size *= ew->symtab.num_entries; /* Fix up sh_link to point to string table. */ sec->shdr.sh_link = section_index(ew, ew->strtab_sec); /* sh_info is supposed to be 1 greater than symbol table * index of last local binding. Just use max symbols. */ sec->shdr.sh_info = ew->symtab.num_entries; } buffer_set_size(&sec->content, sec->shdr.sh_size); } static void fixup_relocations(struct elf_writer *ew) { int i; Elf64_Xword type; switch (ew->ehdr.e_machine) { case EM_386: type = R_386_32; break; case EM_ARM: type = R_ARM_ABS32; break; case EM_AARCH64: type = R_AARCH64_ABS64; break; case EM_MIPS: type = R_MIPS_32; break; case EM_RISCV: type = R_RISCV_32; break; case EM_PPC64: type = R_PPC64_ADDR32; break; default: ERROR("Unable to handle relocations for e_machine %x\n", ew->ehdr.e_machine); return; } for (i = 0; i < MAX_SECTIONS; i++) { struct elf_writer_rel *rel_sec = &ew->rel_sections[i]; struct elf_writer_section *sec = rel_sec->sec; struct buffer writer; size_t j; if (sec == NULL) continue; /* Update section header size as well as content size. */ buffer_init(&sec->content, sec->content.name, rel_sec->rels, rel_sec->num_entries * sec->shdr.sh_entsize); sec->shdr.sh_size = buffer_size(&sec->content); buffer_clone(&writer, &sec->content); /* To make xdr happy. */ buffer_set_size(&writer, 0); for (j = 0; j < ew->rel_sections[i].num_entries; j++) { /* Make copy as we're overwriting backing store. */ Elf64_Rel rel = rel_sec->rels[j]; rel.r_info = ELF64_R_INFO(ELF64_R_SYM(rel.r_info), ELF64_R_TYPE(type)); if (ew->bit64) { ew->xdr->put64(&writer, rel.r_offset); ew->xdr->put64(&writer, rel.r_info); } else { Elf32_Rel rel32; rel32.r_offset = rel.r_offset; rel32.r_info = ELF32_R_INFO(ELF64_R_SYM(rel.r_info), ELF64_R_TYPE(rel.r_info)); ew->xdr->put32(&writer, rel32.r_offset); ew->xdr->put32(&writer, rel32.r_info); } } } } /* * Serialize the ELF file to the output buffer. Return < 0 on error, * 0 on success. */ int elf_writer_serialize(struct elf_writer *ew, struct buffer *out) { Elf64_Half i; Elf64_Xword metadata_size; Elf64_Xword program_size; Elf64_Off shstroffset; size_t shstrlen; struct buffer metadata; struct buffer phdrs; struct buffer data; struct buffer *strtab; INFO("Writing %zu sections.\n", ew->num_secs); /* Perform any necessary work for special sections. */ fixup_symbol_table(ew); fixup_relocations(ew); /* Determine size of sections to be written. */ program_size = 0; /* Start with 1 byte for first byte of section header string table. */ shstrlen = 1; for (i = 0; i < ew->num_secs; i++) { struct elf_writer_section *sec = &ew->sections[i]; if (sec->shdr.sh_flags & SHF_ALLOC) { if (!section_consecutive(ew, i)) ew->ehdr.e_phnum++; } program_size += buffer_size(&sec->content); /* Keep track of the length sections' names. */ if (sec->name != NULL) { sec->shdr.sh_name = shstrlen; shstrlen += strlen(sec->name) + 1; } } ew->ehdr.e_shnum = ew->num_secs; metadata_size = 0; metadata_size += ew->ehdr.e_ehsize; metadata_size += ew->ehdr.e_shnum * ew->ehdr.e_shentsize; metadata_size += ew->ehdr.e_phnum * ew->ehdr.e_phentsize; shstroffset = metadata_size; /* Align up section header string size and metadata size to 4KiB */ metadata_size = ALIGN(metadata_size + shstrlen, 4096); if (buffer_create(out, metadata_size + program_size, "elfout")) { ERROR("Could not create output buffer for ELF.\n"); return -1; } INFO("Created %zu output buffer for ELF file.\n", buffer_size(out)); /* * Write out ELF header. Section headers come right after ELF header * followed by the program headers. Buffers need to be created first * to do the writing. */ ew->ehdr.e_shoff = ew->ehdr.e_ehsize; ew->ehdr.e_phoff = ew->ehdr.e_shoff + ew->ehdr.e_shnum * ew->ehdr.e_shentsize; buffer_splice(&metadata, out, 0, metadata_size); buffer_splice(&phdrs, out, ew->ehdr.e_phoff, ew->ehdr.e_phnum * ew->ehdr.e_phentsize); buffer_splice(&data, out, metadata_size, program_size); /* Set up the section header string table contents. */ strtab = &ew->shstrtab_sec->content; buffer_splice(strtab, out, shstroffset, shstrlen); ew->shstrtab_sec->shdr.sh_size = shstrlen; /* Reset current locations. */ buffer_set_size(&metadata, 0); buffer_set_size(&data, 0); buffer_set_size(&phdrs, 0); buffer_set_size(strtab, 0); /* ELF Header */ ehdr_write(ew, &metadata); /* Write out section headers, section strings, section content, and * program headers. */ ew->xdr->put8(strtab, 0); for (i = 0; i < ew->num_secs; i++) { struct elf_writer_section *sec = &ew->sections[i]; /* Update section offsets. Be