QEMU ELF_LOAER分析[基于MIPS]

本文用于记录对QEMU对ELF文件加载函数进行分析。根据“函数使用->函数定义->函数实现->函数实现的分析”的顺序进行分析，最终提取出ELF文件加载的代码。

1. load_elf

mips malta中，对load_elf的使用如下：

    if (load_elf(loaderparams.kernel_filename, cpu_mips_kseg0_to_phys, NULL,
                 (uint64_t *)&kernel_entry, NULL, (uint64_t *)&kernel_high,
                 big_endian, ELF_MACHINE, ) < ) {
        fprintf(stderr, "qemu: could not load kernel '%s'\n",
                loaderparams.kernel_filename);
        exit();
    }    

load_elf在头文件include/hw/loader.h中，函数定义如下：

int load_elf(const char *filename, uint64_t (*translate_fn)(void *, uint64_t),
             void *translate_opaque, uint64_t *pentry, uint64_t *lowaddr,
             uint64_t *highaddr, int big_endian, int elf_machine,
             int clear_lsb); 

load_elf的具体实现在hw/core/loader.c中，函数体如下：

int load_elf(const char *filename, uint64_t (*translate_fn)(void *, uint64_t),
             void *translate_opaque, uint64_t *pentry, uint64_t *lowaddr,
             uint64_t *highaddr, int big_endian, int elf_machine, int clear_lsb)
{
    int fd, data_order, target_data_order, must_swab, ret = ELF_LOAD_FAILED;
    uint8_t e_ident[EI_NIDENT];

    fd = open(filename, O_RDONLY | O_BINARY);
    if (fd < ) {
        perror(filename);
        return -;
    }
    if (read(fd, e_ident, sizeof(e_ident)) != sizeof(e_ident))
        goto fail;
    if (e_ident[] != ELFMAG0 ||
        e_ident[] != ELFMAG1 ||
        e_ident[] != ELFMAG2 ||
        e_ident[] != ELFMAG3) {
        ret = ELF_LOAD_NOT_ELF;
        goto fail;
    }
#ifdef HOST_WORDS_BIGENDIAN
    data_order = ELFDATA2MSB;
#else
    data_order = ELFDATA2LSB;
#endif
    must_swab = data_order != e_ident[EI_DATA];
    if (big_endian) {
        target_data_order = ELFDATA2MSB;
    } else {
        target_data_order = ELFDATA2LSB;
    }

    if (target_data_order != e_ident[EI_DATA]) {
        ret = ELF_LOAD_WRONG_ENDIAN;
        goto fail;
    }

    lseek(fd, , SEEK_SET);
    if (e_ident[EI_CLASS] == ELFCLASS64) {
        ret = load_elf64(filename, fd, translate_fn, translate_opaque, must_swab,
                         pentry, lowaddr, highaddr, elf_machine, clear_lsb);
    } else {
        ret = load_elf32(filename, fd, translate_fn, translate_opaque, must_swab,
                         pentry, lowaddr, highaddr, elf_machine, clear_lsb);
    }

 fail:
    close(fd);
    return ret;
}

在load_elf中，对elf文件进行读取分析的核心函数为load_elf64和load_elf32，下面将把它们进行展开。

2、load_elf64/load_elf32

load_elf64和load_elf32是通过glue(load_elf, SZ)来进行定义的，所在文件include/hw/elf_ops.h，具体函数如下：

static int glue(load_elf, SZ)(const char *name, int fd,
                              uint64_t (*translate_fn)(void *, uint64_t),
                              void *translate_opaque,
                              int must_swab, uint64_t *pentry,
                              uint64_t *lowaddr, uint64_t *highaddr,
                              int elf_machine, int clear_lsb)
{
    struct elfhdr ehdr;
    struct elf_phdr *phdr = NULL, *ph;
    int size, i, total_size;
    elf_word mem_size, file_size;
    uint64_t addr, low = (uint64_t)-, high = ;
    uint8_t *data = NULL;
    char label[];
    int ret = ELF_LOAD_FAILED; 

    if (read(fd, &ehdr, sizeof(ehdr)) != sizeof(ehdr))
        goto fail;
    if (must_swab) {
        glue(bswap_ehdr, SZ)(&ehdr);
    } 

