#8: loader v0.5 與結果輸出（改善與小討論）

时间：2009-04-10 来源：AppleDragon

jollen 發表於 December 6, 2006 1:12 AM

我們接續 loader v0.4 的工作，強化一下輸出結果的可讀性；先來比較一下 loader v0.4 與 loader v0.5 的輸出畫面。

$ ./loader-0.4 loader-0.4

$ ./loader-0.5 loader-0.4

ELF Identification
 Class: 32-bit objects
Machine: Intel 80386
Num of secionts: 34
.interp
.note.ABI-tag
.hash
.dynsym
.dynstr
.gnu.version
.gnu.version_r
.rel.dyn
.rel.plt
.init
.plt
.text
.fini
.rodata
.eh_frame
.data
.dynamic
.ctors
.dtors
.jcr
.got
.bss
.comment
.debug_aranges
.debug_pubnames
.debug_info
.debug_abbrev
.debug_line
.debug_frame
.debug_str
.shstrtab
.symtab
.strtab

ELF Identification
 Class: 32-bit objects
Machine: Intel 80386
Name Size FileOff
[01] .interp 19 244
[02] .note.ABI-tag 32 264
[03] .hash 56 296
[04] .dynsym 144 352
[05] .dynstr 98 496
[06] .gnu.version 18 594
[07] .gnu.version_r 32 612
[08] .rel.dyn 8 644
[09] .rel.plt 48 652
[10] .init 23 700
[11] .plt 112 724
[12] .text 1292 836
[13] .fini 27 2128
[14] .rodata 348 2156
[15] .eh_frame 4 2504
[16] .data 12 2508
[17] .dynamic 200 2520
[18] .ctors 8 2720
[19] .dtors 8 2728
[20] .jcr 4 2736
[21] .got 40 2740
[22] .bss 4 2780
[23] .comment 306 2780
[24] .debug_aranges 120 3088
[25] .debug_pubnames 37 3208
[26] .debug_info 2692 3245
[27] .debug_abbrev 312 5937
[28] .debug_line 636 6249
[29] .debug_frame 20 6888
[30] .debug_str 1722 6908
[31] .shstrtab 299 8630
[32] .symtab 1856 10292
[33] .strtab 1132 12148

