fs, close_range: add flag CLOSE_RANGE_CLOEXEC

When the flag CLOSE_RANGE_CLOEXEC is set, close_range doesn't
immediately close the files but it sets the close-on-exec bit.

It is useful for e.g. container runtimes that usually install a
seccomp profile "as late as possible" before execv'ing the container
process itself.  The container runtime could either do:
  1                                  2
- install_seccomp_profile();       - close_range(MIN_FD, MAX_INT, 0);
- close_range(MIN_FD, MAX_INT, 0); - install_seccomp_profile();
- execve(...);                     - execve(...);

Both alternative have some disadvantages.

In the first variant the seccomp_profile cannot block the close_range
syscall, as well as opendir/read/close/... for the fallback on older
kernels.
In the second variant, close_range() can be used only on the fds
that are not going to be needed by the runtime anymore, and it must be
potentially called multiple times to account for the different ranges
that must be closed.

Using close_range(..., ..., CLOSE_RANGE_CLOEXEC) solves these issues.
The runtime is able to use the existing open fds, the seccomp profile
can block close_range() and the syscalls used for its fallback.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
Link: https://lore.kernel.org/r/20201118104746.873084-2-gscrivan@redhat.com
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

include/uapi/linux/close_range.h

@@ -5,5 +5,8 @@
 /* Unshare the file descriptor table before closing file descriptors. */
 #define CLOSE_RANGE_UNSHARE	(1U << 1)
 
+/* Set the FD_CLOEXEC bit instead of closing the file descriptor. */
+#define CLOSE_RANGE_CLOEXEC	(1U << 2)
+
 #endif /* _UAPI_LINUX_CLOSE_RANGE_H */