CODEWITHSUNDEEP
clinuxgccelfpic
zhen lee
zhen lee

Reputation: 333

How to genernate position-dependent code for executable file when linking with shared library in gcc?

I'm learning ELF.I want to find the difference of elf format between position-dependent executable file and position-independent executable file when linking shared library.

But I can't genernate position-dependent code for executable file when linking with shared library.

/*Lib.c*/
static int a;
extern int b;
int c=1;
extern void exit();
void set_value()
{
    a=1;
    b=1;
    c=1;
}
void run()
{
    set_value();
    exit();
}

Fist,use gcc to genrnate a shared dynamic library:

gcc -m32 -nostdlib -o Lib.so Lib.c

Note that I don't use -fpic to generate position-independent code for Lib.so.

Now，I have another file main.c which need to link with Lib.so:

/*main.c*/
extern void run();
int b=2;
void nomain()
{
    run();
}
void exit()
{
    asm("int $0x80 \n\t"
        ::"a"(1),"b"(42));

}

use following command to link main.c with Lib.so:

gcc  -m32 -e nomain  -nostartfiles  -fno-builtin  -o a.out main.c ./Lib.so

Howerve,gcc will compile main.c to position-independent code defaultly when link with shared library even this library don't use pic.

I wonder whether gcc have some option to genernate position-dependent for executable file?

I post the section information for Lib.so and a.out.We can see there are '.plt' and '.got.plt' section in a.out which mean a.out use PIC.

  /*Section for Lib.so*/
  Section Headers:
  [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al
  [ 0]                   NULL            00000000 000000 000000 00      0   0  0
  [ 1] .gnu.hash         GNU_HASH        000000b4 0000b4 00003c 04   A  2   0  4
  [ 2] .dynsym           DYNSYM          000000f0 0000f0 000090 10   A  3   1  4
  [ 3] .dynstr           STRTAB          00000180 000180 000030 00   A  0   0  1
  [ 4] .rel.dyn          REL             000001b0 0001b0 000028 08   A  2   0  4
  [ 5] .text             PROGBITS        000001d8 0001d8 000033 00  AX  0   0  4
  [ 6] .dynamic          DYNAMIC         0000120c 00020c 000078 08  WA  3   0  4
  [ 7] .got.plt          PROGBITS        00001284 000284 00000c 04  WA  0   0  4
  [ 8] .data             PROGBITS        00001290 000290 000004 00  WA  0   0  4
  [ 9] .bss              NOBITS          00001294 000294 000004 00  WA  0   0  4
  [10] .comment          PROGBITS        00000000 000294 00002e 00      0   0  1
  [11] .shstrtab         STRTAB          00000000 0002c2 00006a 00      0   0  1
  [12] .symtab           SYMTAB          00000000 00055c 000170 10     13  15  4
  [13] .strtab           STRTAB          00000000 0006cc 000057 00      0   0  1
  /*Section for a.out*/
  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] .gnu.hash         GNU_HASH        08048108 000108 000034 04   A  3   0  4
  [ 3] .dynsym           DYNSYM          0804813c 00013c 000070 10   A  4   1  4
  [ 4] .dynstr           STRTAB          080481ac 0001ac 000037 00   A  0   0  1
  [ 5] .rel.plt          REL             080481e4 0001e4 000008 08   A  3   6  4
  [ 6] .plt              PROGBITS        080481ec 0001ec 000020 04  AX  0   0  4
  [ 7] .text             PROGBITS        0804820c 00020c 000020 00  AX  0   0  4
  [ 8] .dynamic          DYNAMIC         0804922c 00022c 000090 08  WA  4   0  4
  [ 9] .got.plt          PROGBITS        080492bc 0002bc 000010 04  WA  0   0  4
  [10] .data             PROGBITS        080492cc 0002cc 000004 00  WA  0   0  4
  [11] .comment          PROGBITS        00000000 0002d0 00002e 00      0   0  1
  [12] .shstrtab         STRTAB          00000000 0002fe 00006d 00      0   0  1
  [13] .symtab           SYMTAB          00000000 0005c4 000160 10     14  15  4
  [14] .strtab           STRTAB          00000000 000724 000051 00      0   0  1

Upvotes: 2

Views: 1787

Answers (1)

perror
perror

Reputation: 7426

You should compile your Lib.so with gcc -Wall -Wextra -m32 -shared -Wl,-soname,Lib.so -o Lib.so Lib.c.

What you did here is just to generate a normal program without relocation information.

Look at this tutorial to know more about dynamic libraries.

Note: Also, don't forget to set your LD_LIBRARY_PATH environment variable in order to point to Lib.so.

Upvotes: 0

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