我可以对 LD_PRELOAD 加载的系统函数进行嵌套调用吗

Question

Hi I am trying to override sscanf() which gets called from localtime() , But its not calling sscanf() of my library loaded rather its calling sscanf() of glibc.

Is there something I am missing?

Files content mentioned below:

//preload_localtime.c
#include <stdarg.h>
#include <stdio.h>
#include <time.h>
#include <dlfcn.h>


__attribute__((force_align_arg_pointer)) int sscanf(const char *str, const char *format, ...)
{
  int ret;
  va_list ap;
  printf("test\n");
  va_start(ap, format);
  ret = vsscanf(str, format, ap);
  va_end(ap);
  return ret;
}

__attribute__((force_align_arg_pointer)) int __isoc99_sscanf(const char *str, const char *format, ...)
{
  int ret;
  va_list ap;
  printf("test\n");
  va_start(ap, format);
  ret = vsscanf(str, format, ap);
  va_end(ap);
  return ret;
}
__attribute__((force_align_arg_pointer)) struct tm *
localtime(const time_t *timep)
{
    static struct tm *(*real_localtime)(const time_t *) = (void*)0;

    printf("my_local\n");
   if (!real_localtime)
        real_localtime = dlsym(RTLD_NEXT, "localtime");
    return real_localtime(timep);
}


//foo.c
#include <stdio.h>
#include <time.h>
int main(void)
{
        int i;
        //sscanf("42", "%d", &i);
        //printf("%d\n", i);

time_t rawtime;
  struct tm * timeinfo;

  time (&rawtime);
 timeinfo = localtime (&rawtime);
printf ("Current local time and date: %s", asctime(timeinfo));
        return 0;
}

Steps I am doing:

# gcc -fPIC -shared preload_localtime.c -o preload_localtime.so -ldl -D_GNU_SOURCE=1
#export LD_PRELOAD=$PWD/preload_localtime.so
gcc foo.c -o foo

output I am getting:

# ./foo
my_local
Current local time and date: Wed Mar 10 18:20:47 2021

What is expected output: It should print test as well, from printf("test\n"); of sscanf() overriden. Since localtime() internally has call to sscanf() and I have overridden and instructed to use sscanf() as well.

IS there any way to override another system call like in this example sscanf() which is internally gets called from localtime() .

Call hierarchy as below:

localtime()->__tz_convert() ->__tzfile_compute() -->__tzset_parse_tz() -->parse_offset() -->sscanf()

Answer 1

LD_PRELOAD works by being the first symbol of the same name found by the dynamic linker - so if a symbol usage can be resolved in compile time, the dynamic linking won't affect it because localtime and sscanf exist in the same binary the call may very well be a static one - the call can be resolved in compile time and the dynamic linker isn't involved in this call chain.