ELF Msingi wa Taarifa

Reading time: 14 minutes

Vichwa vya Programu

Vinavyoelezea kwa loader jinsi ya kupakia ELF kwenye kumbukumbu:

readelf -lW lnstat

Elf file type is DYN (Position-Independent Executable file)
Entry point 0x1c00
There are 9 program headers, starting at offset 64

Program Headers:
Type           Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
PHDR           0x000040 0x0000000000000040 0x0000000000000040 0x0001f8 0x0001f8 R   0x8
INTERP         0x000238 0x0000000000000238 0x0000000000000238 0x00001b 0x00001b R   0x1
[Requesting program interpreter: /lib/ld-linux-aarch64.so.1]
LOAD           0x000000 0x0000000000000000 0x0000000000000000 0x003f7c 0x003f7c R E 0x10000
LOAD           0x00fc48 0x000000000001fc48 0x000000000001fc48 0x000528 0x001190 RW  0x10000
DYNAMIC        0x00fc58 0x000000000001fc58 0x000000000001fc58 0x000200 0x000200 RW  0x8
NOTE           0x000254 0x0000000000000254 0x0000000000000254 0x0000e0 0x0000e0 R   0x4
GNU_EH_FRAME   0x003610 0x0000000000003610 0x0000000000003610 0x0001b4 0x0001b4 R   0x4
GNU_STACK      0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 RW  0x10
GNU_RELRO      0x00fc48 0x000000000001fc48 0x000000000001fc48 0x0003b8 0x0003b8 R   0x1

Section to Segment mapping:
Segment Sections...
00
01     .interp
02     .interp .note.gnu.build-id .note.ABI-tag .note.package .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt .init .plt .text .fini .rodata .eh_frame_hdr .eh_frame
03     .init_array .fini_array .dynamic .got .data .bss
04     .dynamic
05     .note.gnu.build-id .note.ABI-tag .note.package
06     .eh_frame_hdr
07
08     .init_array .fini_array .dynamic .got

Programu ya awali ina vichwa vya programu 9, kisha, ramani ya sehemu inaonyesha katika kichwa gani cha programu (kuanzia 00 hadi 08) kila sehemu iko.

PHDR - Kichwa cha Programu

Inajumuisha meza za vichwa vya programu na metadata yenyewe.

INTERP

Inaonyesha njia ya loader inayotumika kupakia binary kwenye kumbukumbu.

LOAD

Vichwa hivi vinatumika kuonyesha jinsi ya kupakia binary kwenye kumbukumbu.
Kila kichwa cha LOAD kinaonyesha eneo la kumbukumbu (ukubwa, ruhusa na usawa) na inaonyesha bytes za ELF binary za kunakili huko.

Kwa mfano, ya pili ina ukubwa wa 0x1190, inapaswa kuwa katika 0x1fc48 ikiwa na ruhusa za kusoma na kuandika na itajazwa na 0x528 kutoka kwa ofset 0xfc48 (haijaza nafasi yote iliyohifadhiwa). Kumbukumbu hii itakuwa na sehemu .init_array .fini_array .dynamic .got .data .bss.

DYNAMIC

Kichwa hiki husaidia kuunganisha programu na utegemezi wa maktaba zao na kutekeleza uhamasishaji. Angalia sehemu .dynamic.

NOTE

Hii inahifadhi taarifa za metadata za muuzaji kuhusu binary.

GNU_EH_FRAME

Inafafanua eneo la meza za kuondoa stack, zinazotumiwa na debuggers na kazi za usimamizi wa makosa za C++.

GNU_STACK

Inajumuisha usanidi wa ulinzi wa kuzuia utekelezaji wa stack. Ikiwa imewezeshwa, binary haitakuwa na uwezo wa kutekeleza msimbo kutoka kwenye stack.

GNU_RELRO

Inaonyesha usanidi wa RELRO (Relocation Read-Only) wa binary. Ulinzi huu utaashiria kama isiyo na kuandika sehemu fulani za kumbukumbu (kama GOT au meza za init na fini) baada ya programu kupakiwa na kabla ya kuanza kutekelezwa.

