Using LD_PRELOAD to override a function

This was blatantly stolen from technovelty, kept here because I hate it when my bookmarks die.

For some reason, people seem to get this quite wrong a lot of the time. Certainly one should not be playing with symbols that start with __ unless you really know what you’re doing with them.

ianw@lime:~/tmp/override$ cat override.c
#define _GNU_SOURCE 1
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <dlfcn.h>

pid_t getpid(void)
        pid_t (*orig_getpid)(void) = dlsym(RTLD_NEXT, "getpid");
        printf("Calling GETPID\n");

        return orig_getpid();

ianw@lime:~/tmp/override$ cat test.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(void)
        printf("%d\n", getpid());

ianw@lime:~/tmp/override$ gcc -shared -fPIC -o override.c -ldl
ianw@lime:~/tmp/override$ gcc -o test test.c
ianw@lime:~/tmp/override$ LD_PRELOAD=./ ./test
Calling GETPID

1 Comment

  • sysadmin says:

    I’ve used a similar and rather ugly trick to run old mysqldump util from mysql 5.0 package on a machine with mysql 5.6 installed:

    $ cat /opt/mysql-5.0.95/

    LD_PRELOAD=/opt/mysql-5.0.95/lib64/mysql/ /opt/mysql-5.0.95/bin/mysqldump “$@”

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