sure to not update SHN_UNDEF. */ if (sec == ew->shstrtab_sec) sec->shdr.sh_offset = shstroffset; else if (i != SHN_UNDEF) sec->shdr.sh_offset = buffer_size(&data) + metadata_size; shdr_write(ew, i, &metadata); /* Add section name to string table. */ if (sec->name != NULL) bputs(strtab, sec->name, strlen(sec->name) + 1); /* Output section data for all sections but SHN_UNDEF and * section header string table. */ if (i != SHN_UNDEF && sec != ew->shstrtab_sec) bputs(&data, buffer_get(&sec->content), buffer_size(&sec->content)); } write_phdrs(ew, &phdrs); return 0; } /* Add a string to the string table returning index on success, < 0 on error. */ static int elf_writer_add_string(struct elf_writer *ew, const char *new) { size_t current_offset; size_t new_len; for (current_offset = 0; current_offset < ew->strtab.next_offset; ) { const char *str = ew->strtab.buffer + current_offset; size_t len = strlen(str) + 1; if (!strcmp(str, new)) return current_offset; current_offset += len; } new_len = strlen(new) + 1; if (current_offset + new_len > ew->strtab.max_size) { ERROR("No space for string in .strtab.\n"); return -1; } memcpy(ew->strtab.buffer + current_offset, new, new_len); ew->strtab.next_offset = current_offset + new_len; return current_offset; } static int elf_writer_section_index(struct elf_writer *ew, const char *name) { size_t i; for (i = 0; i < ew->num_secs; i++) { if (ew->sections[i].name == NULL) continue; if (!strcmp(ew->sections[i].name, name)) return i; } ERROR("ELF Section not found: %s\n", name); return -1; } int elf_writer_add_symbol(struct elf_writer *ew, const char *name, const char *section_name, Elf64_Addr value, Elf64_Word size, int binding, int type) { int i; Elf64_Sym sym = { .st_value = value, .st_size = size, .st_info = ELF64_ST_INFO(binding, type), }; if (ew->symtab.max_entries == ew->symtab.num_entries) { ERROR("No more symbol entries left.\n"); return -1; } i = elf_writer_add_string(ew, name); if (i < 0) return -1; sym.st_name = i; i = elf_writer_section_index(ew, section_name); if (i < 0) return -1; sym.st_shndx = i; ew->symtab.syms[ew->symtab.num_entries++] = sym; return 0; } static int elf_sym_index(struct elf_writer *ew, const char *sym) { int j; size_t i; Elf64_Word st_name; /* Determine index of symbol in the string table. */ j = elf_writer_add_string(ew, sym); if (j < 0) return -1; st_name = j; for (i = 0; i < ew->symtab.num_entries; i++) if (ew->symtab.syms[i].st_name == st_name) return i; return -1; } static struct elf_writer_rel *rel_section(struct elf_writer *ew, const Elf64_Rel *r) { Elf64_Sym *sym; struct elf_writer_rel *rel; Elf64_Shdr shdr; struct buffer b; sym = &ew->symtab.syms[ELF64_R_SYM(r->r_info)]; /* Determine if section has been initialized yet. */ rel = &ew->rel_sections[sym->st_shndx]; if (rel->sec != NULL) return rel; memset(&shdr, 0, sizeof(shdr)); shdr.sh_type = SHT_REL; shdr.sh_link = section_index(ew, ew->symtab_sec); shdr.sh_info = sym->st_shndx; if (ew->bit64) { shdr.sh_addralign = sizeof(Elf64_Addr); shdr.sh_entsize = sizeof(Elf64_Rel); } else { shdr.sh_addralign = sizeof(Elf32_Addr); shdr.sh_entsize = sizeof(Elf32_Rel); } if ((strlen(".rel") + strlen(ew->sections[sym->st_shndx].name) + 1) > MAX_REL_NAME) { ERROR("Rel Section name won't fit\n"); return NULL; } strcat(rel->name, ".rel"); strcat(rel->name, ew->sections[sym->st_shndx].name); buffer_init(&b, rel->name, NULL, 0); elf_writer_add_section(ew, &shdr, &b, rel->name); rel->sec = last_section(ew); return rel; } static int add_rel(struct elf_writer_rel *rel_sec, const Elf64_Rel *rel) { if (rel_sec->num_entries == rel_sec->max_entries) { size_t num = rel_sec->max_entries * 2; Elf64_Rel *old_rels; if (num == 0) num = 128; old_rels = rel_sec->rels; rel_sec->rels = calloc(num, sizeof(Elf64_Rel)); memcpy(rel_sec->rels, old_rels, rel_sec->num_entries * sizeof(Elf64_Rel)); free(old_rels); rel_sec->max_entries = num; } rel_sec->rels[rel_sec->num_entries] = *rel; rel_sec->num_entries++; return 0; } int elf_writer_add_rel(struct elf_writer *ew, const char *sym, Elf64_Addr addr) { Elf64_Rel rel; Elf64_Xword sym_info; int sym_index; struct elf_writer_rel *rel_sec; sym_index = elf_sym_index(ew, sym); if (sym_index < 0) { ERROR("Unable to locate symbol: %s\n", sym); return -1; } sym_info = sym_index; /* The relocation type will get fixed prior to serialization. */ rel.r_offset = addr; rel.r_info = ELF64_R_INFO(sym_info, 0); rel_sec = rel_section(ew, &rel); if (rel_sec == NULL) return -1; return add_rel(rel_sec, &rel); }