There is a lot to explain, but I try to make it as short as
possible. I'd start by pasting some parts of sys/stat.h:

extern int stat (const char *__restrict __file,
		 struct stat *__restrict __buf) __THROW __nonnull ((1, 2));

extern int __REDIRECT_NTH (stat, (const char *__restrict __file,
				  struct stat *__restrict __buf), stat64)
     __nonnull ((1, 2));

__extern_inline int
__NTH (stat (const char *__path, struct stat *__statbuf))
{
  return __xstat (_STAT_VER, __path, __statbuf);
}

Only one of these is effective at once, due to some usage of
the mess we are dealing with in here. So, basically, while
compiling or linking stat() in our code can be transformed into
some other func. Or a dragon.
Now, if you read stat(2) manpage, esp. "C library/kernel
differences" section, you'll learn that glibc uses some tricks
for older applications to work. I haven't gotten around actual
code that does this, but based on my observations, if 'stat'
symbol is found, glibc assumes it's dealing with ancient
application. Unfortunately, it can be just ours stat coming from
our mock. Therefore, calling stat() from a test will end up in
our mock. But since glibc is not exposing the symbol anymore, our
call of real_stat() will SIGSEGV immediately as the pointer to
function is NULL. Therefore, we should expose only those symbols
we know glibc has.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>