libc_init_static.cpp

/*
 * Copyright (C) 2008 The Android Open Source Project
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *  * Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *  * Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
 * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */
/*
 * libc_init_static.c
 *
 * The program startup function __libc_init() defined here is
 * used for static executables only (i.e. those that don't depend
 * on shared libraries). It is called from arch-$ARCH/bionic/crtbegin_static.S
 * which is directly invoked by the kernel when the program is launched.
 *
 * The 'structors' parameter contains pointers to various initializer
 * arrays that must be run before the program's 'main' routine is launched.
 */

#include <elf.h>
#include <errno.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/auxv.h>
#include <sys/mman.h>

#include "atexit.h"
#include "bionic_tls.h"
#include "KernelArgumentBlock.h"
#include "libc_init_common.h"
#include "pthread_internal.h"

// Returns the address of the page containing address 'x'.
#define PAGE_START(x)  ((x) & PAGE_MASK)

// Returns the address of the next page after address 'x', unless 'x' is
// itself at the start of a page.
#define PAGE_END(x)    PAGE_START((x) + (PAGE_SIZE-1))

static void call_array(void(**list)()) {
  // First element is -1, list is null-terminated
  while (*++list) {
    (*list)();
  }
}

static void apply_gnu_relro() {
  Elf_Phdr* phdr_start = reinterpret_cast<Elf_Phdr*>(getauxval(AT_PHDR));
  unsigned long int phdr_ct = getauxval(AT_PHNUM);

  for (Elf_Phdr* phdr = phdr_start; phdr < (phdr_start + phdr_ct); phdr++) {
    if (phdr->p_type != PT_GNU_RELRO) {
      continue;
    }

    Elf_Addr seg_page_start = PAGE_START(phdr->p_vaddr);
    Elf_Addr seg_page_end = PAGE_END(phdr->p_vaddr + phdr->p_memsz);

    // Check return value here? What do we do if we fail?
    mprotect(reinterpret_cast<void*>(seg_page_start), seg_page_end - seg_page_start, PROT_READ);
  }
}

__noreturn void __libc_init(void* raw_args,
                            void (*onexit)(void),
                            int (*slingshot)(int, char**, char**),
                            structors_array_t const * const structors) {
  KernelArgumentBlock args(raw_args);
  __libc_init_tls(args);
  __libc_init_common(args);

  apply_gnu_relro();

  // Several Linux ABIs don't pass the onexit pointer, and the ones that
  // do never use it.  Therefore, we ignore it.

  call_array(structors->preinit_array);
  call_array(structors->init_array);

  // The executable may have its own destructors listed in its .fini_array
  // so we need to ensure that these are called when the program exits
  // normally.
  if (structors->fini_array != NULL) {
    __cxa_atexit(__libc_fini,structors->fini_array,NULL);
  }

  exit(slingshot(args.argc, args.argv, args.envp));
}