    switch (elf_machine) {
        case EM_PPC64:
            if (EM_PPC64 != ehdr.e_machine)
                if (EM_PPC != ehdr.e_machine) {
                    ret = ELF_LOAD_WRONG_ARCH;
                    goto fail;
                }
            break;
        case EM_X86_64:
            if (EM_X86_64 != ehdr.e_machine)
                if (EM_386 != ehdr.e_machine) {
                    ret = ELF_LOAD_WRONG_ARCH;
                    goto fail;
                }
            break;
        case EM_MICROBLAZE:
            if (EM_MICROBLAZE != ehdr.e_machine)
                if (EM_MICROBLAZE_OLD != ehdr.e_machine) {
                    ret = ELF_LOAD_WRONG_ARCH;
                    goto fail;
                }
            break;
        default:
            if (elf_machine != ehdr.e_machine) {
                ret = ELF_LOAD_WRONG_ARCH;
                goto fail;
            }
    } 

    if (pentry)
    *pentry = (uint64_t)(elf_sword)ehdr.e_entry; 

    glue(load_symbols, SZ)(&ehdr, fd, must_swab, clear_lsb); 

    size = ehdr.e_phnum * sizeof(phdr[]);
    if (lseek(fd, ehdr.e_phoff, SEEK_SET) != ehdr.e_phoff) {
        goto fail;
    }
    phdr = g_malloc0(size);
    if (!phdr)
        goto fail;
    if (read(fd, phdr, size) != size)
        goto fail;
    if (must_swab) {
        for(i = ; i < ehdr.e_phnum; i++) {
            ph = &phdr[i];
            glue(bswap_phdr, SZ)(ph);
        }
    } 

    total_size = ;
    for(i = ; i < ehdr.e_phnum; i++) {
        ph = &phdr[i];
        if (ph->p_type == PT_LOAD) {
            mem_size = ph->p_memsz; /* Size of the ROM */
            file_size = ph->p_filesz; /* Size of the allocated data */
            data = g_malloc0(file_size);
            if (ph->p_filesz > ) {
                if (lseek(fd, ph->p_offset, SEEK_SET) < ) {
                    goto fail;
                }
                if (read(fd, data, file_size) != file_size) {
                    goto fail;
                }
            }
            /* address_offset is hack for kernel images that are
               linked at the wrong physical address.  */
            if (translate_fn) {
                addr = translate_fn(translate_opaque, ph->p_paddr);
                glue(elf_reloc, SZ)(&ehdr, fd, must_swab,  translate_fn,
                                    translate_opaque, data, ph, elf_machine);
            } else {
                addr = ph->p_paddr;
            } 

            /* the entry pointer in the ELF header is a virtual
             * address, if the text segments paddr and vaddr differ
             * we need to adjust the entry */
            if (pentry && !translate_fn &&
                    ph->p_vaddr != ph->p_paddr &&
                    ehdr.e_entry >= ph->p_vaddr &&
                    ehdr.e_entry < ph->p_vaddr + ph->p_filesz &&
                    ph->p_flags & PF_X) {
                *pentry = ehdr.e_entry - ph->p_vaddr + ph->p_paddr;
            } 

            snprintf(label, sizeof(label), "phdr #%d: %s", i, name); 

            /* rom_add_elf_program() seize the ownership of 'data' */
            rom_add_elf_program(label, data, file_size, mem_size, addr); 

            total_size += mem_size;
            if (addr < low)
                low = addr;
            if ((addr + mem_size) > high)
                high = addr + mem_size; 

            data = NULL;
        }
    }
    g_free(phdr);
    if (lowaddr)
        *lowaddr = (uint64_t)(elf_sword)low;
    if (highaddr)
        *highaddr = (uint64_t)(elf_sword)high;
    return total_size;
 fail:
    g_free(data);
    g_free(phdr);
    return ret;
} 

其中，glue在文件include/qemu/compiler.h中，定义如下：

#ifndef glue
#define xglue(x, y) x ## y
#define glue(x, y) xglue(x, y)
#define stringify(s)    tostring(s)
#define tostring(s) #s
#endif

根据定义，我们可以知道，“glue(load_elf, SZ)”经过展开，会变成“load_elfSZ”，当SZ为32和64的时候，结果就是load_elf32和load_elf64了。

