[PATCH] Add support for empty path in openat and openat2 syscalls

[Jori Koolstra]

To get an operable version of an O_PATH file descriptors, it is possible
to use openat(fd, ".", O_DIRECTORY) for directories, but other files
currently require going through open("/proc/<pid>/fd/<nr>") which
depends on a functioning procfs.

This patch adds the O_EMPTY_PATH flag to openat and openat2. If passed
LOOKUP_EMPTY is set at path resolve time.

Signed-off-by: Jori Koolstra <jkoolstra@xxxxxxxxx>
---
 fs/fcntl.c                       | 2 +-
 fs/open.c                        | 6 ++++--
 include/linux/fcntl.h            | 2 +-
 include/uapi/asm-generic/fcntl.h | 4 ++++
 4 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index f93dbca08435..62ab4ad2b6f5 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -1169,7 +1169,7 @@ 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(20 - 1 /* for O_RDONLY being 0 */ !=
+	BUILD_BUG_ON(21 - 1 /* for O_RDONLY being 0 */ !=
 		HWEIGHT32(
 			(VALID_OPEN_FLAGS & ~(O_NONBLOCK | O_NDELAY)) |
 			__FMODE_EXEC));
diff --git a/fs/open.c b/fs/open.c
index 91f1139591ab..32865822ca1c 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -1160,7 +1160,7 @@ struct file *kernel_file_open(const struct path *path, int flags,
 EXPORT_SYMBOL_GPL(kernel_file_open);
 
 #define WILL_CREATE(flags)	(flags & (O_CREAT | __O_TMPFILE))
-#define O_PATH_FLAGS		(O_DIRECTORY | O_NOFOLLOW | O_PATH | O_CLOEXEC)
+#define O_PATH_FLAGS		(O_DIRECTORY | O_NOFOLLOW | O_PATH | O_CLOEXEC | O_EMPTY_PATH)
 
 inline struct open_how build_open_how(int flags, umode_t mode)
 {
@@ -1277,6 +1277,8 @@ inline int build_open_flags(const struct open_how *how, struct open_flags *op)
 		}
 	}
 
+	if (flags & O_EMPTY_PATH)
+		lookup_flags |= LOOKUP_EMPTY;
 	if (flags & O_DIRECTORY)
 		lookup_flags |= LOOKUP_DIRECTORY;
 	if (!(flags & O_NOFOLLOW))
@@ -1362,7 +1364,7 @@ static int do_sys_openat2(int dfd, const char __user *filename,
 	if (unlikely(err))
 		return err;
 
-	CLASS(filename, name)(filename);
+	CLASS(filename_flags, name)(filename, op.lookup_flags);
 	return FD_ADD(how->flags, do_file_open(dfd, name, &op));
 }
 
diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h
index a332e79b3207..ce742f67bf60 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_EMPTY_PATH | __O_TMPFILE)
 
 /* 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 613475285643..8e4e796ad212 100644
--- a/include/uapi/asm-generic/fcntl.h
+++ b/include/uapi/asm-generic/fcntl.h
@@ -88,6 +88,10 @@
 #define __O_TMPFILE	020000000
 #endif
 
+#ifndef O_EMPTY_PATH
+#define O_EMPTY_PATH	0100000000
+#endif
+
 /* a horrid kludge trying to make sure that this will fail on old kernels */
 #define O_TMPFILE (__O_TMPFILE | O_DIRECTORY)
 
-- 
2.53.0