Introduce O_CLOEXEC

The problem is as follows: in multi-threaded code (or more correctly: all
code using clone() with CLONE_FILES) we have a race when exec'ing.

   thread #1                       thread #2

   fd=open()

                                   fork + exec

  fcntl(fd,F_SETFD,FD_CLOEXEC)

In some applications this can happen frequently.  Take a web browser.  One
thread opens a file and another thread starts, say, an external PDF viewer.
 The result can even be a security issue if that open file descriptor
refers to a sensitive file and the external program can somehow be tricked
into using that descriptor.

Just adding O_CLOEXEC support to open() doesn't solve the whole set of
problems.  There are other ways to create file descriptors (socket,
epoll_create, Unix domain socket transfer, etc).  These can and should be
addressed separately though.  open() is such an easy case that it makes not
much sense putting the fix off.

The test program:

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

#ifndef O_CLOEXEC
# define O_CLOEXEC 02000000
#endif

int
main (int argc, char *argv[])
{
  int fd;
  if (argc > 1)
    {
      fd = atol (argv[1]);
      printf ("child: fd = %d\n", fd);
      if (fcntl (fd, F_GETFD) == 0 || errno != EBADF)
        {
          puts ("file descriptor valid in child");
          return 1;
        }
      return 0;
    }

  fd = open ("/proc/self/exe", O_RDONLY | O_CLOEXEC);
  printf ("in parent: new fd = %d\n", fd);
  char buf[20];
  snprintf (buf, sizeof (buf), "%d", fd);
  execl ("/proc/self/exe", argv[0], buf, NULL);
  puts ("execl failed");
  return 1;
}

[kyle@parisc-linux.org: parisc fix]
Signed-off-by: Ulrich Drepper <drepper@redhat.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: Michael Kerrisk <mtk-manpages@gmx.net>
Cc: Chris Zankel <chris@zankel.net>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/open.c b/fs/open.c
index 0d515d1619747b00e724471a407f2a6cf6ea51d7..e6991c1b587432109c3b7e99cef941d62ba9b57b 100644 (file)
--- a/fs/open.c
+++ b/fs/open.c
@@ -855,7 +855,7 @@ EXPORT_SYMBOL(dentry_open);
 /*
  * Find an empty file descriptor entry, and mark it busy.
  */
-int get_unused_fd(void)
+static int get_unused_fd_flags(int flags)
 {
        struct files_struct * files = current->files;
        int fd, error;
@@ -891,7 +891,10 @@ repeat:
        }
 
        FD_SET(fd, fdt->open_fds);
-       FD_CLR(fd, fdt->close_on_exec);
+       if (flags & O_CLOEXEC)
+               FD_SET(fd, fdt->close_on_exec);
+       else
+               FD_CLR(fd, fdt->close_on_exec);
        files->next_fd = fd + 1;
 #if 1
        /* Sanity check */
@@ -907,6 +910,11 @@ out:
        return error;
 }
 
+int get_unused_fd(void)
+{
+       return get_unused_fd_flags(0);
+}
+
 EXPORT_SYMBOL(get_unused_fd);
 
 static void __put_unused_fd(struct files_struct *files, unsigned int fd)
@@ -959,7 +967,7 @@ long do_sys_open(int dfd, const char __user *filename, int flags, int mode)
        int fd = PTR_ERR(tmp);
 
        if (!IS_ERR(tmp)) {
-               fd = get_unused_fd();
+               fd = get_unused_fd_flags(flags);
                if (fd >= 0) {
                        struct file *f = do_filp_open(dfd, tmp, flags, mode);
                        if (IS_ERR(f)) {

diff --git a/include/asm-generic/fcntl.h b/include/asm-generic/fcntl.h
index c154b9d6e7e51a8593d06b0d6e3104aa7008e5d4..b8477414c5c85858de58d6489673da501f93d7bf 100644 (file)
--- a/include/asm-generic/fcntl.h
+++ b/include/asm-generic/fcntl.h
@@ -48,6 +48,9 @@
 #ifndef O_NOATIME
 #define O_NOATIME      01000000
 #endif
+#ifndef O_CLOEXEC
+#define O_CLOEXEC      02000000        /* set close_on_exec */
+#endif
 #ifndef O_NDELAY
 #define O_NDELAY       O_NONBLOCK
 #endif

diff --git a/include/asm-parisc/fcntl.h b/include/asm-parisc/fcntl.h
index 317851fa78f3f99b702ba30fff49dea37da728ef..1e1c824764ee1f0202c9012090e06f532eb75e42 100644 (file)
--- a/include/asm-parisc/fcntl.h
+++ b/include/asm-parisc/fcntl.h
@@ -3,21 +3,22 @@
 
 /* open/fcntl - O_SYNC is only implemented on blocks devices and on files
    located on an ext2 file system */
-#define O_APPEND       00000010
-#define O_BLKSEEK      00000100 /* HPUX only */
-#define O_CREAT                00000400 /* not fcntl */
-#define O_EXCL         00002000 /* not fcntl */
-#define O_LARGEFILE    00004000
-#define O_SYNC         00100000
-#define O_NONBLOCK     00200004 /* HPUX has separate NDELAY & NONBLOCK */
-#define O_NOCTTY       00400000 /* not fcntl */
-#define O_DSYNC                01000000 /* HPUX only */
-#define O_RSYNC                02000000 /* HPUX only */
-#define O_NOATIME      04000000
+#define O_APPEND       000000010
+#define O_BLKSEEK      000000100 /* HPUX only */
+#define O_CREAT                000000400 /* not fcntl */
+#define O_EXCL         000002000 /* not fcntl */
+#define O_LARGEFILE    000004000
+#define O_SYNC         000100000
+#define O_NONBLOCK     000200004 /* HPUX has separate NDELAY & NONBLOCK */
+#define O_NOCTTY       000400000 /* not fcntl */
+#define O_DSYNC                001000000 /* HPUX only */
+#define O_RSYNC                002000000 /* HPUX only */
+#define O_NOATIME      004000000
+#define O_CLOEXEC      010000000 /* set close_on_exec */
 
-#define O_DIRECTORY    00010000 /* must be a directory */
-#define O_NOFOLLOW     00000200 /* don't follow links */
-#define O_INVISIBLE    04000000 /* invisible I/O, for DMAPI/XDSM */
+#define O_DIRECTORY    000010000 /* must be a directory */
+#define O_NOFOLLOW     000000200 /* don't follow links */
+#define O_INVISIBLE    004000000 /* invisible I/O, for DMAPI/XDSM */
 
 #define F_GETLK64      8
 #define F_SETLK64      9