Cool！做出了 'objdump -x' 的部份功能。頗為有趣，那麼用 objdump 來互相比較一下看看。

loader v0.5 v.s. objdump v.s. readelf

$ objdump -x loader-0.4（只擷取 section 輸出結果）

$ ./loader-0.5 loader-0.4

Sections:
Idx Name Size VMA LMA File off Algn
 0 .interp 00000013 080480f4 080480f4 000000f4 2**0
 CONTENTS, ALLOC, LOAD, READONLY, DATA
 1 .note.ABI-tag 00000020 08048108 08048108 00000108 2**2
 CONTENTS, ALLOC, LOAD, READONLY, DATA
 2 .hash 00000038 08048128 08048128 00000128 2**2
 CONTENTS, ALLOC, LOAD, READONLY, DATA
 3 .dynsym 00000090 08048160 08048160 00000160 2**2
 CONTENTS, ALLOC, LOAD, READONLY, DATA
 4 .dynstr 00000062 080481f0 080481f0 000001f0 2**0
 CONTENTS, ALLOC, LOAD, READONLY, DATA
 5 .gnu.version 00000012 08048252 08048252 00000252 2**1
 CONTENTS, ALLOC, LOAD, READONLY, DATA
 6 .gnu.version_r 00000020 08048264 08048264 00000264 2**2
 CONTENTS, ALLOC, LOAD, READONLY, DATA
 7 .rel.dyn 00000008 08048284 08048284 00000284 2**2
 CONTENTS, ALLOC, LOAD, READONLY, DATA
 8 .rel.plt 00000030 0804828c 0804828c 0000028c 2**2
 CONTENTS, ALLOC, LOAD, READONLY, DATA
 9 .init 00000017 080482bc 080482bc 000002bc 2**2
 CONTENTS, ALLOC, LOAD, READONLY, CODE
 10 .plt 00000070 080482d4 080482d4 000002d4 2**2
 CONTENTS, ALLOC, LOAD, READONLY, CODE
 11 .text 0000050c 08048344 08048344 00000344 2**2
 CONTENTS, ALLOC, LOAD, READONLY, CODE
 12 .fini 0000001b 08048850 08048850 00000850 2**2
 CONTENTS, ALLOC, LOAD, READONLY, CODE
 13 .rodata 0000015c 0804886c 0804886c 0000086c 2**2
 CONTENTS, ALLOC, LOAD, READONLY, DATA
 14 .eh_frame 00000004 080489c8 080489c8 000009c8 2**2
 CONTENTS, ALLOC, LOAD, READONLY, DATA
 15 .data 0000000c 080499cc 080499cc 000009cc 2**2
 CONTENTS, ALLOC, LOAD, DATA
 16 .dynamic 000000c8 080499d8 080499d8 000009d8 2**2
 CONTENTS, ALLOC, LOAD, DATA
 17 .ctors 00000008 08049aa0 08049aa0 00000aa0 2**2
 CONTENTS, ALLOC, LOAD, DATA
 18 .dtors 00000008 08049aa8 08049aa8 00000aa8 2**2
 CONTENTS, ALLOC, LOAD, DATA
 19 .jcr 00000004 08049ab0 08049ab0 00000ab0 2**2
 CONTENTS, ALLOC, LOAD, DATA
 20 .got 00000028 08049ab4 08049ab4 00000ab4 2**2
 CONTENTS, ALLOC, LOAD, DATA
 21 .bss 00000004 08049adc 08049adc 00000adc 2**2
 ALLOC
 22 .comment 00000132 00000000 00000000 00000adc 2**0
 CONTENTS, READONLY
 23 .debug_aranges 00000078 00000000 00000000 00000c10 2**3
 CONTENTS, READONLY, DEBUGGING
 24 .debug_pubnames 00000025 00000000 00000000 00000c88 2**0
 CONTENTS, READONLY, DEBUGGING
 25 .debug_info 00000a84 00000000 00000000 00000cad 2**0
 CONTENTS, READONLY, DEBUGGING
 26 .debug_abbrev 00000138 00000000 00000000 00001731 2**0
 CONTENTS, READONLY, DEBUGGING
 27 .debug_line 0000027c 00000000 00000000 00001869 2**0
 CONTENTS, READONLY, DEBUGGING
 28 .debug_frame 00000014 00000000 00000000 00001ae8 2**2
 CONTENTS, READONLY, DEBUGGING
 29 .debug_str 000006ba 00000000 00000000 00001afc 2**0
 CONTENTS, READONLY, DEBUGGING

ELF Identification
 Class: 32-bit objects
Machine: Intel 80386
Name Size FileOff
[00] .interp 19 244
[01] .note.ABI-tag 32 264
[02] .hash 56 296
[03] .dynsym 144 352
[04] .dynstr 98 496
[05] .gnu.version 18 594
[06] .gnu.version_r 32 612
[07] .rel.dyn 8 644
[08] .rel.plt 48 652
[09] .init 23 700
[10] .plt 112 724
[11] .text 1292 836
[12] .fini 27 2128
[13] .rodata 348 2156
[14] .eh_frame 4 2504
[15] .data 12 2508
[16] .dynamic 200 2520
[17] .ctors 8 2720
[18] .dtors 8 2728
[19] .jcr 4 2736
[20] .got 40 2740
[21] .bss 4 2780
[22] .comment 306 2780
[23] .debug_aranges 120 3088
[24] .debug_pubnames 37 3208
[25] .debug_info 2692 3245
[26] .debug_abbrev 312 5937
[27] .debug_line 636 6249
[28] .debug_frame 20 6888
[29] .debug_str 1722 6908
[30] .shstrtab 299 8630
[31] .symtab 1856 10292
[32] .strtab 1132 12148