Katika mfano wa awali inakopi bytes 0x3b8 hadi 0x1fc48 kama zisizo na kuandika zikihusisha sehemu .init_array .fini_array .dynamic .got .data .bss.

Kumbuka kwamba RELRO inaweza kuwa ya sehemu au kamili, toleo la sehemu halilindi sehemu .plt.got, ambayo inatumika kwa lazy binding na inahitaji nafasi hii ya kumbukumbu kuwa na ruhusa za kuandika ili kuandika anwani za maktaba wakati wa kwanza mahali pake inatafutwa.

TLS

Inafafanua meza ya entries za TLS, ambayo inahifadhi taarifa kuhusu mabadiliko ya ndani ya thread.

Vichwa vya Sehemu

Vichwa vya sehemu vinatoa mtazamo wa kina zaidi wa binary ya ELF.

objdump lnstat -h

lnstat:     file format elf64-littleaarch64

Sections:
Idx Name          Size      VMA               LMA               File off  Algn
0 .interp       0000001b  0000000000000238  0000000000000238  00000238  2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
1 .note.gnu.build-id 00000024  0000000000000254  0000000000000254  00000254  2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
2 .note.ABI-tag 00000020  0000000000000278  0000000000000278  00000278  2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
3 .note.package 0000009c  0000000000000298  0000000000000298  00000298  2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
4 .gnu.hash     0000001c  0000000000000338  0000000000000338  00000338  2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
5 .dynsym       00000498  0000000000000358  0000000000000358  00000358  2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
6 .dynstr       000001fe  00000000000007f0  00000000000007f0  000007f0  2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
7 .gnu.version  00000062  00000000000009ee  00000000000009ee  000009ee  2**1
CONTENTS, ALLOC, LOAD, READONLY, DATA
8 .gnu.version_r 00000050  0000000000000a50  0000000000000a50  00000a50  2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
9 .rela.dyn     00000228  0000000000000aa0  0000000000000aa0  00000aa0  2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
10 .rela.plt     000003c0  0000000000000cc8  0000000000000cc8  00000cc8  2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
11 .init         00000018  0000000000001088  0000000000001088  00001088  2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
12 .plt          000002a0  00000000000010a0  00000000000010a0  000010a0  2**4
CONTENTS, ALLOC, LOAD, READONLY, CODE
13 .text         00001c34  0000000000001340  0000000000001340  00001340  2**6
CONTENTS, ALLOC, LOAD, READONLY, CODE
14 .fini         00000014  0000000000002f74  0000000000002f74  00002f74  2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
15 .rodata       00000686  0000000000002f88  0000000000002f88  00002f88  2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
16 .eh_frame_hdr 000001b4  0000000000003610  0000000000003610  00003610  2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
17 .eh_frame     000007b4  00000000000037c8  00000000000037c8  000037c8  2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
18 .init_array   00000008  000000000001fc48  000000000001fc48  0000fc48  2**3
CONTENTS, ALLOC, LOAD, DATA
19 .fini_array   00000008  000000000001fc50  000000000001fc50  0000fc50  2**3
CONTENTS, ALLOC, LOAD, DATA
20 .dynamic      00000200  000000000001fc58  000000000001fc58  0000fc58  2**3
CONTENTS, ALLOC, LOAD, DATA
21 .got          000001a8  000000000001fe58  000000000001fe58  0000fe58  2**3
CONTENTS, ALLOC, LOAD, DATA
22 .data         00000170  0000000000020000  0000000000020000  00010000  2**3
CONTENTS, ALLOC, LOAD, DATA
23 .bss          00000c68  0000000000020170  0000000000020170  00010170  2**3
ALLOC
24 .gnu_debugaltlink 00000049  0000000000000000  0000000000000000  00010170  2**0
CONTENTS, READONLY
25 .gnu_debuglink 00000034  0000000000000000  0000000000000000  000101bc  2**2
CONTENTS, READONLY

Inayoonyesha pia mahali, ofseti, ruhusa lakini pia aina ya data ambayo sehemu hiyo ina.

