It seems to work!

[preload-hacks] / uopen.c

#define _GNU_SOURCE
#undef sun

#include <errno.h>
#include <stdarg.h>
#include <stdio.h>
#include <string.h>

#include <unistd.h>
#include <dlfcn.h>
#include <fcntl.h>

#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/un.h>

#define IMPORTS(_)							\
  _(open, int, (const char *, int, ...))				\
  _(open64, int, (const char *, int, ...))				\
  _(fopen, FILE *, (const char *, const char *))			\
  _(freopen, FILE *, (const char *, const char *, FILE *))

#define DECL(imp, ret, args) static ret (*real_##imp) args;
IMPORTS(DECL)
#undef DECL

static void setup(void) __attribute__((constructor));
static void import(void)
{
#define IMPORT(imp, ret, args)						\
    real_##imp = (ret (*)args)dlsym(RTLD_NEXT, #imp);
  IMPORTS(IMPORT)
#undef IMPORT
}

#define SA(sa) ((struct sockaddr *)(sa))

#define PRESERVING_ERRNO(body) do {					\
  int _err = errno; { body } errno = _err;				\
} while (0)

static int maybe_connect(const char *fn, int *fdp)
{
  int fd;
  struct sockaddr_un sun;
  struct stat st;

  if (stat(fn, &st) || !S_ISSOCK(st.st_mode))
    return (0);
  *fdp = -1;
  if (strlen(fn) >= sizeof(sun.sun_path)) {
    errno = EINVAL;
    return (1);
  }
  strncpy(sun.sun_path, fn, sizeof(sun.sun_path));
  sun.sun_family = AF_UNIX;
  if ((fd = socket(PF_UNIX, SOCK_STREAM, 0)) < 0 ||
      connect(fd, SA(&sun), SUN_LEN(&sun))) {
    PRESERVING_ERRNO( if (fd >= 0) close(fd); );
    return (1);
  }
  *fdp = fd;
  return (1);
}

#define OPEN_VENEER(open)						\
  int open(const char *fn, int how, ...)				\
  {									\
    int mode = 0;							\
    int fd;								\
    va_list ap;								\
									\
    PRESERVING_ERRNO({							\
      if (how & O_CREAT)						\
	{ va_start(ap, how); mode = va_arg(ap, int); va_end(ap); }	\
      if (!maybe_connect(fn, &fd)) fd = real_##open(fn, how, mode);	\
      if (fd < 0) return (-1);						\
    });									\
    return (fd);							\
  }
OPEN_VENEER(open)
OPEN_VENEER(open64)

FILE *fopen(const char *fn, const char *how)
{
  int fd;
  FILE *fp = 0;

  PRESERVING_ERRNO({
    if (!maybe_connect(fn, &fd))
      fp = real_fopen(fn, how);
    else if (fd >= 0 && (fp = fdopen(fd, how)) == 0)
      PRESERVING_ERRNO( close(fd); );
  });
  return (fp);
}

FILE *freopen(const char *fn, const char *how, FILE *fp)
{
  int fd;

  PRESERVING_ERRNO({
    if (!maybe_connect(fn, &fd))
      fp = real_freopen(fn, how, fp);
    else if (fd >= 0) {
      if (fflush(fp) || dup2(fd, fileno(fp))) fp = 0;
      PRESERVING_ERRNO( close(fd); );
    }
  });
  return (fp);
}

static void setup(void)
{
  PRESERVING_ERRNO({
    import();
  });
}