./protected.sh GOPLT

disassembly.txt

# protected -O3
0000000000000710 <bar>:
 710:   48 8d 3d 0e 00 00 00    lea    0xe(%rip),%rdi        # 725 <_fini+0x9>
 717:   e9 b4 fe ff ff          jmpq   5d0 <puts@plt>

# default -O3
0000000000000730 <bar>:
 730:   e9 cb fe ff ff          jmpq   600 <foo@plt>
 735:   0f 1f 00                nopl   (%rax)

output.txt

# Let's say I comment the last line a we keep liba.so around...

$ bash protected.sh
original
overriden
$ objdump -R liba.so | grep JUMP_SLOT
00000000002009c8 R_X86_64_JUMP_SLOT  puts
00000000002009d0 R_X86_64_JUMP_SLOT  __gmon_start__
00000000002009d8 R_X86_64_JUMP_SLOT  __cxa_finalize
$ bash protected.sh GOPLT
overriden
overriden
$ objdump -R liba.so | grep JUMP_SLOT
00000000002009f8 R_X86_64_JUMP_SLOT  puts
0000000000200a00 R_X86_64_JUMP_SLOT  __gmon_start__
0000000000200a08 R_X86_64_JUMP_SLOT  foo
0000000000200a10 R_X86_64_JUMP_SLOT  __cxa_finalize

protected.sh

#/bin/bash

# with GOPLT argument one can override calls to a function from within the same library

cat > a.h <<EOF
#ifndef A_H__
#define A_H__

extern void foo (void);
extern void bar (void);

#endif
EOF

cat > a.c <<EOF
#include <stdio.h>
#include "a.h"

#ifdef GOPLT
#define VISIBLE
#else
#define VISIBLE __attribute__ ((visibility ("protected")))
#endif

void VISIBLE foo (void)
{
        puts ("original");
}

void bar (void)
{
        foo ();
}
EOF

cat > b.c <<EOF
#include <stdio.h>

void foo (void)
{
        puts ("overriden");
}
EOF

cat > main.c <<EOF
#include "a.h"

int main (void)
{
        bar ();
        foo ();

        return 0;
}
EOF

gcc -D${1:-NOTHING} -O0 -shared -fPIC -Wl,-soname,liba.so a.c -o liba.so
gcc -O0 -shared -fPIC -Wl,-soname,libb.so b.c -o libb.so
gcc -O0 main.c -o main -I. -L. -la
LD_PRELOAD=libb.so LD_LIBRARY_PATH=. ./main

rm -rf liba.so libb.so main a.c b.c main.c a.h