Sehemu za Meta

• String table: Inayo kila nyuzi inayohitajika na faili la ELF (lakini si zile zinazotumiwa na programu). Kwa mfano, ina majina ya sehemu kama .text au .data. Na ikiwa .text iko kwenye ofseti 45 katika jedwali la nyuzi itatumia nambari 45 katika uwanja wa jina.
• Ili kupata mahali ambapo jedwali la nyuzi liko, ELF ina kiashiria kwa jedwali la nyuzi.
• Symbol table: Inayo taarifa kuhusu alama kama jina (ofseti katika jedwali la nyuzi), anwani, ukubwa na metadata zaidi kuhusu alama hiyo.

Sehemu Kuu

• .text: Maagizo ya programu ya kukimbia.
• .data: Vigezo vya kimataifa vyenye thamani iliyofafanuliwa katika programu.
• .bss: Vigezo vya kimataifa vilivyotelekezwa (au kuanzishwa kuwa sifuri). Vigezo hapa vinakaguliwa kiotomatiki kuwa sifuri hivyo kuzuia sifuri zisizohitajika kuongezwa kwenye binary.
• .rodata: Vigezo vya kimataifa visivyobadilika (sehemu ya kusoma tu).
• .tdata na .tbss: Kama .data na .bss wakati vigezo vya ndani ya thread vinapotumika (__thread_local katika C++ au __thread katika C).
• .dynamic: Angalia hapa chini.

Alama

Alama ni mahali lenye jina katika programu ambalo linaweza kuwa kazi, kitu cha data cha kimataifa, vigezo vya ndani ya thread...

readelf -s lnstat

Symbol table '.dynsym' contains 49 entries:
Num:    Value          Size Type    Bind   Vis      Ndx Name
0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
1: 0000000000001088     0 SECTION LOCAL  DEFAULT   12 .init
2: 0000000000020000     0 SECTION LOCAL  DEFAULT   23 .data
3: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND strtok@GLIBC_2.17 (2)
4: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND s[...]@GLIBC_2.17 (2)
5: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND strlen@GLIBC_2.17 (2)
6: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND fputs@GLIBC_2.17 (2)
7: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND exit@GLIBC_2.17 (2)
8: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND _[...]@GLIBC_2.34 (3)
9: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND perror@GLIBC_2.17 (2)
10: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_deregisterT[...]
11: 0000000000000000     0 FUNC    WEAK   DEFAULT  UND _[...]@GLIBC_2.17 (2)
12: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND putc@GLIBC_2.17 (2)
[...]

Kila kipengele cha alama kina:

• Jina
• Sifa za uhusiano (dhaifu, za ndani au za kimataifa): Alama ya ndani inaweza kufikiwa tu na programu yenyewe wakati alama za kimataifa zinashirikiwa nje ya programu. Kitu dhaifu ni mfano wa kazi ambayo inaweza kubadilishwa na nyingine tofauti.
• Aina: NOTYPE (aina haijabainishwa), OBJECT (kibadilisha data cha kimataifa), FUNC (kazi), SECTION (sehemu), FILE (faili ya msimbo wa chanzo kwa ajili ya wadhibiti), TLS (kibadilisha cha ndani cha nyuzi), GNU_IFUNC (kazi isiyo ya moja kwa moja kwa ajili ya uhamasishaji)
• Sehemu index ambapo iko
• Thamani (anwani katika kumbukumbu)
• Ukubwa