在文件hw/core/loader.c中，

#define SZ      32
#define elf_word        uint32_t
#define elf_sword        int32_t
#define bswapSZs    bswap32s
#include "hw/elf_ops.h"

#undef elfhdr
#undef elf_phdr
#undef elf_shdr
#undef elf_sym
#undef elf_rela
#undef elf_note
#undef elf_word
#undef elf_sword
#undef bswapSZs
#undef SZ
#define elfhdr      elf64_hdr
#define elf_phdr    elf64_phdr
#define elf_note    elf64_note
#define elf_shdr    elf64_shdr
#define elf_sym     elf64_sym
#define elf_rela        elf64_rela
#define elf_word        uint64_t
#define elf_sword        int64_t
#define bswapSZs    bswap64s
#define SZ      64
#include "hw/elf_ops.h"

我们可以看到，loader.c在包涵elf_ops.h的时候，先对SZ等进行了宏定义。通过两次宏定义和包含，就得到了load_elf32和load_elf64。

在glue(load_elf, SZ)中，使用了以下4个函数：

glue(bswap_ehdr, SZ)(&ehdr);

glue(load_symbols, SZ)(&ehdr, fd, must_swab, clear_lsb);

glue(bswap_phdr, SZ)(ph);

glue(elf_reloc, SZ)(&ehdr, fd, must_swab,  translate_fn,
                    translate_opaque, data, ph, elf_machine);

它们都在文件include/hw/elf_ops.h中。elf_ops.h中的其它函数也会在解析elf文件时用到，所以我们将elf_ops.h全部贴出来

3、 include/hw/elf_ops.h，文件内容如下：

static void glue(bswap_ehdr, SZ)(struct elfhdr *ehdr)
{
    bswap16s(&ehdr->e_type);			/* Object file type */
    bswap16s(&ehdr->e_machine);		/* Architecture */
    bswap32s(&ehdr->e_version);		/* Object file version */
    bswapSZs(&ehdr->e_entry);		/* Entry point virtual address */
    bswapSZs(&ehdr->e_phoff);		/* Program header table file offset */
    bswapSZs(&ehdr->e_shoff);		/* Section header table file offset */
    bswap32s(&ehdr->e_flags);		/* Processor-specific flags */
    bswap16s(&ehdr->e_ehsize);		/* ELF header size in bytes */
    bswap16s(&ehdr->e_phentsize);		/* Program header table entry size */
    bswap16s(&ehdr->e_phnum);		/* Program header table entry count */
    bswap16s(&ehdr->e_shentsize);		/* Section header table entry size */
    bswap16s(&ehdr->e_shnum);		/* Section header table entry count */
    bswap16s(&ehdr->e_shstrndx);		/* Section header string table index */
}

static void glue(bswap_phdr, SZ)(struct elf_phdr *phdr)
{
    bswap32s(&phdr->p_type);			/* Segment type */
    bswapSZs(&phdr->p_offset);		/* Segment file offset */
    bswapSZs(&phdr->p_vaddr);		/* Segment virtual address */
    bswapSZs(&phdr->p_paddr);		/* Segment physical address */
    bswapSZs(&phdr->p_filesz);		/* Segment size in file */
    bswapSZs(&phdr->p_memsz);		/* Segment size in memory */
    bswap32s(&phdr->p_flags);		/* Segment flags */
    bswapSZs(&phdr->p_align);		/* Segment alignment */
}

static void glue(bswap_shdr, SZ)(struct elf_shdr *shdr)
{
    bswap32s(&shdr->sh_name);
    bswap32s(&shdr->sh_type);
    bswapSZs(&shdr->sh_flags);
    bswapSZs(&shdr->sh_addr);
    bswapSZs(&shdr->sh_offset);
    bswapSZs(&shdr->sh_size);
    bswap32s(&shdr->sh_link);
    bswap32s(&shdr->sh_info);
    bswapSZs(&shdr->sh_addralign);
    bswapSZs(&shdr->sh_entsize);
}

static void glue(bswap_sym, SZ)(struct elf_sym *sym)
{
    bswap32s(&sym->st_name);
    bswapSZs(&sym->st_value);
    bswapSZs(&sym->st_size);
    bswap16s(&sym->st_shndx);
}

static void glue(bswap_rela, SZ)(struct elf_rela *rela)
{
    bswapSZs(&rela->r_offset);
    bswapSZs(&rela->r_info);
    bswapSZs((elf_word *)&rela->r_addend);
}

