c - 静态计算地址的对齐页面

Question

我需要静态计算包含精灵文本段的第一页的地址，以便使用mprotect()并使文本段可写。

Section Headers: [Nr] Name Type Addr Off Size ES Flg Lk Inf Al .. [14] .text PROGBITS 08048380 000380 0002e0 00 AX 0 0 128

有任何想法吗？

Answer 1

这个程序怎么样，编译正常，不会崩溃。

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <unistd.h>
#include <sys/mman.h>

extern char __executable_start;
extern char __etext;

int
main (int argc, char **argv)
{
  int pagesize = sysconf (_SC_PAGE_SIZE);
  char *start =
    (char *) (((uintptr_t) & __executable_start) & ~(pagesize - 1));
  char *end =
    (char *) (((uintptr_t) & __etext + pagesize - 1) & ~(pagesize - 1));
  mprotect (start, end - start, PROT_READ | PROT_WRITE | PROT_EXEC);
  printf ("Hello world\n");
  void *m = main;
  *((char *) m) = 0;
  exit (0);
}

我用过__executable_startand __etext，但你可能会更好地看看你是否能让这些工作，这至少记录在手册页中：

姓名

  `etext`, `edata`, `end` - end of program segments

概要

  extern etext;
  extern edata;
  extern end;

描述

  The addresses of these symbols indicate the end of various program segments:

  `etext`  This is the first address past the end of the text segment (the program
           code).

  `edata`  This is the first address past the end of the initialized data segment.

  `end`    This  is the first address past the end of the uninitialized data
           segment (also known as the BSS segment).

符合

  Although these symbols have long been provided on most UNIX systems, they are
  not standardized; use with caution.