【发布时间】：2021-01-01 08:24:04
【问题描述】：
我们正在努力重现与参考相同的 .elf 文件。我们快到了，文件是相同的，但评论部分的地址（其余是 100% 相同的二进制方式）。
使用readelf -e 的其中一个.elf 文件的转储如下（目标是powerpc，但我认为它并不重要）：
ELF Header:
Magic: 7f 45 4c 46 01 02 01 00 00 00 00 00 00 00 00 00
Class: ELF32
Data: 2's complement, big endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: EXEC (Executable file)
Machine: PowerPC
Version: 0x1
Entry point address: 0x1800f88
Start of program headers: 52 (bytes into file)
Start of section headers: 1367724 (bytes into file)
Flags: 0x0
Size of this header: 52 (bytes)
Size of program headers: 32 (bytes)
Number of program headers: 5
Size of section headers: 40 (bytes)
Number of section headers: 20
Section header string table index: 19
Section Headers:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 0] NULL 00000000 000000 000000 00 0 0 0
[ 1] .interp PROGBITS 018000f4 0000f4 000011 00 A 0 0 1
[ 2] .hash HASH 01800108 000108 000248 04 A 3 0 4
[ 3] .dynsym DYNSYM 01800350 000350 0004d0 10 A 4 1 4
[ 4] .dynstr STRTAB 01800820 000820 000473 00 A 0 0 1
[ 5] .rela.plt RELA 01800c94 000c94 0002dc 0c A 3 16 4
[ 6] .rela.sbss RELA 01800f70 000f70 000018 0c A 3 15 4
[ 7] .text PROGBITS 01800f88 000f88 0e4f78 00 AX 0 0 4
[ 8] .rodata PROGBITS 018e5f00 0e5f00 064708 00 A 0 0 8
[ 9] .sdata2 PROGBITS 0194a608 14a608 000000 00 A 0 0 4
[10] .data PROGBITS 0198a608 14a608 000b5c 00 WA 0 0 8
[11] .dynamic DYNAMIC 0198b164 14b164 000090 08 WA 4 0 4
[12] .got2 PROGBITS 0198b1f4 14b1f4 000024 00 WA 0 0 1
[13] .got PROGBITS 0198b218 14b218 000010 04 WA 0 0 4
[14] .sdata PROGBITS 0198b228 14b228 001814 00 WA 0 0 8
[15] .sbss NOBITS 0198ca40 14ca40 00064c 00 WA 0 0 8
[16] .plt NOBITS 0198d08c 14ca40 000324 00 WAX 0 0 4
[17] .bss NOBITS 0198d3b0 14ca40 223920 00 WA 0 0 16
[18] .comment PROGBITS 0026a8bd 14ca40 0013cb 00 0 0 1
[19] .shstrtab STRTAB 00000000 14de0b 00009e 00 0 0 1
地址为0 的部分有意义：加载程序时未映射这些部分。而其他的0x18xxxxx 值由链接器规范文件指定。
但是为什么注释部分有一个非零地址（其中包含编译器版本和执行平台等详细信息），链的哪一部分使用它（调试器？），以及链接器如何决定放在哪里吗？
链接器脚本甚至没有提到这个.comment 部分：
OUTPUT_FORMAT("elf32-powerpc", "elf32-powerpc",
"elf32-powerpc")
OUTPUT_ARCH(powerpc)
ENTRY(_start)
SEARCH_DIR(/opt/gnu/powerpc-wrs-vxworks/lib);
/* Do we need any of these for elf?