static struct elf_shdr *glue(find_section, SZ)(struct elf_shdr *shdr_table,
                                               int n, int type)
{
    int i;
    for(i=0;i<n;i++) {
        if (shdr_table[i].sh_type == type)
            return shdr_table + i;
    }
    return NULL;
}

static int glue(symfind, SZ)(const void *s0, const void *s1)
{
    hwaddr addr = *(hwaddr *)s0;
    struct elf_sym *sym = (struct elf_sym *)s1;
    int result = 0;
    if (addr < sym->st_value) {
        result = -1;
    } else if (addr >= sym->st_value + sym->st_size) {
        result = 1;
    }
    return result;
}

static const char *glue(lookup_symbol, SZ)(struct syminfo *s,
                                           hwaddr orig_addr)
{
    struct elf_sym *syms = glue(s->disas_symtab.elf, SZ);
    struct elf_sym *sym;

    sym = bsearch(&orig_addr, syms, s->disas_num_syms, sizeof(*syms),
                  glue(symfind, SZ));
    if (sym != NULL) {
        return s->disas_strtab + sym->st_name;
    }

    return "";
}

static int glue(symcmp, SZ)(const void *s0, const void *s1)
{
    struct elf_sym *sym0 = (struct elf_sym *)s0;
    struct elf_sym *sym1 = (struct elf_sym *)s1;
    return (sym0->st_value < sym1->st_value)
        ? -1
        : ((sym0->st_value > sym1->st_value) ? 1 : 0);
}

static int glue(load_symbols, SZ)(struct elfhdr *ehdr, int fd, int must_swab,
                                  int clear_lsb)
{
    struct elf_shdr *symtab, *strtab, *shdr_table = NULL;
    struct elf_sym *syms = NULL;
    struct syminfo *s;
    int nsyms, i;
    char *str = NULL;

    shdr_table = load_at(fd, ehdr->e_shoff,
                         sizeof(struct elf_shdr) * ehdr->e_shnum);
    if (!shdr_table)
        return -1;

    if (must_swab) {
        for (i = 0; i < ehdr->e_shnum; i++) {
            glue(bswap_shdr, SZ)(shdr_table + i);
        }
    }

    symtab = glue(find_section, SZ)(shdr_table, ehdr->e_shnum, SHT_SYMTAB);
    if (!symtab)
        goto fail;
    syms = load_at(fd, symtab->sh_offset, symtab->sh_size);
    if (!syms)
        goto fail;

    nsyms = symtab->sh_size / sizeof(struct elf_sym);

    i = 0;
    while (i < nsyms) {
        if (must_swab)
            glue(bswap_sym, SZ)(&syms[i]);
        /* We are only interested in function symbols.
           Throw everything else away.  */
        if (syms[i].st_shndx == SHN_UNDEF ||
                syms[i].st_shndx >= SHN_LORESERVE ||
                ELF_ST_TYPE(syms[i].st_info) != STT_FUNC) {
            nsyms--;
            if (i < nsyms) {
                syms[i] = syms[nsyms];
            }
            continue;
        }
        if (clear_lsb) {
            /* The bottom address bit marks a Thumb or MIPS16 symbol.  */
            syms[i].st_value &= ~(glue(glue(Elf, SZ), _Addr))1;
        }
        i++;
    }
    syms = g_realloc(syms, nsyms * sizeof(*syms));

    qsort(syms, nsyms, sizeof(*syms), glue(symcmp, SZ));
    for (i = 0; i < nsyms - 1; i++) {
        if (syms[i].st_size == 0) {
            syms[i].st_size = syms[i + 1].st_value - syms[i].st_value;
        }
    }

    /* String table */
    if (symtab->sh_link >= ehdr->e_shnum)
        goto fail;
    strtab = &shdr_table[symtab->sh_link];

    str = load_at(fd, strtab->sh_offset, strtab->sh_size);
    if (!str)
        goto fail;