Sehemu ya Kihisia

readelf -d lnstat

Dynamic section at offset 0xfc58 contains 28 entries:
Tag        Type                         Name/Value
0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
0x0000000000000001 (NEEDED)             Shared library: [ld-linux-aarch64.so.1]
0x000000000000000c (INIT)               0x1088
0x000000000000000d (FINI)               0x2f74
0x0000000000000019 (INIT_ARRAY)         0x1fc48
0x000000000000001b (INIT_ARRAYSZ)       8 (bytes)
0x000000000000001a (FINI_ARRAY)         0x1fc50
0x000000000000001c (FINI_ARRAYSZ)       8 (bytes)
0x000000006ffffef5 (GNU_HASH)           0x338
0x0000000000000005 (STRTAB)             0x7f0
0x0000000000000006 (SYMTAB)             0x358
0x000000000000000a (STRSZ)              510 (bytes)
0x000000000000000b (SYMENT)             24 (bytes)
0x0000000000000015 (DEBUG)              0x0
0x0000000000000003 (PLTGOT)             0x1fe58
0x0000000000000002 (PLTRELSZ)           960 (bytes)
0x0000000000000014 (PLTREL)             RELA
0x0000000000000017 (JMPREL)             0xcc8
0x0000000000000007 (RELA)               0xaa0
0x0000000000000008 (RELASZ)             552 (bytes)
0x0000000000000009 (RELAENT)            24 (bytes)
0x000000000000001e (FLAGS)              BIND_NOW
0x000000006ffffffb (FLAGS_1)            Flags: NOW PIE
0x000000006ffffffe (VERNEED)            0xa50
0x000000006fffffff (VERNEEDNUM)         2
0x000000006ffffff0 (VERSYM)             0x9ee
0x000000006ffffff9 (RELACOUNT)          15
0x0000000000000000 (NULL)               0x0

Direktori ya NEEDED inaonyesha kwamba programu inahitaji kupakia maktaba iliyoelezwa ili kuendelea. Direktori ya NEEDED inakamilika mara tu maktaba inayo shared inakuwa na kazi kamili na tayari kwa matumizi.

Mabadiliko

Mchambuzi pia lazima ahamasishe utegemezi baada ya kuyapakia. Mabadiliko haya yanaonyeshwa katika jedwali la mabadiliko katika muundo wa REL au RELA na idadi ya mabadiliko inatolewa katika sehemu za dynamic RELSZ au RELASZ.

readelf -r lnstat

Relocation section '.rela.dyn' at offset 0xaa0 contains 23 entries:
Offset          Info           Type           Sym. Value    Sym. Name + Addend
00000001fc48  000000000403 R_AARCH64_RELATIV                    1d10
00000001fc50  000000000403 R_AARCH64_RELATIV                    1cc0
00000001fff0  000000000403 R_AARCH64_RELATIV                    1340
000000020008  000000000403 R_AARCH64_RELATIV                    20008
000000020010  000000000403 R_AARCH64_RELATIV                    3330
000000020030  000000000403 R_AARCH64_RELATIV                    3338
000000020050  000000000403 R_AARCH64_RELATIV                    3340
000000020070  000000000403 R_AARCH64_RELATIV                    3348
000000020090  000000000403 R_AARCH64_RELATIV                    3350
0000000200b0  000000000403 R_AARCH64_RELATIV                    3358
0000000200d0  000000000403 R_AARCH64_RELATIV                    3360
0000000200f0  000000000403 R_AARCH64_RELATIV                    3370
000000020110  000000000403 R_AARCH64_RELATIV                    3378
000000020130  000000000403 R_AARCH64_RELATIV                    3380
000000020150  000000000403 R_AARCH64_RELATIV                    3388
00000001ffb8  000a00000401 R_AARCH64_GLOB_DA 0000000000000000 _ITM_deregisterTM[...] + 0
00000001ffc0  000b00000401 R_AARCH64_GLOB_DA 0000000000000000 __cxa_finalize@GLIBC_2.17 + 0
00000001ffc8  000f00000401 R_AARCH64_GLOB_DA 0000000000000000 stderr@GLIBC_2.17 + 0
00000001ffd0  001000000401 R_AARCH64_GLOB_DA 0000000000000000 optarg@GLIBC_2.17 + 0
00000001ffd8  001400000401 R_AARCH64_GLOB_DA 0000000000000000 stdout@GLIBC_2.17 + 0
00000001ffe0  001e00000401 R_AARCH64_GLOB_DA 0000000000000000 __gmon_start__ + 0
00000001ffe8  001f00000401 R_AARCH64_GLOB_DA 0000000000000000 __stack_chk_guard@GLIBC_2.17 + 0
00000001fff8  002e00000401 R_AARCH64_GLOB_DA 0000000000000000 _ITM_registerTMCl[...] + 0

