linux - 为什么 ELF 部分之间有未使用的空白空间？

Question

似乎在 Linux(Ubuntu 15.04，32 位)上使用 gcc 4.9.2 创建的二进制文件在 .eh_frame 和 .init_array 部分之间有几千个未使用的字节>。 objdump -h 的简单可执行文件输出示例:

Sections:
Idx Name          Size      VMA       LMA       File off  Algn
[...]
 16 .eh_frame     000000c0  080484ac  080484ac  000004ac  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
 17 .init_array   00000004  08049f08  08049f08  00000f08  2**2
                  CONTENTS, ALLOC, LOAD, DATA
[...]

.eh_frame 在文件偏移量 0x56c 处结束，但 .init_array 从 0xf08 处开始，留下 0x99c = 2460 字节的空洞。所有其他部分在上一节结束后立即开始。

未使用空间的大小各不相同，因此很难观察某些更改如何影响代码大小。

这个洞是从哪里来的？有办法避免吗？

更新 ld --verbose 的输出:

$ cat so.c

int main() {
    return 0;
}

$ gcc so.c -Wl,--verbose -o so
GNU ld (GNU Binutils for Ubuntu) 2.25
  Supported emulations:
   elf_i386
   i386linux
   elf32_x86_64
   elf_x86_64
   elf_l1om
   elf_k1om
   i386pep
   i386pe
using internal linker script:
==================================================
/* Script for -z combreloc: combine and sort reloc sections */
/* Copyright (C) 2014 Free Software Foundation, Inc.
   Copying and distribution of this script, with or without modification,
   are permitted in any medium without royalty provided the copyright
   notice and this notice are preserved.  */
OUTPUT_FORMAT("elf32-i386", "elf32-i386",
              "elf32-i386")
OUTPUT_ARCH(i386)
ENTRY(_start)
SEARCH_DIR("=/usr/i686-linux-gnu/lib32"); SEARCH_DIR("=/usr/local/lib32"); SEARCH_DIR("=/lib32"); SEARCH_DIR("=/usr/lib32"); SEARCH_DIR("=/usr/i686-linux-gnu/lib"); SEARCH_DIR("=/usr/local/lib/i386-linux-gnu"); SEARCH_DIR("=/usr/local/lib"); SEARCH_DIR("=/lib/i386-linux-gnu"); SEARCH_DIR("=/lib"); SEARCH_DIR("=/usr/lib/i386-linux-gnu"); SEARCH_DIR("=/usr/lib");
SECTIONS
{
  /* Read-only sections, merged into text segment: */
  PROVIDE (__executable_start = SEGMENT_START("text-segment", 0x08048000)); . = SEGMENT_START("text-segment", 0x08048000) + SIZEOF_HEADERS;
  .interp         : { *(.interp) }
  .note.gnu.build-id : { *(.note.gnu.build-id) }
  .hash           : { *(.hash) }
  .gnu.hash       : { *(.gnu.hash) }
  .dynsym         : { *(.dynsym) }
  .dynstr         : { *(.dynstr) }
  .gnu.version    : { *(.gnu.version) }
  .gnu.version_d  : { *(.gnu.version_d) }
  .gnu.version_r  : { *(.gnu.version_r) }
  .rel.dyn        :
    {
      *(.rel.init)
      *(.rel.text .rel.text.* .rel.gnu.linkonce.t.*)
      *(.rel.fini)
      *(.rel.rodata .rel.rodata.* .rel.gnu.linkonce.r.*)
      *(.rel.data.rel.ro .rel.data.rel.ro.* .rel.gnu.linkonce.d.rel.ro.*)
      *(.rel.data .rel.data.* .rel.gnu.linkonce.d.*)
      *(.rel.tdata .rel.tdata.* .rel.gnu.linkonce.td.*)
      *(.rel.tbss .rel.tbss.* .rel.gnu.linkonce.tb.*)
      *(.rel.ctors)
      *(.rel.dtors)
      *(.rel.got)
      *(.rel.bss .rel.bss.* .rel.gnu.linkonce.b.*)
      *(.rel.ifunc)
    }
  .rel.plt        :
    {
      *(.rel.plt)
      PROVIDE_HIDDEN (__rel_iplt_start = .);
      *(.rel.iplt)
      PROVIDE_HIDDEN (__rel_iplt_end = .);
    }
  .init           :
  {
    KEEP (*(SORT_NONE(.init)))
  }
  .plt            : { *(.plt) *(.iplt) }
