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分类: LINUX

2009-03-22 12:32:21

static void elf_write(void)
{
    int nsections, align;
    int scount;
    char *p;
    int commlen;
    char comment[64];
    int i;

    struct SAA *symtab;
    long symtablen, symtablocal;

    /*
     * Work out how many sections we will have. We have SHN_UNDEF,
     * then the flexible user sections, then the four fixed
     * sections `.comment', `.shstrtab', `.symtab' and `.strtab',
     * then optionally relocation sections for the user sections.
     */

    if (of_elf.current_dfmt == &df_stabs)
        nsections = 8;
    else
        nsections = 5; /* SHN_UNDEF and the fixed ones */

    add_sectname("", ".comment");
    add_sectname("", ".shstrtab");
    add_sectname("", ".symtab");
    add_sectname("", ".strtab");
    /* 计算有几个 section */
    for (i = 0; i < nsects; i++) {
        nsections++; /* for the section itself */
        if (sects[i]->head) {
            nsections++; /* for its relocations */
            add_sectname(".rel", sects[i]->name);
        }
    }

    if (of_elf.current_dfmt == &df_stabs) {
        /* in case the debug information is wanted, just add these three sections... */
        add_sectname("", ".stab");
        add_sectname("", ".stabstr");
        add_sectname(".rel", ".stab");
    }

    /*
     * Do the comment.
     */

    *comment = '\0';
    commlen = 2 + sprintf(comment + 1, "The Netwide Assembler %s", NASM_VER);

    /*
     * Output the ELF header.
     */

    fwrite("\177ELF\1\1\1\0\0\0\0\0\0\0\0\0", 16, 1, elffp);
    fwriteshort(1, elffp); /* ET_REL relocatable file */
    fwriteshort(3, elffp); /* EM_386 processor ID */
    fwritelong(1L, elffp); /* EV_CURRENT file format version */
    fwritelong(0L, elffp); /* no entry point */
    fwritelong(0L, elffp); /* no program header table */
    fwritelong(0x40L, elffp); /* section headers straight after
                                 * ELF header plus alignment */

    fwritelong(0L, elffp); /* 386 defines no special flags */
    fwriteshort(0x34, elffp); /* size of ELF header */
    fwriteshort(0, elffp); /* no program header table, again */
    fwriteshort(0, elffp); /* still no program header table */
    fwriteshort(0x28, elffp); /* size of section header */
    fwriteshort(nsections, elffp); /* number of sections */
    fwriteshort(nsects + 2, elffp); /* string table section index for
                                         * section header table */

    fwritelong(0L, elffp); /* align to 0x40 bytes */
    fwritelong(0L, elffp);
    fwritelong(0L, elffp);

    /*
     * Build the symbol table and relocation tables.
     */

    symtab = elf_build_symtab(&symtablen, &symtablocal);
    for (i = 0; i < nsects; i++)
        if (sects[i]->head)
            sects[i]->rel = elf_build_reltab(&sects[i]->rellen, sects[i]->head);

    /*
     * Now output the section header table.
     */


    elf_foffs = 0x40 + 0x28 * nsections; /* elf_foffs 存放的是 section 的内容存放的其实位置。 */
    align = ((elf_foffs + SEG_ALIGN_1) & ~SEG_ALIGN_1) - elf_foffs;
    elf_foffs += align;
    elf_nsect = 0;
    elf_sects = nasm_malloc(sizeof(*elf_sects) * (2 * nsects + 10));

    elf_section_header(0, 0, 0, NULL, FALSE, 0L, 0, 0, 0, 0); /* SHN_UNDEF */
    scount = 1; /* needed for the stabs debugging to track the symtable section */
    p = shstrtab + 1; /* 存放 section name 的字符串 */
    /* 处理 segment 的 section */
    for (i = 0; i < nsects; i++) {
        elf_section_header(p - shstrtab, sects[i]->type, sects[i]->flags, (sects[i]->type == SHT_PROGBITS ? sects[i]->data : NULL), TRUE, sects[i]->len, 0, 0, sects[i]->align, 0);
        p += strlen(p) + 1;
        scount++; /* dito */
    }
    /* commnet */
    elf_section_header(p - shstrtab, 1, 0, comment, FALSE, (long)commlen, 0, 0, 1, 0); /* .comment */
    scount++; /* dito */
    p += strlen(p) + 1;
    /* 存放 section 名字的 section */
    elf_section_header(p - shstrtab, 3, 0, shstrtab, FALSE, (long)shstrtablen, 0, 0, 1, 0); /* .shstrtab */
    scount++; /* dito */
    p += strlen(p) + 1;
    /* 存放 符号表实例的 section */
    elf_section_header(p - shstrtab, 2, 0, symtab, TRUE, symtablen, nsects + 4, symtablocal, 4, 16); /* .symtab */
    symtabsection = scount; /* now we got the symtab section index in the ELF file */
    p += strlen(p) + 1;
    /* 存放字符串的 section */
    elf_section_header(p - shstrtab, 3, 0, strs, TRUE, strslen, 0, 0, 1, 0); /* .strtab */
    /* 处理重定位 section */
    for (i = 0; i < nsects; i++)
        if (sects[i]->head) {
            p += strlen(p) + 1;
            elf_section_header(p - shstrtab, 9, 0, sects[i]->rel, TRUE, sects[i]->rellen, nsects + 3, i + 1, 4, 8);
        }
    if (of_elf.current_dfmt == &df_stabs) {
        /* for debugging information, create the last three sections
           which are the .stab , .stabstr and .rel.stab sections respectively */


