out.c

   1 #include <elf.h>
   2 #include <string.h>
   3 #include <unistd.h>
   4 #include "gen.h"
   5
   6 #define MAXSECS         (1 << 7)
   7 #define MAXSYMS         (1 << 10)
   8 #define MAXRELA         (1 << 10)
   9 #define SEC_SYMS                1
  10 #define SEC_SYMSTR              2
  11 #define SEC_BEG                 3
  12
  13 static Elf64_Ehdr ehdr;
  14 static Elf64_Shdr shdr[MAXSECS];
  15 static int nshdr = SEC_BEG;
  16 static Elf64_Sym syms[MAXSYMS];
  17 static int nsyms;
  18 static char symstr[MAXSYMS * 8];
  19 static int nsymstr = 1;
  20
  21 static struct sec {
  22         char buf[SECSIZE];
  23         int len;
  24         Elf64_Sym *sym;
  25         Elf64_Rela rela[MAXRELA];
  26         int nrela;
  27         Elf64_Shdr *sec_shdr;
  28         Elf64_Shdr *rel_shdr;
  29 } secs[MAXSECS];
  30 static int nsecs;
  31 static struct sec *sec;
  32
  33 static char *putstr(char *s, char *r)
  34 {
  35         while (*r)
  36                 *s++ = *r++;
  37         *s++ = '\0';
  38         return s;
  39 }
  40
  41 static Elf64_Sym *put_sym(char *name)
  42 {
  43         Elf64_Sym *sym = &syms[nsyms++];
  44         sym->st_name = nsymstr;
  45         nsymstr = putstr(symstr + nsymstr, name) - symstr;
  46         sym->st_info = ELF64_ST_INFO(STB_GLOBAL, STT_FUNC);
  47         return sym;
  48 }
  49
  50 static int sym_find(char *name)
  51 {
  52         int i;
  53         Elf64_Sym *sym;
  54         for (i = 0; i < nsyms; i++)
  55                 if (!strcmp(name, symstr + syms[i].st_name))
  56                         return i;
  57         sym = put_sym(name);
  58         sym->st_shndx = SHN_UNDEF;
  59         return sym - syms;
  60 }
  61
  62 void out_func_beg(char *name)
  63 {
  64         sec = &secs[nsecs++];
  65         sec->sym = put_sym(name);
  66         sec->sym->st_shndx = nshdr;
  67         sec->sec_shdr = &shdr[nshdr++];
  68         sec->rel_shdr = &shdr[nshdr++];
  69         sec->rel_shdr->sh_link = SEC_SYMS;
  70         sec->rel_shdr->sh_info = sec->sec_shdr - shdr;
  71 }
  72
  73 void out_func_end(char *buf, int len)
  74 {
  75         memcpy(sec->buf, buf, len);
  76         sec->len = len;
  77         sec->sym->st_size = len;
  78 }
  79
  80 void out_rela(char *name, int off)
  81 {
  82         Elf64_Rela *rela = &sec->rela[sec->nrela++];
  83         rela->r_offset = off;
  84         rela->r_info = ELF64_R_INFO(sym_find(name), R_X86_64_32);
  85 }
  86
  87 void out_write(int fd)
  88 {
  89         Elf64_Shdr *symstr_shdr = &shdr[SEC_SYMSTR];
  90         Elf64_Shdr *syms_shdr = &shdr[SEC_SYMS];
  91         unsigned long offset = sizeof(ehdr);
  92         int i;
  93
  94         ehdr.e_ident[0] = 0x7f;
  95         ehdr.e_ident[1] = 'E';
  96         ehdr.e_ident[2] = 'L';
  97         ehdr.e_ident[3] = 'F';
  98         ehdr.e_ident[4] = ELFCLASS64;
  99         ehdr.e_ident[5] = ELFDATA2LSB;
 100         ehdr.e_ident[6] = EV_CURRENT;
 101         ehdr.e_type = ET_REL;
 102         ehdr.e_machine = EM_X86_64;
 103         ehdr.e_version = EV_CURRENT;
 104         ehdr.e_ehsize = sizeof(ehdr);
 105         ehdr.e_shentsize = sizeof(shdr[0]);
 106         ehdr.e_shoff = offset;
 107         ehdr.e_shnum = nshdr;
 108         ehdr.e_shstrndx = SEC_SYMSTR;
 109         offset += sizeof(shdr[0]) * nshdr;
 110
 111         syms_shdr->sh_type = SHT_SYMTAB;
 112         syms_shdr->sh_offset = offset;
 113         syms_shdr->sh_size = nsyms * sizeof(syms[0]);
 114         syms_shdr->sh_entsize = sizeof(syms[0]);
 115         syms_shdr->sh_link = SEC_SYMSTR;
 116         offset += syms_shdr->sh_size;
 117
 118         symstr_shdr->sh_type = SHT_STRTAB;
 119         symstr_shdr->sh_offset = offset;
 120         symstr_shdr->sh_size = nsymstr;
 121         symstr_shdr->sh_entsize = 1;
 122         offset += symstr_shdr->sh_size;
 123
 124         for (i = 0; i < nsecs; i++) {
 125                 struct sec *sec = &secs[i];
 126
 127                 sec->sec_shdr->sh_type = SHT_PROGBITS;
 128                 sec->sec_shdr->sh_flags = SHF_EXECINSTR;
 129                 sec->sec_shdr->sh_offset = offset;
 130                 sec->sec_shdr->sh_size = sec->len;
 131                 sec->sec_shdr->sh_entsize = 1;
 132                 offset += sec->sec_shdr->sh_size;
 133
 134                 sec->rel_shdr->sh_type = SHT_RELA;
 135                 sec->rel_shdr->sh_offset = offset;
 136                 sec->rel_shdr->sh_size = sec->nrela * sizeof(sec->rela[0]);
 137                 sec->rel_shdr->sh_entsize = sizeof(sec->rela[0]);
 138                 offset += sec->rel_shdr->sh_size;
 139         }
 140
 141         write(fd, &ehdr, sizeof(ehdr));
 142         write(fd, shdr, sizeof(shdr[0]) * nshdr);
 143         write(fd, syms, sizeof(syms[0]) * nsyms);
 144         write(fd, symstr, nsymstr);
 145         for (i = 0; i < nsecs; i++) {
 146                 struct sec *sec = &secs[i];
 147                 write(fd, sec->buf, sec->len);
 148                 write(fd, sec->rela, sec->nrela * sizeof(sec->rela[0]));
 149         }
 150 }