__DYNAMIC = 0; */
PROVIDE (__stack = 0);
SECTIONS
{
/* Read-only sections, merged into text segment: */
. = 0x01800000 + SIZEOF_HEADERS;
.interp : { *(.interp) }
.hash : { *(.hash) }
.dynsym : { *(.dynsym) }
.dynstr : { *(.dynstr) }
.gnu.version : { *(.gnu.version) }
.gnu.version_d : { *(.gnu.version_d) }
.gnu.version_r : { *(.gnu.version_r) }
.rela.text :
{ *(.rela.text) *(.rela.gnu.linkonce.t*) }
.rela.data :
{ *(.rela.data) *(.rela.gnu.linkonce.d*) }
.rela.rodata :
{ *(.rela.rodata) *(.rela.gnu.linkonce.r*) }
.rela.got : { *(.rela.got) }
.rela.got1 : { *(.rela.got1) }
.rela.got2 : { *(.rela.got2) }
.rela.ctors : { *(.rela.ctors) }
.rela.dtors : { *(.rela.dtors) }
.rela.init : { *(.rela.init) }
.rela.fini : { *(.rela.fini) }
.rela.bss : { *(.rela.bss) }
.rela.plt : { *(.rela.plt) }
.rela.sdata : { *(.rela.sdata) }
.rela.sbss : { *(.rela.sbss) }
.rela.sdata2 : { *(.rela.sdata2) }
.rela.sbss2 : { *(.rela.sbss2) }
.text :
{
*(.text)
/* .gnu.warning sections are handled specially by elf32.em. */
*(.gnu.warning)
*(.gnu.linkonce.t*)
} =0
.init : { *(.init) } =0
.fini : { *(.fini) } =0
.rodata : { *(.rodata) *(.gnu.linkonce.r*) }
.rodata1 : { *(.rodata1) }
_etext = .;
PROVIDE (etext = .);
.sdata2 : { *(.sdata2) }
.sbss2 : { *(.sbss2) }
/* Adjust the address for the data segment. We want to adjust up to
the same address within the page on the next page up. It would
be more correct to do this:
. = ALIGN(0x40000) + (ALIGN(8) & (0x40000 - 1));
The current expression does not correctly handle the case of a
text segment ending precisely at the end of a page; it causes the
data segment to skip a page. The above expression does not have
this problem, but it will currently (2/95) cause BFD to allocate
a single segment, combining both text and data, for this case.
This will prevent the text segment from being shared among
multiple executions of the program; I think that is more
important than losing a page of the virtual address space (note
that no actual memory is lost; the page which is skipped can not
be referenced). */
. = ALIGN(8) + 0x40000;
.data :
{
*(.data)
*(.gnu.linkonce.d*)
CONSTRUCTORS
}
.data1 : { *(.data1) }
.got1 : { *(.got1) }
.dynamic : { *(.dynamic) }
/* Put .ctors and .dtors next to the .got2 section, so that the pointers
get relocated with -mrelocatable. Also put in the .fixup pointers.
The current compiler no longer needs this, but keep it around for 2.7.2 */
PROVIDE (_GOT2_START_ = .);
.got2 : { *(.got2) }
PROVIDE (__CTOR_LIST__ = .);
.ctors : { *(.ctors) }
PROVIDE (__CTOR_END__ = .);
PROVIDE (__DTOR_LIST__ = .);
.dtors : { *(.dtors) }
PROVIDE (__DTOR_END__ = .);
PROVIDE (_FIXUP_START_ = .);
.fixup : { *(.fixup) }
PROVIDE (_FIXUP_END_ = .);
PROVIDE (_GOT2_END_ = .);
PROVIDE (_GOT_START_ = .);
.got : { *(.got) }
.got.plt : { *(.got.plt) }
PROVIDE (_GOT_END_ = .);
/* We want the small data sections together, so single-instruction offsets
can access them all, and initialized data all before uninitialized, so
we can shorten the on-disk segment size. */
.sdata : { *(.sdata) }
_edata = .;
PROVIDE (edata = .);
.sbss :
{
PROVIDE (__sbss_start = .);
*(.sbss)
*(.scommon)
*(.dynsbss)
PROVIDE (__sbss_end = .);
}
.plt : { *(.plt) }
.bss :
{
PROVIDE (__bss_start = .);
*(.dynbss)
*(.bss)
*(COMMON)
}
_end = . ;
PROVIDE (end = .);
/* These are needed for ELF backends which have not yet been
converted to the new style linker. */
.stab 0 : { *(.stab) }
.stabstr 0 : { *(.stabstr) }
/* 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) }
.debug_abbrev 0 : { *(.debug_abbrev) }
.debug_line 0 : { *(.debug_line) }
.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) }
/* These must appear regardless of . */
}
链接器版本也很旧（是的，我知道...）：
GNU ld 2.9.1
Copyright 1997 Free Software Foundation, Inc.
【问题讨论】：
-
答案很可能在您系统的链接器脚本中找到。与
gcc -v -Wl,--verbose链接，你会得到它的转储。 -
AFAIK
.comment部分没有被任何工具特别使用；如果他们需要知道使用的编译器版本，程序员可以手动检查（使用objdump或readelf）。 -
我也是这么想的，但在那种情况下，为什么这个奇怪的地址？它应该为零。
-
是的，我也不知道地址的原因，但我怀疑它是由链接描述文件直接或间接确定的。
-
添加了链接描述文件。没有
.comment的踪迹。 AFAIR 如果有一个没有映射的部分，链接器选择放置它选择的任何内容（除非我们丢弃它）
标签： cross-compiling executable elf