        /* this function call creates the stab sections in memory */
        stabs_generate();

        if ((stabbuf) && (stabstrbuf) && (stabrelbuf)) {
            p += strlen(p) + 1;
            elf_section_header(p - shstrtab, 1, 0, stabbuf, 0, stablen, nsections - 2, 0, 4, 12);

            p += strlen(p) + 1;
            elf_section_header(p - shstrtab, 3, 0, stabstrbuf, 0, stabstrlen, 0, 0, 4, 0);

            p += strlen(p) + 1;
            /* link -> symtable info -> section to refer to */
            elf_section_header(p - shstrtab, 9, 0, stabrelbuf, 0, stabrellen, symtabsection, nsections - 3, 4, 8);
        }
    }
    fwrite(align_str, align, 1, elffp);

    /*
     * Now output the sections.
     */

    elf_write_sections();

    nasm_free(elf_sects);
    saa_free(symtab);
}

static struct SAA *elf_build_symtab(long *len, long *local)
{
    struct SAA *s = saa_init(1L);
    struct Symbol *sym;
    unsigned char entry[16], *p;
    int i;

    *len = *local = 0;

    /*
     * First, an all-zeros entry, required by the ELF spec.
     */

    saa_wbytes(s, NULL, 16L); /* null symbol table entry */
    *len += 16;
    (*local)++;

    /*
     * Next, an entry for the file name.
     */

    p = entry;
    WRITELONG(p, 1); /* we know it's 1st thing in strtab */
    WRITELONG(p, 0); /* no value */
    WRITELONG(p, 0); /* no size either */
    WRITESHORT(p, 4); /* type FILE */
    WRITESHORT(p, SHN_ABS);
    saa_wbytes(s, entry, 16L);
    *len += 16;
    (*local)++;

    /*
     * Now some standard symbols defining the segments, for relocation
     * purposes.
     */

    for (i = 1; i <= nsects + 1; i++) {
        p = entry;
        WRITELONG(p, 0); /* no symbol name */
        WRITELONG(p, 0); /* offset zero */
        WRITELONG(p, 0); /* size zero */
        WRITESHORT(p, 3); /* local section-type thing */
        WRITESHORT(p, (i == 1 ? SHN_ABS : i - 1)); /* the section id */
        saa_wbytes(s, entry, 16L);
        *len += 16;
        (*local)++;
    }

    /*
     * Now the other local symbols.
     */

    saa_rewind(syms);
    while ((sym = saa_rstruct(syms))) {
        if (sym->type & SYM_GLOBAL)
            continue;
        p = entry;
        WRITELONG(p, sym->strpos);
        WRITELONG(p, sym->value);
        WRITELONG(p, sym->size);
        WRITESHORT(p, sym->type); /* local non-typed thing */
        WRITESHORT(p, sym->section);
        saa_wbytes(s, entry, 16L);
        *len += 16;
        (*local)++;
    }

    /*
     * Now the global symbols.
     */

    saa_rewind(syms);
    while ((sym = saa_rstruct(syms))) {
        if (!(sym->type & SYM_GLOBAL))
            continue;
        p = entry;
        WRITELONG(p, sym->strpos);
        WRITELONG(p, sym->value);
        WRITELONG(p, sym->size);
        WRITESHORT(p, sym->type); /* global non-typed thing */
        WRITESHORT(p, sym->section);
        saa_wbytes(s, entry, 16L);
        *len += 16;
    }

    return s;
}

static struct SAA *elf_build_reltab(long *len, struct Reloc *r)
{
    struct SAA *s;
    unsigned char *p, entry[8];

    if (!r)
        return NULL;

    s = saa_init(1L);
    *len = 0;

    while (r) {
        long sym = r->symbol;

        if (sym >= GLOBAL_TEMP_BASE) /* extern func3
                                             * call func3
                                             */

            sym += -GLOBAL_TEMP_BASE + (nsects + 3) + nlocals;

        p = entry;
        WRITELONG(p, r->address);
        WRITELONG(p, (sym << 8) + r->type);
        saa_wbytes(s, entry, 8L);
        *len += 8;

        r = r->next;
    }

    return s;
}



/* 输入 section header */
static void elf_section_header(int name, int type, int flags, void *data, int is_saa, long datalen, int link, int info, int align, int eltsize)
{
    elf_sects[elf_nsect].data = data;
    elf_sects[elf_nsect].len = datalen;
    elf_sects[elf_nsect].is_saa = is_saa;
    elf_nsect++;

    fwritelong((long)name, elffp);
    fwritelong((long)type, elffp);
    fwritelong((long)flags, elffp);
    fwritelong(0L, elffp); /* no address, ever, in object files */
    fwritelong(type == 0 ? 0L : elf_foffs, elffp);
    fwritelong(datalen, elffp);
    if (data)
        elf_foffs += (datalen + SEG_ALIGN_1) & ~SEG_ALIGN_1;
    fwritelong((long)link, elffp);
    fwritelong((long)info, elffp);
    fwritelong((long)align, elffp);
    fwritelong((long)eltsize, elffp);
}

static void elf_write_sections(void)
{
    int i;
    for (i = 0; i < elf_nsect; i++)
        if (elf_sects[i].data) {
            long len = elf_sects[i].len;
            long reallen = (len + SEG_ALIGN_1) & ~SEG_ALIGN_1;
            long align = reallen - len;
            if (elf_sects[i].is_saa)
                saa_fpwrite(elf_sects[i].data, elffp);
            else
                fwrite(elf_sects[i].data, len, 1, elffp);
            fwrite(align_str, align, 1, elffp); /* 不足之数以 0 垫尾 */
        }
}

static void elf_sect_write(struct Section *sect, const unsigned char *data, unsigned long len)
{
    saa_wbytes(sect->data, data, len);
    sect->len += len;
}

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