ELF weak import / fallback stubs for glibc functions

Question

I am trying to make our program runnable on some old Linux versions. One common import that prevents it is __longjmp_chk, added in glibc 2.11 but missing in older ones. One "solution" is to use -D_FORTIFY_SOURCE=0 but this turns off other fortify functions (__printf_chk etc) which are present in the target libc. Is there a way to make __longjmp_chk a "weak import" which would use the function from libc.so.6 if present, and fall back to local stub if not?

Answer 1

I am trying to make our program runnable on some old Linux versions.

There are only a few ways to make this work, and most of them are enumerated here.

Is there a way to make __longjmp_chk a "weak import".

No.

Answer 2

Is there a way to make __longjmp_chk a "weak import" which would use the function from libc.so.6 if present, and fall back to local stub if not?

I'd say yes, using dlsym() to check for __longjmp_chk and acting accordingly:

/* cc -ldl */
#define _GNU_SOURCE
#include <setjmp.h>
#include <stdio.h>
#include <dlfcn.h>

void __longjmp_chk(sigjmp_buf env, int val)
{
    void (*p)(sigjmp_buf, int) = dlsym(RTLD_NEXT, "__longjmp_chk");
    if (p)
        printf("use the function from libc\n"),
        p(env, val);
    else
    {
        printf("falling back to local stub\n");
        /* local stub - whatever that may be */
    }
}

main()
{   // try it
    sigjmp_buf env;
    while (!setjmp(env)) __longjmp_chk(env, 1);
    return 0;
}