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Maelezo Muhimu ya ELF

Jifunze kuhusu kudukua AWS kutoka sifuri hadi shujaa na htARTE (Mtaalam wa Timu Nyekundu ya AWS ya HackTricks)!

Vichwa vya Programu

Hivi hufafanua kwa mzigo 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 iliyopita ina vichwa vya programu 9, kisha, upangaji wa sehemu unaonyesha katika kichwa cha programu gani (kutoka 00 hadi 08) kila sehemu inapatikana.

PHDR - Kichwa cha Programu

Ina meza za vichwa vya programu na metadata yenyewe.

INTERP

Inaonyesha njia ya loader ya kutumia kusoma faili ya binary kwenye kumbukumbu.

LOAD

Vichwa hivi hutumiwa kuonyesha jinsi ya kusoma faili ya binary kwenye kumbukumbu.
Kila kichwa cha LOAD huonyesha eneo la kumbukumbu (ukubwa, ruhusa, na usawazishaji) na inaonyesha baits za ELF binary za kunakili hapo.

Kwa mfano, la pili lina ukubwa wa 0x1190, linapaswa kuwa kwenye 0x1fc48 na ruhusa za kusoma na kuandika na litajazwa na 0x528 kutoka kwa offset 0xfc48 (hailazi nafasi yote iliyohifadhiwa). Kumbukumbu hii italeta sehemu .init_array .fini_array .dynamic .got .data .bss.

DYNAMIC

Kichwa hiki husaidia kuunganisha programu na mahitaji yake ya maktaba na kutumia marekebisho. Angalia sehemu ya .dynamic.

NOTE

Hii hifadhi habari za metadata za muuzaji kuhusu binary.

GNU_EH_FRAME

Inaainisha eneo la meza za kufungua stack, zinazotumiwa na wachunguzi na kazi za kutunza mizunguko ya C++.

GNU_STACK

Ina mazingira ya ulinzi wa kuzuia utekelezaji wa stack. Ikiwa imewezeshwa, binary haitaweza kutekeleza nambari kutoka kwenye stack.

GNU_RELRO

Inaonyesha usanidi wa RELRO (Relocation Read-Only) wa binary. Ulinzi huu utaweka sehemu fulani za kumbukumbu kama za kusoma tu (kama GOT au meza za init na fini) baada ya programu kupakiwa na kabla haijaanza kukimbia.

Katika mfano uliopita inaiga baits 0x3b8 hadi 0x1fc48 kama kusoma tu ikigusa sehemu .init_array .fini_array .dynamic .got .data .bss.

Tambua kuwa RELRO inaweza kuwa ya sehemu au kamili, toleo la sehemu halilindi sehemu .plt.got, ambayo hutumiwa kwa kufunga uvivu na inahitaji nafasi hii ya kumbukumbu kuwa na ruhusa za kuandika kuandika anwani za maktaba mara ya kwanza wanapopatikana.

TLS

Inaainisha meza ya vipengele vya TLS, ambavyo huhifadhi habari kuhusu pembejeo za mnyororo wa ndani.

Vichwa vya Sehemu

Vichwa vya sehemu hutoa 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

Sehemu za Meta

  • Jedwali la String: Inaleta pamoja strings zote zinazohitajika na faili ya ELF (lakini sio zile zinazotumiwa na programu). Kwa mfano, inaleta majina ya sehemu kama vile .text au .data. Na kama .text iko kwenye offset 45 katika jedwali la strings itatumia nambari 45 katika uga wa jina.
  • Ili kupata mahali ambapo jedwali la string liko, ELF ina pointer kuelekea jedwali la string.
  • Jedwali la Alama: Lina habari kuhusu alama kama vile jina (offset katika jedwali la strings), anwani, saizi na metadata zaidi kuhusu alama.

Sehemu Kuu

  • .text: Maelekezo ya programu ya kukimbia.
  • .data: Variables za kimataifa zenye thamani iliyowekwa wazi katika programu.
  • .bss: Variables za kimataifa zilizoachwa bila kuanzishwa (au kuanzishwa kuwa sifuri). Variables hapa zinaanzishwa moja kwa moja kuwa sifuri hivyo kuzuia sifuri zisizohitajika kuongezwa kwenye binary.
  • .rodata: Variables za kimataifa zenye thamani zisizobadilika (sehemu isiyoweza kusomwa).
  • .tdata na .tbss: Kama .data na .bss wakati variables za thread-local zinapotumiwa (__thread_local katika C++ au __thread katika C).
  • .dynamic: Angalia chini.

Alama

Alama ni eneo lenye jina katika programu ambalo linaweza kuwa function, object la data la kimataifa, variables za thread-local...

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 kuingia ishara ina:

  • Jina
  • Vipengele vya kufunga (dhaifu, la ndani au la kawaida): Ishara ya ndani inaweza kupatikana tu na programu yenyewe wakati ishara ya kawaida inashirikiwa nje ya programu. Kitu dhaifu ni kwa mfano kazi inayoweza kubadilishwa na moja tofauti.
  • Aina: NOTYPE (aina haikufafanuliwa), OBJECT (data kuu ya ulimwengu), FUNC (kazi), SECTION (sehemu), FILE (faili ya msingi ya nambari ya chanzo kwa wachunguzi wa hitilafu), TLS (kigezo cha mada cha mada), GNU_IFUNC (kazi isiyo ya moja kwa moja kwa ajili ya uhamishaji)
  • Sehemu ya index ambapo iko
  • Thamani (anwani kwenye kumbukumbu)
  • Ukubwa

Sehemu ya Kudumu

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

The NEEDED directory inaonyesha kwamba programu inahitaji kupakia maktaba iliyotajwa ili iendelee. Direktori ya NEEDED inakamilika mara tu maktaba ya pamoja inapokuwa tayari kikamilifu na inaweza kutumika.

