[PATCH] clean dup2() up a bit

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/fcntl.c b/fs/fcntl.c
index 2e40799..ac4f7db 100644 (file)
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -63,31 +63,35 @@ asmlinkage long sys_dup3(unsigned int oldfd, unsigned int newfd, int flags)
                return -EINVAL;
 
        spin_lock(&files->file_lock);
-       if (!(file = fcheck(oldfd)))
-               goto out_unlock;
-       get_file(file);                 /* We are now finished with oldfd */
-
        err = expand_files(files, newfd);
+       file = fcheck(oldfd);
+       if (unlikely(!file))
+               goto Ebadf;
        if (unlikely(err < 0)) {
                if (err == -EMFILE)
-                       err = -EBADF;
-               goto out_fput;
+                       goto Ebadf;
+               goto out_unlock;
        }
-
-       /* To avoid races with open() and dup(), we will mark the fd as
-        * in-use in the open-file bitmap throughout the entire dup2()
-        * process.  This is quite safe: do_close() uses the fd array
-        * entry, not the bitmap, to decide what work needs to be
-        * done.  --sct */
-       /* Doesn't work. open() might be there first. --AV */
-
-       /* Yes. It's a race. In user space. Nothing sane to do */
+       /*
+        * We need to detect attempts to do dup2() over allocated but still
+        * not finished descriptor.  NB: OpenBSD avoids that at the price of
+        * extra work in their equivalent of fget() - they insert struct
+        * file immediately after grabbing descriptor, mark it larval if
+        * more work (e.g. actual opening) is needed and make sure that
+        * fget() treats larval files as absent.  Potentially interesting,
+        * but while extra work in fget() is trivial, locking implications
+        * and amount of surgery on open()-related paths in VFS are not.
+        * FreeBSD fails with -EBADF in the same situation, NetBSD "solution"
+        * deadlocks in rather amusing ways, AFAICS.  All of that is out of
+        * scope of POSIX or SUS, since neither considers shared descriptor
+        * tables and this condition does not arise without those.
+        */
        err = -EBUSY;
        fdt = files_fdtable(files);
        tofree = fdt->fd[newfd];
        if (!tofree && FD_ISSET(newfd, fdt->open_fds))
-               goto out_fput;
-
+               goto out_unlock;
+       get_file(file);
        rcu_assign_pointer(fdt->fd[newfd], file);
        FD_SET(newfd, fdt->open_fds);
        if (flags & O_CLOEXEC)
@@ -98,17 +102,14 @@ asmlinkage long sys_dup3(unsigned int oldfd, unsigned int newfd, int flags)
 
        if (tofree)
                filp_close(tofree, files);
-       err = newfd;
-out:
-       return err;
-out_unlock:
-       spin_unlock(&files->file_lock);
-       goto out;
 
-out_fput:
+       return newfd;
+
+Ebadf:
+       err = -EBADF;
+out_unlock:
        spin_unlock(&files->file_lock);
-       fput(file);
-       goto out;
+       return err;
 }
 
 asmlinkage long sys_dup2(unsigned int oldfd, unsigned int newfd)