[PATCH 04/10] file: let dup_fd() drop everything outside of the range

[Christian Brauner <brauner@kernel.org>]

Add FD_RANGE_EXCEPT. It turns the meaning of range around. Instead of
indicating that the descriptors in the range are the ones that are left
out of the copy they indicate the range that makes it into the copy. All
other files are left behind. The clone only has to reach the last open
descriptor inside the range.

Nothing passes the flag yet.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/file.c               | 15 ++++++++++-----
 include/linux/fdtable.h |  6 ++++++
 2 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index 6234548be88b..ae7d021398dd 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -367,19 +367,24 @@ static unsigned long fd_range_word(struct fd_range *range, unsigned int i)
 /* Bits of word @i that dup_fd() leaves behind. */
 static unsigned long dup_fd_dropped_word(unsigned int i, struct fd_range *range)
 {
+	unsigned long dropped;
+
 	if (!range)
 		return 0;
-	return fd_range_word(range, i);
+	dropped = fd_range_word(range, i);
+	if (range->flags & FD_RANGE_EXCEPT)
+		dropped = ~dropped;
+	return dropped;
 }
 
 /*
  * Note that a sane fdtable size always has to be a multiple of
  * BITS_PER_LONG, since we have bitmaps that are sized by this.
  *
- * range is optional - when close_range() is asked to unshare
- * and close, dup_fd() leaves the descriptors in that range behind,
- * so the cloned table only has to reach the last open descriptor
- * outside of it.
+ * range is optional. When close_range() is asked to unshare dup_fd()
+ * will leave any files behind according to the range and its flags. The
+ * cloned table only has to reach the last open descriptor that is
+ * carried over.
  */
 static unsigned int sane_fdtable_size(struct fdtable *fdt, struct fd_range *range)
 {
diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index c45306a9f007..d6c6c7a3400d 100644
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -101,8 +101,14 @@ struct task_struct;
 
 void put_files_struct(struct files_struct *fs);
 int unshare_files(void);
+enum fd_range_flags {
+	/* Leave behind all descriptors outside of the specified range. */
+	FD_RANGE_EXCEPT		= (1U << 0),
+};
+
 struct fd_range {
 	unsigned int from, to;
+	enum fd_range_flags flags;
 };
 struct files_struct *dup_fd(struct files_struct *, struct fd_range *) __latent_entropy;
 void do_close_on_exec(struct files_struct *);

-- 
2.53.0