[PATCH] clean dup2() up a bit

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

1 files changed, 27 insertions, 26 deletions

diff --git a/fs/fcntl.c b/fs/fcntl.c
index 2e40799daad6..ac4f7db9f134 100644
--- 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)