    /* Commit */
    s = g_malloc0(sizeof(*s));
    s->lookup_symbol = glue(lookup_symbol, SZ);
    glue(s->disas_symtab.elf, SZ) = syms;
    s->disas_num_syms = nsyms;
    s->disas_strtab = str;
    s->next = syminfos;
    syminfos = s;
    g_free(shdr_table);
    return 0;
 fail:
    g_free(syms);
    g_free(str);
    g_free(shdr_table);
    return -1;
}

static int glue(elf_reloc, SZ)(struct elfhdr *ehdr, int fd, int must_swab,
                               uint64_t (*translate_fn)(void *, uint64_t),
                               void *translate_opaque, uint8_t *data,
                               struct elf_phdr *ph, int elf_machine)
{
    struct elf_shdr *reltab, *shdr_table = NULL;
    struct elf_rela *rels = NULL;
    int nrels, i, ret = -1;
    elf_word wordval;
    void *addr;

    shdr_table = load_at(fd, ehdr->e_shoff,
                         sizeof(struct elf_shdr) * ehdr->e_shnum);
    if (!shdr_table) {
        return -1;
    }
    if (must_swab) {
        for (i = 0; i < ehdr->e_shnum; i++) {
            glue(bswap_shdr, SZ)(&shdr_table[i]);
        }
    }

    reltab = glue(find_section, SZ)(shdr_table, ehdr->e_shnum, SHT_RELA);
    if (!reltab) {
        goto fail;
    }
    rels = load_at(fd, reltab->sh_offset, reltab->sh_size);
    if (!rels) {
        goto fail;
    }
    nrels = reltab->sh_size / sizeof(struct elf_rela);

    for (i = 0; i < nrels; i++) {
        if (must_swab) {
            glue(bswap_rela, SZ)(&rels[i]);
        }
        if (rels[i].r_offset < ph->p_vaddr ||
            rels[i].r_offset >= ph->p_vaddr + ph->p_filesz) {
            continue;
        }
        addr = &data[rels[i].r_offset - ph->p_vaddr];
        switch (elf_machine) {
        case EM_S390:
            switch (rels[i].r_info) {
            case R_390_RELATIVE:
                wordval = *(elf_word *)addr;
                if (must_swab) {
                    bswapSZs(&wordval);
                }
                wordval = translate_fn(translate_opaque, wordval);
                if (must_swab) {
                    bswapSZs(&wordval);
                }
                *(elf_word *)addr = wordval;
                break;
            default:
                fprintf(stderr, "Unsupported relocation type %i!\n",
                        (int)rels[i].r_info);
            }
        }
    }

    ret = 0;
fail:
    g_free(rels);
    g_free(shdr_table);
    return ret;
}

static int glue(load_elf, SZ)(const char *name, int fd,
                              uint64_t (*translate_fn)(void *, uint64_t),
                              void *translate_opaque,
                              int must_swab, uint64_t *pentry,
                              uint64_t *lowaddr, uint64_t *highaddr,
                              int elf_machine, int clear_lsb)
{
    struct elfhdr ehdr;
    struct elf_phdr *phdr = NULL, *ph;
    int size, i, total_size;
    elf_word mem_size, file_size;
    uint64_t addr, low = (uint64_t)-1, high = 0;
    uint8_t *data = NULL;
    char label[128];
    int ret = ELF_LOAD_FAILED;

    if (read(fd, &ehdr, sizeof(ehdr)) != sizeof(ehdr))
        goto fail;
    if (must_swab) {
        glue(bswap_ehdr, SZ)(&ehdr);
    }

    switch (elf_machine) {
        case EM_PPC64:
            if (EM_PPC64 != ehdr.e_machine)
                if (EM_PPC != ehdr.e_machine) {
                    ret = ELF_LOAD_WRONG_ARCH;
                    goto fail;
                }
            break;
        case EM_X86_64:
            if (EM_X86_64 != ehdr.e_machine)
                if (EM_386 != ehdr.e_machine) {
                    ret = ELF_LOAD_WRONG_ARCH;
                    goto fail;
                }
            break;
        case EM_MICROBLAZE:
            if (EM_MICROBLAZE != ehdr.e_machine)
                if (EM_MICROBLAZE_OLD != ehdr.e_machine) {
                    ret = ELF_LOAD_WRONG_ARCH;
                    goto fail;
                }
            break;
        default:
            if (elf_machine != ehdr.e_machine) {
                ret = ELF_LOAD_WRONG_ARCH;
                goto fail;
            }
    }

    if (pentry)
   	*pentry = (uint64_t)(elf_sword)ehdr.e_entry;

