why is library loaded via LD_PRELOAD operating before initialization? - linux

Question

In the following minimal example, a library loaded via LD_PRELOAD with functions to intercept fopen and openat is apparently operating before its initialization. (Linux is CentOS 7.3). Why??
library file comm.c:
#define _GNU_SOURCE
#include <dlfcn.h>
#include <stdarg.h>
#include <stdio.h>
#include <fcntl.h>
typedef FILE *(*fopen_type)(const char *, const char *);
// initialize to invalid value (non-NULL)
// init() should initialize this correctly
fopen_type g_orig_fopen = (fopen_type) 1;
typedef int (*openat_type)(int, const char *, int, ...);
openat_type g_orig_openat;
void init() {
g_orig_fopen = (fopen_type)dlsym(RTLD_NEXT,"fopen");
g_orig_openat = (openat_type)dlsym(RTLD_NEXT,"openat");
}
FILE *fopen(const char *filename, const char *mode) {
// have to do this here because init is not called yet???
FILE * const ret = ((fopen_type)dlsym(RTLD_NEXT,"fopen"))(filename, mode);
printf("g_orig_fopen %p fopen file %s\n", g_orig_fopen, filename);
return ret;
}
int openat(int dirfd, const char* pathname, int flags, ...) {
int fd;
va_list ap;
printf("g_orig_fopen %p openat file %s\n", g_orig_fopen, pathname);
if (flags & (O_CREAT)) {
va_start(ap, flags);
fd = g_orig_openat(dirfd, pathname, flags, va_arg(ap, mode_t));
}
else
fd = g_orig_openat(dirfd, pathname, flags);
return fd;
}
compiled with:
gcc -shared -fPIC -Wl,-init,init -ldl comm.c -o comm.so
I have an empty subdirectory subdir. Then it appears the library function fopen is called before init:
#LD_PRELOAD=./comm.so find subdir
g_orig_fopen 0x1 fopen file /proc/filesystems
g_orig_fopen 0x1 fopen file /proc/mounts
subdir
g_orig_fopen 0x7f7b2e574620 openat file subdir

Answer 1

Obviously, fopen is called before initialization of comm.so. It is interesting to place a breakpoint in fopen() in order to understand (check this link in order to get debug symbols of various packages). I get this backtrace:
(gdb) bt
#0 fopen (filename=0x7ffff79cd2e7 "/proc/filesystems", mode=0x7ffff79cd159 "r") at comm.c:28
#1 0x00007ffff79bdb0e in selinuxfs_exists_internal () at init.c:64
#2 0x00007ffff79b5d98 in init_selinuxmnt () at init.c:99
#3 init_lib () at init.c:154
#4 0x00007ffff7de88aa in call_init (l=<optimized out>, argc=argc#entry=1, argv=argv#entry=0x7fffffffdf58, env=env#entry=0x7fffffffdf68) at dl-init.c:72
#5 0x00007ffff7de89bb in call_init (env=0x7fffffffdf68, argv=0x7fffffffdf58, argc=1, l=<optimized out>) at dl-init.c:30
#6 _dl_init (main_map=0x7ffff7ffe170, argc=1, argv=0x7fffffffdf58, env=0x7fffffffdf68) at dl-init.c:120
#7 0x00007ffff7dd9c5a in _dl_start_user () from /lib64/ld-linux-x86-64.so.2
#8 0x0000000000000001 in ?? ()
#9 0x00007fffffffe337 in ?? ()
#10 0x0000000000000000 in ?? ()
It is obvious, comm.so depends on other libraries (libdl.so that requires libselinux.so). And comm.so is not the only library that declare an init function. libdl.so and libselinux.so also declare ones.
So, comm.so is the first library to be loaded (because it is declared with LD_PRELOAD) but, comm.so depends on libdl.so (because of -ldl during compilation) and libdl.so depends on libselinux.so. So, in order to load comm.so, init functions from libdl.so and libselinux.so are called before. And finally, init function from libselinux.so call fopen()
Personally, I usually resolve dynamic symbols during first call to the symbol. Like this:
FILE *fopen(const char *filename, const char *mode) {
static FILE *(*real_fopen)(const char *filename, const char *mode) = NULL;
if (!real_fopen)
real_fopen = dlsym(RTLD_NEXT, "fopen");
return real_fopen(filename, mode);
}