  .text           :
  {
    *(.text.unlikely .text.*_unlikely .text.unlikely.*)
    *(.text.exit .text.exit.*)
    *(.text.startup .text.startup.*)
    *(.text.hot .text.hot.*)
    *(.text .stub .text.* .gnu.linkonce.t.*)
    /* .gnu.warning sections are handled specially by elf32.em.  */
    *(.gnu.warning)
  }
  .fini           :
  {
    KEEP (*(SORT_NONE(.fini)))
  }
  PROVIDE (__etext = .);
  PROVIDE (_etext = .);
  PROVIDE (etext = .);
  .rodata         : { *(.rodata .rodata.* .gnu.linkonce.r.*) }
  .rodata1        : { *(.rodata1) }
  .eh_frame_hdr : { *(.eh_frame_hdr) }
  .eh_frame       : ONLY_IF_RO { KEEP (*(.eh_frame)) }
  .gcc_except_table   : ONLY_IF_RO { *(.gcc_except_table
  .gcc_except_table.*) }
  /* These sections are generated by the Sun/Oracle C++ compiler.  */
  .exception_ranges   : ONLY_IF_RO { *(.exception_ranges
  .exception_ranges*) }
  /* Adjust the address for the data segment.  We want to adjust up to
     the same address within the page on the next page up.  */
  . = ALIGN (CONSTANT (MAXPAGESIZE)) - ((CONSTANT (MAXPAGESIZE) - .) & (CONSTANT (MAXPAGESIZE) - 1)); . = DATA_SEGMENT_ALIGN (CONSTANT (MAXPAGESIZE), CONSTANT (COMMONPAGESIZE));
  /* Exception handling  */
  .eh_frame       : ONLY_IF_RW { KEEP (*(.eh_frame)) }
  .gcc_except_table   : ONLY_IF_RW { *(.gcc_except_table .gcc_except_table.*) }
  .exception_ranges   : ONLY_IF_RW { *(.exception_ranges .exception_ranges*) }
  /* Thread Local Storage sections  */
  .tdata          : { *(.tdata .tdata.* .gnu.linkonce.td.*) }
  .tbss           : { *(.tbss .tbss.* .gnu.linkonce.tb.*) *(.tcommon) }
  .preinit_array     :
  {
    PROVIDE_HIDDEN (__preinit_array_start = .);
    KEEP (*(.preinit_array))
    PROVIDE_HIDDEN (__preinit_array_end = .);
  }
  .init_array     :
  {
    PROVIDE_HIDDEN (__init_array_start = .);
    KEEP (*(SORT_BY_INIT_PRIORITY(.init_array.*) SORT_BY_INIT_PRIORITY(.ctors.*)))
    KEEP (*(.init_array EXCLUDE_FILE (*crtbegin.o *crtbegin?.o *crtend.o *crtend?.o ) .ctors))
    PROVIDE_HIDDEN (__init_array_end = .);
  }
  .fini_array     :
  {
    PROVIDE_HIDDEN (__fini_array_start = .);
    KEEP (*(SORT_BY_INIT_PRIORITY(.fini_array.*) SORT_BY_INIT_PRIORITY(.dtors.*)))
    KEEP (*(.fini_array EXCLUDE_FILE (*crtbegin.o *crtbegin?.o *crtend.o *crtend?.o ) .dtors))
    PROVIDE_HIDDEN (__fini_array_end = .);
  }
  .ctors          :
  {
    /* gcc uses crtbegin.o to find the start of
       the constructors, so we make sure it is
       first.  Because this is a wildcard, it
       doesn't matter if the user does not
       actually link against crtbegin.o; the
       linker won't look for a file to match a
       wildcard.  The wildcard also means that it
       doesn't matter which directory crtbegin.o
       is in.  */
    KEEP (*crtbegin.o(.ctors))
    KEEP (*crtbegin?.o(.ctors))
    /* We don't want to include the .ctor section from
       the crtend.o file until after the sorted ctors.
       The .ctor section from the crtend file contains the
       end of ctors marker and it must be last */
