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@apsun apsun/hax.c
Last active Jul 16, 2019

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Hook main() using LD_PRELOAD
/*
* Hook main() using LD_PRELOAD, because why not?
* Obviously, this code is not portable. Use at your own risk.
*
* Compile using 'gcc hax.c -o hax.so -fPIC -shared -ldl'
* Then run your program as 'LD_PRELOAD=$PWD/hax.so ./a.out'
*/
#define _GNU_SOURCE
#include <stdio.h>
#include <dlfcn.h>
/* Trampoline for the real main() */
static int (*main_orig)(int, char **, char **);
/* Our fake main() that gets called by __libc_start_main() */
int main_hook(int argc, char **argv, char **envp)
{
for (int i = 0; i < argc; ++i) {
printf("argv[%d] = %s\n", i, argv[i]);
}
printf("--- Before main ---\n");
int ret = main_orig(argc, argv, envp);
printf("--- After main ----\n");
printf("main() returned %d\n", ret);
return ret;
}
/*
* Wrapper for __libc_start_main() that replaces the real main
* function with our hooked version.
*/
int __libc_start_main(
int (*main)(int, char **, char **),
int argc,
char **argv,
int (*init)(int, char **, char **),
void (*fini)(void),
void (*rtld_fini)(void),
void *stack_end)
{
/* Save the real main function address */
main_orig = main;
/* Find the real __libc_start_main()... */
typeof(&__libc_start_main) orig = dlsym(RTLD_NEXT, "__libc_start_main");
/* ... and call it with our custom main function */
return orig(main_hook, argc, argv, init, fini, rtld_fini, stack_end);
}
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