为什么ELF部分之间有空闲空间？

看来在Linux（Ubuntu .eh_frame位）上使用gcc 4.9.2创build的二进制文件在段.eh_frame和.init_array之间有几千个未使用的字节。 从简单的可执行文件objdump -h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.eh_frame处结束，但是.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，很难观察到某些更改会影响代码大小。

这个洞从哪里来？ 有没有办法避免它？

• 为什么ELF头文件与文本段一起被加载到内存中？
• 在共享库中重定位条目
• Linux ELF文件：对于静态和dynamicELF程序，哪个字节会有所不同？
• 预链接仅对相对重定位有影响
• 创buildELF二进制文件，而不使用libelf或其他库

更新： ld --verbose ：

 $ 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 

 [...] .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 (.); 

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