ELF Basic Information - HackTricks

# ELF Basic Information

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## Program Headers

The describe to the loader how to load the ELF into memory:

    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

The previous program has 9 program headers, then, the segment mapping indicates in which program header (from 00 to 08) each section is located.

### PHDR - Program HeaDeR

Contains the program header tables and metadata itself.

### INTERP

Indicates the path of the loader to use to load the binary into memory.

### LOAD

These headers are used to indicate how to load a binary into memory.
Each LOAD header indicates a region of memory (size, permissions and alignment) and indicates the bytes of the ELF binary to copy in there.

For example, the second one has a size of 0x1190, should be located at 0x1fc48 with permissions read and write and will be filled with 0x528 from the offset 0xfc48 (it doesn't fill all the reserved space). This memory will contain the sections `.init_array .fini_array .dynamic .got .data .bss`.

### DYNAMIC

This header helps to link programs to their library dependencies and apply relocations. Check the `.dynamic` section.

### NOTE

This stores vendor metadata information about the binary.

### GNU_EH_FRAME

Defines the location of the stack unwind tables, used by debuggers and C++ exception handling-runtime functions.

### GNU_STACK

Contains the configuration of the stack execution prevention defense. If enabled, the binary won't be able to execute code from the stack.

### GNU_RELRO

Indicates the RELRO (Relocation Read-Only) configuration of the binary. This protection will mark as read-only certain sections of the memory (like the `GOT` or the `init` and `fini` tables) after the program has loaded and before it begins running.

In the previous example it's copying 0x3b8 bytes to 0x1fc48 as read-only affecting the sections `.init_array .fini_array .dynamic .got .data .bss`.

Note that RELRO can be partial or full, the partial version do not protect the section `.plt.got`, which is used for lazy binding and needs this memory space to have write permissions to write the address of the libraries the first time their location is searched.

### TLS

Defines a table of TLS entries, which stores info about thread-local variables.

## Section Headers

Section headers gives a more detailed view of the ELF binary

    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

It also indicates the location, offset, permissions but also the type of data it section has.

### Meta Sections

- String table: It contains all the strings needed by the ELF file (but not the ones actually used by the program). For example it contains sections names like `.text` or `.data`. And if `.text` is at offset 45 in the strings table it will use the number 45 in the name field.
  - In order to find where the string table is, the ELF contains a pointer to the string table.
- Symbol table: It contains info about the symbols like the name (offset in the strings table), address, size and more metadata about the symbol.

### Main Sections

- `.text`: The instruction of the program to run.
- `.data`: Global variables with a defined value in the program.
- `.bss`: Global variables left uninitialized (or init to zero). Variables here are automatically intialized to zero therefore preventing useless zeroes to being added to the binary.
- `.rodata`: Constant global variables (read-only section).
- `.tdata` and `.tbss`: Like the .data and .bss when thread-local variables are used (`__thread_local` in C++ or `__thread` in C).
- `.dynamic`: See below.

## Symbols

Symbols is a named location in the program which could be a function, a global data object, thread-local variables...

    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)
        [...]

Each symbol entry contains:

- Name
- Binding attributes (weak, local or global): A local symbol can only be accessed by the program itself while the global symbol are shared outside the program. A weak object is for example a function that can be overridden by a different one.
- Type: NOTYPE (no type specified), OBJECT (global data var), FUNC (function), SECTION (section), FILE (source-code file for debuggers), TLS (thread-local variable), GNU_IFUNC (indirect function for relocation)
- Section index where it's located
- Value (address sin memory)
- Size

## Dynamic Section

    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 indicates that the program needs to load the mentioned library in order to continue. The NEEDED directory completes once the shared library is fully operational and ready for use.

## Relocations

The loader also must relocate dependencies after having loaded them. These relocations are indicated in the relocation table in formats REL or RELA and the number of relocations is given in the dynamic sections RELSZ or 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

If the program is loaded in a place different from the preferred address (usually 0x400000) because the address is already used or because of ASLR or any other reason, a static relocation corrects pointers that had values expecting the binary to be loaded in the preferred address.

For example any section of type `R_AARCH64_RELATIV` should have modified the address at the relocation bias plus the addend value.

### Dynamic Relocations and GOT

The relocation could also reference an external symbol (like a function from a dependency). Like the function malloc from libC. Then, the loader when loading libC in an address checking where the malloc function is loaded, it will write this address in the GOT (Global Offset Table) table (indicated in the relocation table) where the address of malloc should be specified.

### Procedure Linkage Table

The PLT section allows to perform lazy binding, which means that the resolution of the location of a function will be performed the first time it's accessed.

So when a program calls to malloc, it actually calls the corresponding location of `malloc` in the PLT (`malloc@plt`). The first time it's called it resolves the address of `malloc` and stores it so next time `malloc` is called, that address is used instead of the PLT code.

## Program Initialization

After the program has been loaded it's time for it to run. However, the first code that is run isn't always the `main` function. This is because for example in C++ if a global variable is an object of a class, this object must be initialized before main runs, like in:

    #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;
    }

Note that these global variables are located in `.data` or `.bss` but in the lists `__CTOR_LIST__` and `__DTOR_LIST__` the objects to initialize and destruct are stored in order to keep track of them.

From C code it's possible to obtain the same result using the GNU extensions :

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

From a compiler perspective, to execute these actions before and after the `main` function is executed, it's possible to create a `init` function and a `fini` function which would be referenced in the dynamic section as `INIT` and `FIN`. and are placed in the `init` and `fini` sections of the ELF.

The other option, as mentioned, is to reference the lists `__CTOR_LIST__` and `__DTOR_LIST__` in the `INIT_ARRAY` and `FINI_ARRAY` entries in the dynamic section and the length of these are indicated by `INIT_ARRAYSZ` and `FINI_ARRAYSZ`. Each entry is a function pointer that will be called without arguments.

Moreover, it's also possible to have a `PREINIT_ARRAY` with pointers that will be executed before the `INIT_ARRAY` pointers.

### Initialization Order

1. The program is loaded into memory, static global variables are initialized in `.data` and unitialized ones zeroed in `.bss`.
2. All dependencies for the program or libraries are initialized and the the dynamic linking is executed.
3. `PREINIT_ARRAY` functions are executed.
4. `INIT_ARRAY` functions are executed.
5. If there is a `INIT` entry it's called.
6. If a library, dlopen ends here, if a program, it's time to call the real entry point (`main` function).

## Thread-Local Storage (TLS)

They are defined using the keyword `__thread_local` in C++ or the GNU extension `__thread`.

Each thread will maintain a unique location for this variable so only the thread can access its variable.

When this is used the sections `.tdata` and `.tbss` are used in the ELF. Which are like `.data` (initialized) and `.bss` (not initialized) but for TLS.

Each variable will hace an entry in the TLS header specifying the size and the TLS offset, which is the offset it will use in the thread's local data area.

The `__TLS_MODULE_BASE` is a symbol used to refer to the base address of the thread local storage and points to the area in memory that contains all the thread-local data of a module.

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