F_DUPFD_CLOEXEC implementation

One more small change to extend the availability of creation of file
descriptors with FD_CLOEXEC set.  Adding a new command to fcntl() requires
no new system call and the overall impact on code size if minimal.

If this patch gets accepted we will also add this change to the next
revision of the POSIX spec.

To test the patch, use the following little program.  Adjust the value of
F_DUPFD_CLOEXEC appropriately.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#ifndef F_DUPFD_CLOEXEC
# define F_DUPFD_CLOEXEC 12
#endif

int
main (int argc, char *argv[])
{
  if  (argc > 1)
    {
      if (fcntl (3, F_GETFD) == 0)
	{
	  puts ("descriptor not closed");
	  exit (1);
	}
      if (errno != EBADF)
	{
	  puts ("error not EBADF");
	  exit (1);
	}

      exit (0);
    }
  int fd = fcntl (STDOUT_FILENO, F_DUPFD_CLOEXEC, 0);
  if (fd == -1 && errno == EINVAL)
    {
      puts ("F_DUPFD_CLOEXEC not supported");
      return 0;
    }
  if (fd != 3)
    {
      puts ("program called with descriptors other than 0,1,2");
      return 1;
    }

  execl ("/proc/self/exe", "/proc/self/exe", "1", NULL);
  puts ("execl failed");
  return 1;
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Ulrich Drepper <drepper@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>
Cc: <linux-arch@vger.kernel.org>
Cc: Kyle McMartin <kyle@mcmartin.ca>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 4 deletions

diff --git a/fs/fcntl.c b/fs/fcntl.c
index 78b2ff044054..c9db73fc5e3d 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -110,7 +110,7 @@ out:
 	return error;
 }
 
-static int dupfd(struct file *file, unsigned int start)
+static int dupfd(struct file *file, unsigned int start, int cloexec)
 {
 	struct files_struct * files = current->files;
 	struct fdtable *fdt;
@@ -122,7 +122,10 @@ static int dupfd(struct file *file, unsigned int start)
 		/* locate_fd() may have expanded fdtable, load the ptr */
 		fdt = files_fdtable(files);
 		FD_SET(fd, fdt->open_fds);
-		FD_CLR(fd, fdt->close_on_exec);
+		if (cloexec)
+			FD_SET(fd, fdt->close_on_exec);
+		else
+			FD_CLR(fd, fdt->close_on_exec);
 		spin_unlock(&files->file_lock);
 		fd_install(fd, file);
 	} else {
@@ -195,7 +198,7 @@ asmlinkage long sys_dup(unsigned int fildes)
 	struct file * file = fget(fildes);
 
 	if (file)
-		ret = dupfd(file, 0);
+		ret = dupfd(file, 0, 0);
 	return ret;
 }
 
@@ -319,8 +322,9 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
 
 	switch (cmd) {
 	case F_DUPFD:
+	case F_DUPFD_CLOEXEC:
 		get_file(filp);
-		err = dupfd(filp, arg);
+		err = dupfd(filp, arg, cmd == F_DUPFD_CLOEXEC);
 		break;
 	case F_GETFD:
 		err = get_close_on_exec(fd) ? FD_CLOEXEC : 0;