    KEEP (*(EXCLUDE_FILE (*crtend.o *crtend?.o ) .ctors))
    KEEP (*(SORT(.ctors.*)))
    KEEP (*(.ctors))
  }
  .dtors          :
  {
    KEEP (*crtbegin.o(.dtors))
    KEEP (*crtbegin?.o(.dtors))
    KEEP (*(EXCLUDE_FILE (*crtend.o *crtend?.o ) .dtors))
    KEEP (*(SORT(.dtors.*)))
    KEEP (*(.dtors))
  }
  .jcr            : { KEEP (*(.jcr)) }
  .data.rel.ro : { *(.data.rel.ro.local* .gnu.linkonce.d.rel.ro.local.*) *(.data.rel.ro .data.rel.ro.* .gnu.linkonce.d.rel.ro.*) }
  .dynamic        : { *(.dynamic) }
  .got            : { *(.got) *(.igot) }
  . = DATA_SEGMENT_RELRO_END (SIZEOF (.got.plt) >= 12 ? 12 : 0, .);
  .got.plt        : { *(.got.plt)  *(.igot.plt) }
  .data           :
  {
    *(.data .data.* .gnu.linkonce.d.*)
    SORT(CONSTRUCTORS)
  }
  .data1          : { *(.data1) }
  _edata = .; PROVIDE (edata = .);
  . = .;
  __bss_start = .;
  .bss            :
  {
   *(.dynbss)
   *(.bss .bss.* .gnu.linkonce.b.*)
   *(COMMON)
   /* Align here to ensure that the .bss section occupies space up to
      _end.  Align after .bss to ensure correct alignment even if the
      .bss section disappears because there are no input sections.
      FIXME: Why do we need it? When there is no .bss section, we don't
      pad the .data section.  */
   . = ALIGN(. != 0 ? 32 / 8 : 1);
  }
  . = ALIGN(32 / 8);
  . = SEGMENT_START("ldata-segment", .);
  . = ALIGN(32 / 8);
  _end = .; PROVIDE (end = .);
  . = DATA_SEGMENT_END (.);
  /* Stabs debugging sections.  */
  .stab          0 : { *(.stab) }
  .stabstr       0 : { *(.stabstr) }
  .stab.excl     0 : { *(.stab.excl) }
  .stab.exclstr  0 : { *(.stab.exclstr) }
  .stab.index    0 : { *(.stab.index) }
  .stab.indexstr 0 : { *(.stab.indexstr) }
  .comment       0 : { *(.comment) }
  /* DWARF debug sections.
     Symbols in the DWARF debugging sections are relative to the beginning
     of the section so we begin them at 0.  */
  /* DWARF 1 */
  .debug          0 : { *(.debug) }
  .line           0 : { *(.line) }
  /* GNU DWARF 1 extensions */
  .debug_srcinfo  0 : { *(.debug_srcinfo) }
  .debug_sfnames  0 : { *(.debug_sfnames) }
  /* DWARF 1.1 and DWARF 2 */
  .debug_aranges  0 : { *(.debug_aranges) }
  .debug_pubnames 0 : { *(.debug_pubnames) }
  /* DWARF 2 */
  .debug_info     0 : { *(.debug_info .gnu.linkonce.wi.*) }
  .debug_abbrev   0 : { *(.debug_abbrev) }
  .debug_line     0 : { *(.debug_line .debug_line.* .debug_line_end ) }
  .debug_frame    0 : { *(.debug_frame) }
  .debug_str      0 : { *(.debug_str) }
  .debug_loc      0 : { *(.debug_loc) }
  .debug_macinfo  0 : { *(.debug_macinfo) }
  /* SGI/MIPS DWARF 2 extensions */
  .debug_weaknames 0 : { *(.debug_weaknames) }
  .debug_funcnames 0 : { *(.debug_funcnames) }
  .debug_typenames 0 : { *(.debug_typenames) }
  .debug_varnames  0 : { *(.debug_varnames) }
  /* DWARF 3 */
  .debug_pubtypes 0 : { *(.debug_pubtypes) }
  .debug_ranges   0 : { *(.debug_ranges) }
  /* DWARF Extension.  */
  .debug_macro    0 : { *(.debug_macro) }
  .gnu.attributes 0 : { KEEP (*(.gnu.attributes)) }
  /DISCARD/ : { *(.note.GNU-stack) *(.gnu_debuglink) *(.gnu.lto_*) }
}


