ncv/ncrewrite.c

   1
   2 #include <assert.h>
   3 #include <fcntl.h>
   4 #include <stdio.h>
   5 #include <string.h>
   6 #include <sys/mman.h>
   7 #include <sys/stat.h>
   8 #include <unistd.h>
   9
  10 #include "native_client/include/elf.h"
  11 #include "native_client/ncv/ncdecode.h"
  12
  13
  14 static int write_nops = 0;
  15
  16 static void handle_instruction(const struct NCDecoderState *mstate)
  17 {
  18   /* Rewrites
  19        nop; nop; nop
  20        jmp *%reg
  21      to
  22        and $0xffffffe0, %reg
  23        jmp *%reg
  24   */
  25   if(mstate->opinfo->insttype == NACLi_INDIRECT) {
  26     unsigned char and_instr[3] = {
  27       0x83, /* "and" opcode */
  28       0xe0 | modrm_rm(mstate->inst.maddr[1]), /* operand */
  29       0xe0, /* alignment mask, constant ~31 */
  30     };
  31
  32     if((PreviousInst(mstate, -1) != NULL &&
  33         memcmp(mstate->inst.maddr - 3, and_instr, 3) == 0) ||
  34        (PreviousInst(mstate, -3) != NULL &&
  35         mstate->inst.maddr[-3] == 0x90 &&
  36         mstate->inst.maddr[-2] == 0x90 &&
  37         mstate->inst.maddr[-1] == 0x90)) {
  38       if(write_nops)
  39         memset(mstate->inst.maddr - 3, 0x90, 3);
  40       else
  41         memcpy(mstate->inst.maddr - 3, and_instr, 3);
  42     }
  43     else {
  44       fprintf(stderr, "%08x: cannot rewrite\n", mstate->inst.vaddr);
  45     }
  46   }
  47 }
  48
  49 static void fixup_file(const char *filename)
  50 {
  51   int fd = open(filename, O_RDWR);
  52   if(fd < 0) {
  53     perror("open");
  54     exit(1);
  55   }
  56   struct stat st;
  57   if(fstat(fd, &st) < 0) {
  58     perror("fstat");
  59     exit(1);
  60   }
  61   unsigned char *data = mmap(NULL, st.st_size, PROT_READ | PROT_WRITE,
  62                              MAP_SHARED, fd, 0);
  63   if(data == MAP_FAILED) {
  64     perror("mmap");
  65     exit(1);
  66   }
  67   if(close(fd) < 0) {
  68     perror("close");
  69     exit(1);
  70   }
  71   Elf32_Ehdr *header = (Elf32_Ehdr *) data;
  72   assert(memcmp(header->e_ident, ELFMAG, strlen(ELFMAG)) == 0);
  73   int i;
  74   for(i = 0; i < header->e_shnum; i++) {
  75     Elf32_Shdr *section = (Elf32_Shdr *) (data + header->e_shoff +
  76                                           header->e_shentsize * i);
  77     if((section->sh_flags & SHF_EXECINSTR) != 0) {
  78       NCDecodeSegment(data + section->sh_offset, section->sh_addr,
  79                       section->sh_size, NULL);
  80     }
  81   }
  82   if(munmap(data, st.st_size) < 0) {
  83     perror("munmap");
  84     exit(1);
  85   }
  86 }
  87
  88 int main(int argc, const char *argv[])
  89 {
  90   int i;
  91   NCDecodeRegisterCallbacks(handle_instruction, NULL, NULL, NULL);
  92   for(i = 1; i < argc; i++) {
  93     const char *arg = argv[i];
  94     if(strcmp(arg, "--nop") == 0)
  95       write_nops = 1;
  96     else
  97       fixup_file(arg);
  98   }
  99   return 0;
 100 }