Uhamishaji

Mzigo pia lazima uhamishe mahitaji baada ya kuzipakia. Uhamishaji huu unaonyeshwa kwenye meza ya uhamishaji katika muundo wa REL au RELA na idadi ya uhamishaji inatolewa katika sehemu za kudumu 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
```plaintext
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

Marekebisho ya Stati

Ikiwa programu imepakia mahali tofauti na anwani inayopendelewa (kawaida 0x400000) kwa sababu anwani tayari inatumika au kwa sababu ya ASLR au sababu nyingine yoyote, marekebisho ya stati hurekebisha pointa ambazo zilikuwa na thamani zikiatarajia binary ipakuliwe katika anwani inayopendelewa.

Kwa mfano, sehemu yoyote ya aina R_AARCH64_RELATIV inapaswa kurekebisha anwani kwenye upendeleo wa marekebisho pamoja na thamani ya kuongeza.

Marekebisho ya Kudumu na GOT

Marekebisho yanaweza pia kurejelea alama ya nje (kama kazi kutoka kwa tegemeo). Kama vile kazi ya malloc kutoka libC. Kisha, mzigo unapopakia libC katika anwani ikichunguza wapi kazi ya malloc imepakuliwa, itaandika anwani hii katika jedwali la GOT (Global Offset Table) (inayoonyeshwa katika jedwali la marekebisho) ambapo anwani ya malloc inapaswa kutajwa.

Jedwali la Uunganishaji wa Taratibu

Sehemu ya PLT inaruhusu kufanya uunganishaji wa uvivu, ambao maana yake ni kwamba kutatuliwa kwa eneo la kazi kutafanywa mara ya kwanza inapofikiwa.

Kwa hivyo wakati programu inaita malloc, kimsingi inaita eneo linalofanana la malloc katika PLT (malloc@plt). Mara ya kwanza inaitwa, inatatua anwani ya malloc na kuichukua ili wakati malloc inaitwa tena, anwani hiyo itatumika badala ya msimbo wa PLT.

Uanzishaji wa Programu

Baada ya programu kupakia ni wakati wake wa kukimbia. Walakini, msimbo wa kwanza unaorushwa si mara zote ni main. Hii ni kwa sababu kwa mfano katika C++ ikiwa kigezo cha kawaida ni kitu cha darasa, kipengee hiki lazima kihifadhiwe kabla ya main kukimbia, kama vile:

#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;
}

Tafadhali elewa kwamba hizi variables za kimataifa zinapatikana katika .data au .bss lakini katika orodha __CTOR_LIST__ na __DTOR_LIST__ vitu vya kuanzisha na kuharibu vimehifadhiwa kwa mpangilio ili kufuatilia.

Kutoka kwa nambari ya C ni rahisi kupata matokeo sawa kwa kutumia vifaa vya GNU:

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

Kutoka kwa mtazamo wa compiler, ili kutekeleza hatua hizi kabla na baada ya kazi ya main kutekelezwa, ni rahisi kuunda kazi ya init na kazi ya fini ambazo zitatajwa katika sehemu ya kudumu kama INIT na FIN na kuwekwa katika sehemu za init na fini za ELF.

Chaguo lingine, kama ilivyotajwa, ni kutaja orodha __CTOR_LIST__ na __DTOR_LIST__ katika viingilio vya INIT_ARRAY na FINI_ARRAY katika sehemu ya kudumu na urefu wa hizi unatajwa na INIT_ARRAYSZ na FINI_ARRAYSZ. Kila kuingilio ni kidude cha kazi ambacho kitaitwa bila hoja.

Zaidi ya hayo, ni rahisi pia kuwa na PREINIT_ARRAY na pointers ambazo zitatekelezwa kabla ya kidude cha INIT_ARRAY.

Mpangilio wa Uanzishaji

  1. Programu inapakiwa kumbukani, vitu vya kudumu vya kimataifa vinainishwa katika .data na vile visivyoainishwa vinawekwa sifuri katika .bss.
  2. Mahitaji yote kwa programu au maktaba zina inaishwa na uunganishaji wa kudumu unatekelezwa.
  3. Kazi za PREINIT_ARRAY zinatekelezwa.
  4. Kazi za INIT_ARRAY zinatekelezwa.
  5. Ikiwa kuna kuingilio la INIT linaitwa.
  6. Ikiwa ni maktaba, dlopen inamalizika hapa, ikiwa ni programu, ni wakati wa kuita sehemu ya kuingia halisi (main function).

Uhifadhi wa Wateja wa Wateja (TLS)

Hizi zinatajwa kwa kutumia neno __thread_local katika C++ au kifupisho cha GNU __thread.

Kila wateja atahifadhi eneo la kipekee kwa hii kidude hivyo ni wateja pekee wanaweza kupata kidude chao.

Inapotumiwa, sehemu .tdata na .tbss hutumiwa katika ELF. Ambazo ni kama .data (inaanzishwa) na .bss (haikoanzishwa) lakini kwa TLS.

Kila kidude kitakuwa na kuingilio katika kichwa cha TLS kinachotaja ukubwa na kielekezo cha TLS, ambacho ni kielekezo kitatumika katika eneo la data la wateja wa wateja.

__TLS_MODULE_BASE ni ishara inayotumika kutaja anwani ya msingi ya uhifadhi wa wateja wa wateja na inaelekeza eneo kumbukumbu linalo jumuisha data yote ya wateja wa wateja wa moduli.