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。来自objdump -h 的简单可执行文件的示例输出：

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 以文件偏移量 0x56c 结束，但 .init_array 从 0xf08 开始，留下 0x99c = 2460 字节的空洞。所有其他部分在上一部分结束后立即开始。

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
{
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      *(.rel.ifunc)
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      PROVIDE_HIDDEN (__rel_iplt_start = .);
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      PROVIDE_HIDDEN (__rel_iplt_end = .);
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  .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.*)
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  .fini           :
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  PROVIDE (__etext = .);
  PROVIDE (_etext = .);
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  .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, .);
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  .data           :
  {
    *(.data .data.* .gnu.linkonce.d.*)
    SORT(CONSTRUCTORS)
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  .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
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   . = ALIGN(. != 0 ? 32 / 8 : 1);
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  . = ALIGN(32 / 8);
  . = SEGMENT_START("ldata-segment", .);
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  _end = .; PROVIDE (end = .);
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  /* Stabs debugging sections.  */
  .stab          0 : { *(.stab) }
  .stabstr       0 : { *(.stabstr) }
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  .stab.index    0 : { *(.stab.index) }
  .stab.indexstr 0 : { *(.stab.indexstr) }
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  /* GNU DWARF 1 extensions */
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}


==================================================
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延迟重定位。
• 数据。

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

.init_array是初始化函数的函数指针数组，可以在加载程序/库时解析到它们的绝对地址，然后标记为只读（写入函数指针是常用的漏洞利用方式） ，所以它进入第二个区域。

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 查阅有关 GNU ld 链接器脚本的内置函数的文档。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，如下：

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

导致一页跳转，在您的示例中，这会将您从 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 导致之前的 DATA_SEGMENT_ALIGN 添加足够的填充以导致 @ 的两个参数的总和987654335@ 与新页面对齐。

因此，假设.got.plt 至少有三个指针，前三个指针（加载程序会立即使用）将位于第二个区域，.got.plt 的其余部分位于第三个区域。

DATA_SEGMENT_ALIGN 添加的填充将您从 0x0804956c 移动到 0x08049f08。当所有可以在修复后被 mprotected 只读的东西发出时，您将位于 0x0804a000，在一个新页面中，该页面将保持读写状态。