==================================================
attempt to open /usr/lib/gcc/i686-linux-gnu/4.9/../../../i386-linux-gnu/crt1.o succeeded
/usr/lib/gcc/i686-linux-gnu/4.9/../../../i386-linux-gnu/crt1.o
attempt to open /usr/lib/gcc/i686-linux-gnu/4.9/../../../i386-linux-gnu/crti.o succeeded
/usr/lib/gcc/i686-linux-gnu/4.9/../../../i386-linux-gnu/crti.o
attempt to open /usr/lib/gcc/i686-linux-gnu/4.9/crtbegin.o succeeded
/usr/lib/gcc/i686-linux-gnu/4.9/crtbegin.o
attempt to open /tmp/ccQ0fTTK.o succeeded
/tmp/ccQ0fTTK.o
attempt to open /usr/lib/gcc/i686-linux-gnu/4.9/libgcc.so failed
attempt to open /usr/lib/gcc/i686-linux-gnu/4.9/libgcc.a succeeded
attempt to open /usr/lib/gcc/i686-linux-gnu/4.9/libgcc_s.so succeeded
-lgcc_s (/usr/lib/gcc/i686-linux-gnu/4.9/libgcc_s.so)
attempt to open /usr/lib/gcc/i686-linux-gnu/4.9/libc.so failed
attempt to open /usr/lib/gcc/i686-linux-gnu/4.9/libc.a failed
attempt to open /usr/lib/gcc/i686-linux-gnu/4.9/../../../i386-linux-gnu/libc.so succeeded
opened script file /usr/lib/gcc/i686-linux-gnu/4.9/../../../i386-linux-gnu/libc.so
opened script file /usr/lib/gcc/i686-linux-gnu/4.9/../../../i386-linux-gnu/libc.so
attempt to open /lib/i386-linux-gnu/libc.so.6 succeeded
/lib/i386-linux-gnu/libc.so.6
attempt to open /usr/lib/i386-linux-gnu/libc_nonshared.a succeeded
(/usr/lib/i386-linux-gnu/libc_nonshared.a)elf-init.oS
attempt to open /lib/i386-linux-gnu/ld-linux.so.2 succeeded
/lib/i386-linux-gnu/ld-linux.so.2
/lib/i386-linux-gnu/ld-linux.so.2
attempt to open /usr/lib/gcc/i686-linux-gnu/4.9/libgcc.so failed
attempt to open /usr/lib/gcc/i686-linux-gnu/4.9/libgcc.a succeeded
attempt to open /usr/lib/gcc/i686-linux-gnu/4.9/libgcc_s.so succeeded
-lgcc_s (/usr/lib/gcc/i686-linux-gnu/4.9/libgcc_s.so)
attempt to open /usr/lib/gcc/i686-linux-gnu/4.9/crtend.o succeeded
/usr/lib/gcc/i686-linux-gnu/4.9/crtend.o
attempt to open /usr/lib/gcc/i686-linux-gnu/4.9/../../../i386-linux-gnu/crtn.o succeeded
/usr/lib/gcc/i686-linux-gnu/4.9/../../../i386-linux-gnu/crtn.o
ld-linux.so.2 needed by /lib/i386-linux-gnu/libc.so.6
found ld-linux.so.2 at /lib/i386-linux-gnu/ld-linux.so.2