不過事情好像有點怪異，我們的 loader v0.5 怎麼在最後多出 3 個 section 呢？被附身了。

不過，原來是 objdump 不會把最後這 3 個 section 印出來啦，這 3 個 section 分別是：

1) .shstrtab：section header straing table

2) .symtab：symbol table

3) .strtab：string table

哇塞！原來 symbol table 藏在這裡啊。但是，如果我們改用 readelf 來讀 ELF，就可以看到完整的 ELF headers 了，而且輸出的畫面也比較具可讀性：

$ readelf -S loader-0.4

There are 34 section headers, starting at offset 0x22e4:

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 080480f4 0000f4 000013 00 A 0 0 1
 [ 2] .note.ABI-tag NOTE 08048108 000108 000020 00 A 0 0 4
 [ 3] .hash HASH 08048128 000128 000038 04 A 4 0 4
 [ 4] .dynsym DYNSYM 08048160 000160 000090 10 A 5 1 4
 [ 5] .dynstr STRTAB 080481f0 0001f0 000062 00 A 0 0 1
 [ 6] .gnu.version VERSYM 08048252 000252 000012 02 A 4 0 2
 [ 7] .gnu.version_r VERNEED 08048264 000264 000020 00 A 5 1 4
 [ 8] .rel.dyn REL 08048284 000284 000008 08 A 4 0 4
 [ 9] .rel.plt REL 0804828c 00028c 000030 08 A 4 b 4
 [10] .init PROGBITS 080482bc 0002bc 000017 00 AX 0 0 4
 [11] .plt PROGBITS 080482d4 0002d4 000070 04 AX 0 0 4
 [12] .text PROGBITS 08048344 000344 00050c 00 AX 0 0 4
 [13] .fini PROGBITS 08048850 000850 00001b 00 AX 0 0 4
 [14] .rodata PROGBITS 0804886c 00086c 00015c 00 A 0 0 4
 [15] .eh_frame PROGBITS 080489c8 0009c8 000004 00 A 0 0 4
 [16] .data PROGBITS 080499cc 0009cc 00000c 00 WA 0 0 4
 [17] .dynamic DYNAMIC 080499d8 0009d8 0000c8 08 WA 5 0 4
 [18] .ctors PROGBITS 08049aa0 000aa0 000008 00 WA 0 0 4
 [19] .dtors PROGBITS 08049aa8 000aa8 000008 00 WA 0 0 4
 [20] .jcr PROGBITS 08049ab0 000ab0 000004 00 WA 0 0 4
 [21] .got PROGBITS 08049ab4 000ab4 000028 04 WA 0 0 4
 [22] .bss NOBITS 08049adc 000adc 000004 00 WA 0 0 4
 [23] .comment PROGBITS 00000000 000adc 000132 00 0 0 1
 [24] .debug_aranges PROGBITS 00000000 000c10 000078 00 0 0 8
 [25] .debug_pubnames PROGBITS 00000000 000c88 000025 00 0 0 1
 [26] .debug_info PROGBITS 00000000 000cad 000a84 00 0 0 1
 [27] .debug_abbrev PROGBITS 00000000 001731 000138 00 0 0 1
 [28] .debug_line PROGBITS 00000000 001869 00027c 00 0 0 1
 [29] .debug_frame PROGBITS 00000000 001ae8 000014 00 0 0 4
 [30] .debug_str PROGBITS 00000000 001afc 0006ba 01 MS 0 0 1
 [31] .shstrtab STRTAB 00000000 0021b6 00012b 00 0 0 1
 [32] .symtab SYMTAB 00000000 002834 000740 10 33 54 4
 [33] .strtab STRTAB 00000000 002f74 00046c 00 0 0 1
Key to Flags:
 W (write), A (alloc), X (execute), M (merge), S (strings)
 I (info), L (link order), G (group), x (unknown)
 O (extra OS processing required) o (OS specific), p (processor specific)