From 5e7cdf1817924f8ae0333c0b53c2da32f35b65ea Mon Sep 17 00:00:00 2001
From: Konstantin Belousov <kib@FreeBSD.org>
Date: Thu, 6 May 2021 20:16:53 +0300
Subject: [PATCH] openat(2): add O_EMPTY_PATH

It reopens the passed file descriptor, checking the file backing vnode'
current access rights against open mode. In particular, this flag allows
to convert file descriptor opened with O_PATH, into operable file
descriptor, assuming permissions allow that.

Reviewed by:	markj
Tested by:	Andrew Walker <awalker@ixsystems.com>
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D30148
---
 lib/libc/sys/open.2  | 18 +++++++++++++++++-
 sys/kern/vfs_vnops.c |  2 ++
 sys/sys/fcntl.h      |  3 +++
 3 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/lib/libc/sys/open.2 b/lib/libc/sys/open.2
index a7806df69daf..5a6f161291f6 100644
--- a/lib/libc/sys/open.2
+++ b/lib/libc/sys/open.2
@@ -28,7 +28,7 @@
 .\"     @(#)open.2	8.2 (Berkeley) 11/16/93
 .\" $FreeBSD$
 .\"
-.Dd March 18, 2021
+.Dd May 6, 2021
 .Dt OPEN 2
 .Os
 .Sh NAME
@@ -169,6 +169,7 @@ O_CLOEXEC	set FD_CLOEXEC upon open
 O_VERIFY	verify the contents of the file
 O_RESOLVE_BENEATH	path resolution must not cross the fd directory
 O_PATH		record only the target path in the opened descriptor
+O_EMPTY_PATH	openat, open file referenced by fd if path is empty
 .Ed
 .Pp
 Opening a file with
@@ -355,6 +356,21 @@ are not allowed.
 File opened with the
 .Dv O_PATH
 flag does not prevent non-forced unmount of the volume it belongs to.
+.Pp
+A file descriptor created with the
+.Dv O_PATH
+flag can be opened into normal (operable) file descriptor by
+specifying it as the
+.Fa fd
+argument to
+.Fn openat
+with empty
+.Fa path
+and flag
+.Dv O_EMPTY_PATH .
+Such an open behaves as if the current path of the file referenced by
+.Fa fd
+is passed, except that the path walk permissions are not checked.
 See also the description of
 .Dv AT_EMPTY_PATH
 flag for
diff --git a/sys/kern/vfs_vnops.c b/sys/kern/vfs_vnops.c
index d4396f67a67b..670bc4bf8fcc 100644
--- a/sys/kern/vfs_vnops.c
+++ b/sys/kern/vfs_vnops.c
@@ -200,6 +200,8 @@ open2nameif(int fmode, u_int vn_open_flags)
 	res = ISOPEN | LOCKLEAF;
 	if ((fmode & O_RESOLVE_BENEATH) != 0)
 		res |= RBENEATH;
+	if ((fmode & O_EMPTY_PATH) != 0)
+		res |= EMPTYPATH;
 	if ((vn_open_flags & VN_OPEN_NOAUDIT) == 0)
 		res |= AUDITVNODE1;
 	if ((vn_open_flags & VN_OPEN_NOCAPCHECK) != 0)
diff --git a/sys/sys/fcntl.h b/sys/sys/fcntl.h
index 58d46ae26338..8fa52aeacafd 100644
--- a/sys/sys/fcntl.h
+++ b/sys/sys/fcntl.h
@@ -141,6 +141,9 @@ typedef	__pid_t		pid_t;
 #endif
 
 #define	O_DSYNC		0x01000000	/* POSIX data sync */
+#if __BSD_VISIBLE
+#define	O_EMPTY_PATH	0x02000000
+#endif
 
 /*
  * XXX missing O_RSYNC.