Relocation section '.rela.plt' at offset 0xcc8 contains 40 entries:
Offset          Info           Type           Sym. Value    Sym. Name + Addend
00000001fe70  000300000402 R_AARCH64_JUMP_SL 0000000000000000 strtok@GLIBC_2.17 + 0
00000001fe78  000400000402 R_AARCH64_JUMP_SL 0000000000000000 strtoul@GLIBC_2.17 + 0
00000001fe80  000500000402 R_AARCH64_JUMP_SL 0000000000000000 strlen@GLIBC_2.17 + 0
00000001fe88  000600000402 R_AARCH64_JUMP_SL 0000000000000000 fputs@GLIBC_2.17 + 0
00000001fe90  000700000402 R_AARCH64_JUMP_SL 0000000000000000 exit@GLIBC_2.17 + 0
00000001fe98  000800000402 R_AARCH64_JUMP_SL 0000000000000000 __libc_start_main@GLIBC_2.34 + 0
00000001fea0  000900000402 R_AARCH64_JUMP_SL 0000000000000000 perror@GLIBC_2.17 + 0
00000001fea8  000b00000402 R_AARCH64_JUMP_SL 0000000000000000 __cxa_finalize@GLIBC_2.17 + 0
00000001feb0  000c00000402 R_AARCH64_JUMP_SL 0000000000000000 putc@GLIBC_2.17 + 0
00000001feb8  000d00000402 R_AARCH64_JUMP_SL 0000000000000000 opendir@GLIBC_2.17 + 0
00000001fec0  000e00000402 R_AARCH64_JUMP_SL 0000000000000000 fputc@GLIBC_2.17 + 0
00000001fec8  001100000402 R_AARCH64_JUMP_SL 0000000000000000 snprintf@GLIBC_2.17 + 0
00000001fed0  001200000402 R_AARCH64_JUMP_SL 0000000000000000 __snprintf_chk@GLIBC_2.17 + 0
00000001fed8  001300000402 R_AARCH64_JUMP_SL 0000000000000000 malloc@GLIBC_2.17 + 0
00000001fee0  001500000402 R_AARCH64_JUMP_SL 0000000000000000 gettimeofday@GLIBC_2.17 + 0
00000001fee8  001600000402 R_AARCH64_JUMP_SL 0000000000000000 sleep@GLIBC_2.17 + 0
00000001fef0  001700000402 R_AARCH64_JUMP_SL 0000000000000000 __vfprintf_chk@GLIBC_2.17 + 0
00000001fef8  001800000402 R_AARCH64_JUMP_SL 0000000000000000 calloc@GLIBC_2.17 + 0
00000001ff00  001900000402 R_AARCH64_JUMP_SL 0000000000000000 rewind@GLIBC_2.17 + 0
00000001ff08  001a00000402 R_AARCH64_JUMP_SL 0000000000000000 strdup@GLIBC_2.17 + 0
00000001ff10  001b00000402 R_AARCH64_JUMP_SL 0000000000000000 closedir@GLIBC_2.17 + 0
00000001ff18  001c00000402 R_AARCH64_JUMP_SL 0000000000000000 __stack_chk_fail@GLIBC_2.17 + 0
00000001ff20  001d00000402 R_AARCH64_JUMP_SL 0000000000000000 strrchr@GLIBC_2.17 + 0
00000001ff28  001e00000402 R_AARCH64_JUMP_SL 0000000000000000 __gmon_start__ + 0
00000001ff30  002000000402 R_AARCH64_JUMP_SL 0000000000000000 abort@GLIBC_2.17 + 0
00000001ff38  002100000402 R_AARCH64_JUMP_SL 0000000000000000 feof@GLIBC_2.17 + 0
00000001ff40  002200000402 R_AARCH64_JUMP_SL 0000000000000000 getopt_long@GLIBC_2.17 + 0
00000001ff48  002300000402 R_AARCH64_JUMP_SL 0000000000000000 __fprintf_chk@GLIBC_2.17 + 0
00000001ff50  002400000402 R_AARCH64_JUMP_SL 0000000000000000 strcmp@GLIBC_2.17 + 0
00000001ff58  002500000402 R_AARCH64_JUMP_SL 0000000000000000 free@GLIBC_2.17 + 0
00000001ff60  002600000402 R_AARCH64_JUMP_SL 0000000000000000 readdir64@GLIBC_2.17 + 0
00000001ff68  002700000402 R_AARCH64_JUMP_SL 0000000000000000 strndup@GLIBC_2.17 + 0
00000001ff70  002800000402 R_AARCH64_JUMP_SL 0000000000000000 strchr@GLIBC_2.17 + 0
00000001ff78  002900000402 R_AARCH64_JUMP_SL 0000000000000000 fwrite@GLIBC_2.17 + 0
00000001ff80  002a00000402 R_AARCH64_JUMP_SL 0000000000000000 fflush@GLIBC_2.17 + 0
00000001ff88  002b00000402 R_AARCH64_JUMP_SL 0000000000000000 fopen64@GLIBC_2.17 + 0
00000001ff90  002c00000402 R_AARCH64_JUMP_SL 0000000000000000 __isoc99_sscanf@GLIBC_2.17 + 0
00000001ff98  002d00000402 R_AARCH64_JUMP_SL 0000000000000000 strncpy@GLIBC_2.17 + 0
00000001ffa0  002f00000402 R_AARCH64_JUMP_SL 0000000000000000 __assert_fail@GLIBC_2.17 + 0
00000001ffa8  003000000402 R_AARCH64_JUMP_SL 0000000000000000 fgets@GLIBC_2.17 + 0

