Add a way to get an empty descriptor table with one reference. Let
dup_fd() use that helper. The coredump code will use it too in a bit.

No functional changes.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/file.c               | 49 ++++++++++++++++++++++++++++++++++++++-----------
 include/linux/fdtable.h |  1 +
 2 files changed, 39 insertions(+), 11 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index 608e6a1fa69b..ccbaa71dfbe4 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -375,21 +375,15 @@ static unsigned int sane_fdtable_size(struct fdtable *fdt, struct fd_range *punc
 	return ALIGN(last + 1, BITS_PER_LONG);
 }
 
-/*
- * Allocate a new descriptor table and copy contents from the passed in
- * instance.  Returns a pointer to cloned table on success, ERR_PTR()
- * on failure.  For 'punch_hole' see sane_fdtable_size().
- */
-struct files_struct *dup_fd(struct files_struct *oldf, struct fd_range *punch_hole)
+/* A table with one reference and the embedded fdtable, nothing copied yet. */
+static struct files_struct *alloc_files(gfp_t gfp)
 {
 	struct files_struct *newf;
-	struct file **old_fds, **new_fds;
-	unsigned int open_files, i;
-	struct fdtable *old_fdt, *new_fdt;
+	struct fdtable *new_fdt;
 
-	newf = kmem_cache_alloc(files_cachep, GFP_KERNEL);
+	newf = kmem_cache_alloc(files_cachep, gfp);
 	if (!newf)
-		return ERR_PTR(-ENOMEM);
+		return NULL;
 
 	atomic_set(&newf->count, 1);
 
@@ -404,6 +398,39 @@ struct files_struct *dup_fd(struct files_struct *oldf, struct fd_range *punch_ho
 	new_fdt->full_fds_bits = newf->full_fds_bits_init;
 	new_fdt->fd = &newf->fd_array[0];
 
+	return newf;
+}
+
+/* An empty descriptor table with one reference. */
+struct files_struct *alloc_files_struct(void)
+{
+	struct files_struct *newf;
+
+	newf = alloc_files(GFP_KERNEL | __GFP_ZERO);
+	if (!newf)
+		return NULL;
+
+	rcu_assign_pointer(newf->fdt, &newf->fdtab);
+	return newf;
+}
+
+/*
+ * Allocate a new descriptor table and copy contents from the passed in
+ * instance.  Returns a pointer to cloned table on success, ERR_PTR()
+ * on failure.  For 'punch_hole' see sane_fdtable_size().
+ */
+struct files_struct *dup_fd(struct files_struct *oldf, struct fd_range *punch_hole)
+{
+	struct files_struct *newf;
+	struct file **old_fds, **new_fds;
+	unsigned int open_files, i;
+	struct fdtable *old_fdt, *new_fdt;
+
+	newf = alloc_files(GFP_KERNEL);
+	if (!newf)
+		return ERR_PTR(-ENOMEM);
+	new_fdt = &newf->fdtab;
+
 	spin_lock(&oldf->file_lock);
 	old_fdt = files_fdtable(oldf);
 	open_files = sane_fdtable_size(old_fdt, punch_hole);
diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index 15ee6292e0e3..811994b16bed 100644
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -100,6 +100,7 @@ static inline bool close_on_exec(unsigned int fd, const struct files_struct *fil
 struct task_struct;
 
 void put_files_struct(struct files_struct *fs);
+struct files_struct *alloc_files_struct(void);
 struct files_struct *switch_files_struct(struct task_struct *tsk,
 					 struct files_struct *files);
 int unshare_fd(unsigned long unshare_flags, struct files_struct **new_fdp);

-- 
2.53.0