Answer 1

这里需要考虑三个内存区域:

• 只读数据。
• 可以在加载时修复的非惰性重定位。
• 数据。

现在，.eh_frame 部分被标记为READONLY，因此它进入第一部分。

.init_array 是指向初始化函数的函数指针数组，加载程序/库时可以解析为它们的绝对地址，然后标记为只读(写入函数指针是一个漏洞利用的常用方法)，因此它进入第二个区域。

链接描述文件的相关部分是:

[...]
.eh_frame       : ONLY_IF_RO { KEEP (*(.eh_frame)) }
[...]
/* Adjust the address for the data segment.  We want to adjust up to
   the same address within the page on the next page up.  */
. = ALIGN (CONSTANT (MAXPAGESIZE)) - ((CONSTANT (MAXPAGESIZE) - .) & (CONSTANT (MAXPAGESIZE) - 1));
. = DATA_SEGMENT_ALIGN (CONSTANT (MAXPAGESIZE), CONSTANT (COMMONPAGESIZE));
[...]
.init_array     :
[...]
.got            : { *(.got) *(.igot) }
. = DATA_SEGMENT_RELRO_END (SIZEOF (.got.plt) >= 12 ? 12 : 0, .);
.got.plt        : { *(.got.plt)  *(.igot.plt) }
.data           :
[...]
. = DATA_SEGMENT_END (.);

您可以在 https://sourceware.org/binutils/docs/ld/Builtin-Functions.html 查阅 GNU ld 链接器脚本的内置函数文档。 .但请注意，DATA_SEGMENT_ALIGN 文档不正确，正如 Stephen Kell 在 binutils bug #19203: "DATA_SEGMENT_ALIGN documentation is not consistent with behaviour" 所报告的那样，显然是因为 Jakub Jelinek 的 [PATCH] Fix DATA_SEGMENT_ALIGN . DATA_SEGMENT_ALIGN 本身是在名为 [RFC PATCH] Smarter aligning of data segment 的 binutils 邮件列表线程中引入的.

不知何故，以下内容:

. = ALIGN (CONSTANT (MAXPAGESIZE)) - ((CONSTANT (MAXPAGESIZE) - .) & (CONSTANT (MAXPAGESIZE) - 1));
. = DATA_SEGMENT_ALIGN (CONSTANT (MAXPAGESIZE), CONSTANT (COMMONPAGESIZE));

导致 1 页跳转，在您的示例中会将您从 0x0804856c 移动到 0x0804956c。

当使用链接器选项 -z relro 时，请求在加载时修复的重定位被标记为只读，DATA_SEGMENT_RELRO_END 导致先前的 DATA_SEGMENT_ALIGN 添加足够的填充以使 DATA_SEGMENT_RELRO_END 的两个参数之和与新页面对齐。

因此，假设 .got.plt 至少有三个指针，前三个指针(加载器立即使用)将位于第二个区域，而 的其余部分>.got.plt 第三个。

DATA_SEGMENT_ALIGN 添加的填充将您从 0x0804956c 移动到 0x08049f08。当发出修复后所有可以 mprotected 为只读的内容时，您将位于 0x0804a000，在一个新页面中，该页面将保持读写状态。