    glue(load_symbols, SZ)(&ehdr, fd, must_swab, clear_lsb);

    size = ehdr.e_phnum * sizeof(phdr[0]);
    if (lseek(fd, ehdr.e_phoff, SEEK_SET) != ehdr.e_phoff) {
        goto fail;
    }
    phdr = g_malloc0(size);
    if (!phdr)
        goto fail;
    if (read(fd, phdr, size) != size)
        goto fail;
    if (must_swab) {
        for(i = 0; i < ehdr.e_phnum; i++) {
            ph = &phdr[i];
            glue(bswap_phdr, SZ)(ph);
        }
    }

    total_size = 0;
    for(i = 0; i < ehdr.e_phnum; i++) {
        ph = &phdr[i];
        if (ph->p_type == PT_LOAD) {
            mem_size = ph->p_memsz; /* Size of the ROM */
            file_size = ph->p_filesz; /* Size of the allocated data */
            data = g_malloc0(file_size);
            if (ph->p_filesz > 0) {
                if (lseek(fd, ph->p_offset, SEEK_SET) < 0) {
                    goto fail;
                }
                if (read(fd, data, file_size) != file_size) {
                    goto fail;
                }
            }
            /* address_offset is hack for kernel images that are
               linked at the wrong physical address.  */
            if (translate_fn) {
                addr = translate_fn(translate_opaque, ph->p_paddr);
                glue(elf_reloc, SZ)(&ehdr, fd, must_swab,  translate_fn,
                                    translate_opaque, data, ph, elf_machine);
            } else {
                addr = ph->p_paddr;
            }

            /* the entry pointer in the ELF header is a virtual
             * address, if the text segments paddr and vaddr differ
             * we need to adjust the entry */
            if (pentry && !translate_fn &&
                    ph->p_vaddr != ph->p_paddr &&
                    ehdr.e_entry >= ph->p_vaddr &&
                    ehdr.e_entry < ph->p_vaddr + ph->p_filesz &&
                    ph->p_flags & PF_X) {
                *pentry = ehdr.e_entry - ph->p_vaddr + ph->p_paddr;
            }

            snprintf(label, sizeof(label), "phdr #%d: %s", i, name);

            /* rom_add_elf_program() seize the ownership of 'data' */
            rom_add_elf_program(label, data, file_size, mem_size, addr);

            total_size += mem_size;
            if (addr < low)
                low = addr;
            if ((addr + mem_size) > high)
                high = addr + mem_size;

            data = NULL;
        }
    }
    g_free(phdr);
    if (lowaddr)
        *lowaddr = (uint64_t)(elf_sword)low;
    if (highaddr)
        *highaddr = (uint64_t)(elf_sword)high;
    return total_size;
 fail:
    g_free(data);
    g_free(phdr);
    return ret;
}

其中，bswap16s在文件include/qemu/bswap.h中，其定义如下：

static inline void bswap16s(uint16_t *s)
{
    *s = bswap16(*s);
}

static inline uint16_t bswap16(uint16_t x)
{
    return bswap_16(x);
}

4、load_at

load_at在hw/core/loader.c中，其定义如下：

static void *load_at(int fd, off_t offset, size_t size)
{
    void *ptr;
    if (lseek(fd, offset, SEEK_SET) < 0)
        return NULL;
    ptr = g_malloc(size);
    if (read(fd, ptr, size) != size) {
        g_free(ptr);
        return NULL;
    }
    return ptr;
}

5、ELF文件格式：关于ELF文件格式的内容摘自百度百科

在计算机科学中，是一种用于二进制文件、可执行文件、目标代码、共享库和核心转储的标准文件格式。

是UNIX系统实验室（USL）作为应用程序二进制接口（Application Binary Interface，ABI）而开发和发布的，也是Linux的主要可执行文件格式。

1999年，被86open项目选为x86架构上的类Unix操作系统的二进制文件标准格式，用来取代COFF。因其可扩展性与灵活性，也可应用在其它处理器、计算机系统架构的操作系统上。

ELF文件由4部分组成，分别是ELF头（ELF header）、程序头表（Program header table）、节（Section）和节头表（Section header table）。实际上，一个文件中不一定包含全部内容，而且他们的位置也未必如同所示这样安排，只有ELF头的位置是固定的，其余各部分的位置、大小等信息有ELF头中的各项值来决定。