Static Relocations

Ikiwa programu imepakuliwa mahali tofauti na anwani inayopendelea (kawaida 0x400000) kwa sababu anwani hiyo tayari inatumika au kwa sababu ya ASLR au sababu nyingine yoyote, uhamasishaji wa statiki unarekebisha viashiria ambavyo vilikuwa na thamani zikitarajia binary kupakuliwa katika anwani inayopendelea.

Kwa mfano, sehemu yoyote ya aina R_AARCH64_RELATIV inapaswa kuwa na anwani iliyorekebishwa kwenye bias ya uhamasishaji pamoja na thamani ya kuongeza.

Dynamic Relocations and GOT

Uhamasishaji unaweza pia kurejelea alama ya nje (kama kazi kutoka kwa utegemezi). Kama kazi malloc kutoka libC. Kisha, mpakuwaji anapopakua libC katika anwani akichunguza mahali ambapo kazi ya malloc imepakuliwa, itaandika anwani hii katika jedwali la GOT (Global Offset Table) (linaloonyeshwa katika jedwali la uhamasishaji) ambapo anwani ya malloc inapaswa kufafanuliwa.

Procedure Linkage Table

Sehemu ya PLT inaruhusu kufanya uhusiano wa uvivu, ambayo inamaanisha kwamba ufumbuzi wa mahali pa kazi utafanywa wakati wa kwanza inapoombwa.

Hivyo wakati programu inaita malloc, kwa kweli inaita mahali husika pa malloc katika PLT (malloc@plt). Wakati wa kwanza inapoitwa inatatua anwani ya malloc na kuihifadhi ili wakati wa pili malloc inapoitwa, anwani hiyo inatumika badala ya msimbo wa PLT.

