[PATCH] Add new open(2) flag - O_EMPTY_PATH

[Ameer Hamza]

This patch adds a new flag O_EMPTY_PATH that allows openat and open
system calls to open a file referenced by fd if the path is empty,
and it is very similar to the FreeBSD O_EMPTY_PATH flag. This can be
beneficial in some cases since it would avoid having to grant /proc
access to things like samba containers for reopening files to change
flags in a race-free way.

Signed-off-by: Ameer Hamza <ahamza@xxxxxxxxxxxxx>
---
 fs/fcntl.c                             | 2 +-
 fs/namei.c                             | 4 ++--
 fs/open.c                              | 2 +-
 include/linux/fcntl.h                  | 2 +-
 include/uapi/asm-generic/fcntl.h       | 4 ++++
 tools/include/uapi/asm-generic/fcntl.h | 4 ++++
 6 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index 146c9ab0cd4b..7aac650e16e2 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -1027,7 +1027,7 @@ static int __init fcntl_init(void)
 	 * Exceptions: O_NONBLOCK is a two bit define on parisc; O_NDELAY
 	 * is defined as O_NONBLOCK on some platforms and not on others.
 	 */
-	BUILD_BUG_ON(21 - 1 /* for O_RDONLY being 0 */ !=
+	BUILD_BUG_ON(22 - 1 /* for O_RDONLY being 0 */ !=
 		HWEIGHT32(
 			(VALID_OPEN_FLAGS & ~(O_NONBLOCK | O_NDELAY)) |
 			__FMODE_EXEC | __FMODE_NONOTIFY));
diff --git a/fs/namei.c b/fs/namei.c
index 309ae6fc8c99..2b2735af6d03 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -192,7 +192,7 @@ getname_flags(const char __user *filename, int flags, int *empty)
 	if (unlikely(!len)) {
 		if (empty)
 			*empty = 1;
-		if (!(flags & LOOKUP_EMPTY)) {
+		if (!(flags & (LOOKUP_EMPTY | O_EMPTY_PATH))) {
 			putname(result);
 			return ERR_PTR(-ENOENT);
 		}
@@ -2347,7 +2347,7 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
 	if ((flags & (LOOKUP_RCU | LOOKUP_CACHED)) == LOOKUP_CACHED)
 		return ERR_PTR(-EAGAIN);
 
-	if (!*s)
+	if (!*s && unlikely(!(flags & O_EMPTY_PATH)))
 		flags &= ~LOOKUP_RCU;
 	if (flags & LOOKUP_RCU)
 		rcu_read_lock();
diff --git a/fs/open.c b/fs/open.c
index 82c1a28b3308..b4ec054a418f 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -1301,7 +1301,7 @@ static long do_sys_openat2(int dfd, const char __user *filename,
 	if (fd)
 		return fd;
 
-	tmp = getname(filename);
+	tmp = getname_flags(filename, how->flags & O_EMPTY_PATH, NULL);
 	if (IS_ERR(tmp))
 		return PTR_ERR(tmp);
 
diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h
index a332e79b3207..bf8467bb0bd2 100644
--- a/include/linux/fcntl.h
+++ b/include/linux/fcntl.h
@@ -10,7 +10,7 @@
 	(O_RDONLY | O_WRONLY | O_RDWR | O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC | \
 	 O_APPEND | O_NDELAY | O_NONBLOCK | __O_SYNC | O_DSYNC | \
 	 FASYNC	| O_DIRECT | O_LARGEFILE | O_DIRECTORY | O_NOFOLLOW | \
-	 O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE)
+	 O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE | O_EMPTY_PATH)
 
 /* List of all valid flags for the how->resolve argument: */
 #define VALID_RESOLVE_FLAGS \
diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h
index 1ecdb911add8..a03f4275517b 100644
--- a/include/uapi/asm-generic/fcntl.h
+++ b/include/uapi/asm-generic/fcntl.h
@@ -89,6 +89,10 @@
 #define __O_TMPFILE	020000000
 #endif
 
+#ifndef O_EMPTY_PATH
+#define O_EMPTY_PATH	040000000
+#endif
+
 /* a horrid kludge trying to make sure that this will fail on old kernels */
 #define O_TMPFILE (__O_TMPFILE | O_DIRECTORY)
 #define O_TMPFILE_MASK (__O_TMPFILE | O_DIRECTORY | O_CREAT)      
diff --git a/tools/include/uapi/asm-generic/fcntl.h b/tools/include/uapi/asm-generic/fcntl.h
index b02c8e0f4057..f32a81604296 100644
--- a/tools/include/uapi/asm-generic/fcntl.h
+++ b/tools/include/uapi/asm-generic/fcntl.h
@@ -89,6 +89,10 @@
 #define __O_TMPFILE	020000000
 #endif
 
+#ifndef O_EMPTY_PATH
+#define O_EMPTY_PATH	040000000
+#endif
+
 /* a horrid kludge trying to make sure that this will fail on old kernels */
 #define O_TMPFILE (__O_TMPFILE | O_DIRECTORY)
 #define O_TMPFILE_MASK (__O_TMPFILE | O_DIRECTORY | O_CREAT)      
-- 
2.25.1