IssueTracker

I have forgotten to check the return value of open_output in the "working" code. If I change wopenout to

void wopenout (gzFile f) {
  if (open_output ((FILE**)&(f), "wb"))
    f = gzdopen(fileno((FILE*)f), "wb");
}

I get the same error as with the macro, so I suppose there is a real fd issue in this code.

There's another error in the test program as well, -Wall warns that gzFile fp is uninitialized. After fixing that it works again for the function implementation, but not the macro-implementation. So, this gives an error:

#include <zlib.h>
#include <stdio.h>
int open_output(FILE **f_ptr, char *mode) {
  *f_ptr = fopen ("test.gz", mode);
  return *f_ptr != NULL;
}
#define wopenout(f) (open_output ((FILE**)&(f), "wb") \
		     && (f = gzdopen(fileno((FILE*)f), "wb")))
int main() {
  gzFile fp = NULL;
  wopenout(fp);
  gzclose(fp);
}

and this works:

#include <zlib.h>
#include <stdio.h>
int open_output(FILE **f_ptr, char *mode) {
  *f_ptr = fopen ("test.gz", mode);
  return *f_ptr != NULL;
}
void wopenout (gzFile f) {
  if (open_output ((FILE**)&(f), "wb"))
    f = gzdopen(fileno((FILE*)f), "wb");
}
int main() {
  gzFile fp = NULL;
  wopenout(fp);
  gzclose(fp);
}

and I cannot see why there should be a difference in behaviour. I also noticed that clang 11 gives a working program when using -O0, but -O1 or higher gives the fdsan error. clang 9 (which the ndk uses) gives the error for all optimization levels.

Here's another minimal example, condensed from emacs, that fails:

#include <stdio.h>
int main()
{
  fdopen (2, "w");
  fclose (stderr);
}

My guess is that the sanitizer does not like that we do not use the fd fdopen returns when closing stream 2/stderr. If I change to this:

#include <stdio.h>
int main()
{
  FILE *err = fdopen (2, "w");
  fclose (err);
}

It compiles and runs without issues. Is there an actual problem in the first snippet?

Is there an actual problem in the first snippet?

yes. you have two FILE*s that both think they own file descriptor 2. depending on what you're actually trying to do, you probably meant to use freopen(3) instead? https://man7.org/linux/man-pages/man3/fopen.3.html

Thanks for the feedback. The idea in the code is to open a line buffered copy of stderr, is that valid? I am still having some problems wrapping my head around the issue, so I would be greatful if you could have a look at the failing example below:

#include <stdlib.h>
#include <unistd.h>

/* If FD is not already open, arrange for it to be open with FLAGS.  */
static void
force_open (int fd, int flags)
{
  if (dup2 (fd, fd) < 0)
    {
      int n = open ("/dev/null", flags);
      if (n < 0 || (fd != n && (dup2 (n, fd) < 0 || close (n) == 0 != 0)))
	exit (EXIT_FAILURE);
    }
}

/* A stream that is like stderr, except line buffered.  It is NULL
   during startup, or if line buffering is not in use.  */
static FILE *buferr;

int
main()
{
  /* Make sure stderr are open to something, so that the file
   descriptor is not hijacked by later system calls.  */
  force_open (STDERR_FILENO, O_RDONLY);

  /* Set buferr if possible on platforms defining _PC_PIPE_BUF, as
     they support the notion of atomic writes to pipes.  */
  #ifdef _PC_PIPE_BUF
    buferr = fdopen (STDERR_FILENO, "w");
    if (buferr)
      setvbuf (buferr, NULL, _IOLBF, 0);
  #endif

  int err = buferr && (fflush (buferr) != 0 || ferror (buferr));
  if (err | (fclose (stderr) != 0))
    return 1;
  return 0;
}

Compiling and executing on device gives

fdsan: attempted to close file descriptor 2, expected to be unowned, actually owned by FILE* 0xaba9700c
Aborted

from the fclose (stderr) call, similar to in the previous very minimal example.

The idea in the code is to open a line buffered copy of stderr

in that case you just want setlinebuf(stderr). see https://man7.org/linux/man-pages/man3/setbuf.3.html for the various stdio facilities for manipulating buffering.