Program Initialization

Baada ya programu kupakuliwa ni wakati wa kuendesha. Hata hivyo, msimbo wa kwanza unaotekelezwa sio kila wakati kazi ya main. Hii ni kwa sababu kwa mfano katika C++ ikiwa kigezo cha kimataifa ni kitu cha darasa, kitu hiki lazima kiwe kimeanzishwa kabla ya main kuendesha, kama katika:

#include <stdio.h>
// g++ autoinit.cpp -o autoinit
class AutoInit {
public:
AutoInit() {
printf("Hello AutoInit!\n");
}
~AutoInit() {
printf("Goodbye AutoInit!\n");
}
};

AutoInit autoInit;

int main() {
printf("Main\n");
return 0;
}

Kumbuka kwamba hizi variables za kimataifa ziko katika .data au .bss lakini katika orodha __CTOR_LIST__ na __DTOR_LIST__ vitu vya kuanzisha na kuharibu vinahifadhiwa ili kufuatilia.

Kutoka kwa msimbo wa C inawezekana kupata matokeo sawa kwa kutumia nyongeza za GNU:

__attributte__((constructor)) //Add a constructor to execute before
__attributte__((destructor)) //Add to the destructor list

Kwa mtazamo wa mkusanyiko, ili kutekeleza vitendo hivi kabla na baada ya kazi ya main kutekelezwa, inawezekana kuunda kazi ya init na kazi ya fini ambazo zitarejelewa katika sehemu ya dynamic kama INIT na FIN. na zimewekwa katika sehemu za init na fini za ELF.

Chaguo lingine, kama ilivyotajwa, ni kurejelea orodha __CTOR_LIST__ na __DTOR_LIST__ katika INIT_ARRAY na FINI_ARRAY katika sehemu ya dynamic na urefu wa hizi unatajwa na INIT_ARRAYSZ na FINI_ARRAYSZ. Kila kipengee ni kiashiria cha kazi ambacho kitaitwa bila hoja.

Zaidi ya hayo, inawezekana pia kuwa na PREINIT_ARRAY yenye viashiria ambavyo vitatekelezwa kabla ya viashiria vya INIT_ARRAY.

Agizo la Uanzishaji

1. Programu inawekwa kwenye kumbukumbu, mabadiliko ya kimataifa ya static yanaanzishwa katika .data na yasiyoanzishwa yanawekwa sifuri katika .bss.
2. Mahitaji yote ya programu au maktaba yanaanzishwa na kuunganishwa kwa dynamic kunatekelezwa.
3. Kazi za PREINIT_ARRAY zinafanywa.
4. Kazi za INIT_ARRAY zinafanywa.
5. Ikiwa kuna kipengee cha INIT kinaitwa.
6. Ikiwa ni maktaba, dlopen inamalizika hapa, ikiwa ni programu, ni wakati wa kuita nukta halisi ya kuingia (kazi ya main).

Hifadhi ya Mitaa ya Thread (TLS)

Zimefafanuliwa kwa kutumia neno muhimu __thread_local katika C++ au nyongeza ya GNU __thread.

Kila thread itahifadhi eneo la kipekee kwa ajili ya variable hii hivyo ni thread pekee inayoweza kufikia variable yake.

Wakati hii inatumika sehemu .tdata na .tbss zinatumika katika ELF. Ambazo ni kama .data (iliyowekwa) na .bss (siyo iliyowekwa) lakini kwa TLS.

Kila variable itakuwa na kipengee katika kichwa cha TLS kinachoelezea ukubwa na ofset ya TLS, ambayo ni ofset itakayotumiwa katika eneo la data la ndani la thread.

__TLS_MODULE_BASE ni alama inayotumika kurejelea anwani ya msingi ya hifadhi ya ndani ya thread na inaelekeza kwenye eneo katika kumbukumbu ambalo lina data zote za ndani za thread za moduli.