[RFC PATCH v2 1/3] vfs: add support for empty path to openat2(2)

[Jori Koolstra]

To get an operable version of an O_PATH file descriptor, 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 OPENAT2_EMPTY_PATH flag to openat2(2). If passed,
LOOKUP_EMPTY is set at path resolve time.

Note: This implies that you cannot rely anymore on disabling procfs from
being mounted (e.g. inside a container without procfs mounted and with
CAP_SYS_ADMIN dropped) to prevent O_PATH fds from being re-opened
read-write.

Signed-off-by: Jori Koolstra <jkoolstra@xxxxxxxxx>
---
 fs/fcntl.c                   |  4 ++--
 fs/open.c                    | 11 +++++------
 include/linux/fcntl.h        |  5 ++++-
 include/uapi/linux/openat2.h |  4 ++++
 4 files changed, 15 insertions(+), 9 deletions(-)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index beab8080badf..d9ae3c71edfe 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -1169,8 +1169,8 @@ 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 */ !=
-		HWEIGHT32(
+	BUILD_BUG_ON(21 - 1 /* for O_RDONLY being 0 */ !=
+		HWEIGHT64(
 			(VALID_OPEN_FLAGS & ~(O_NONBLOCK | O_NDELAY)) |
 			__FMODE_EXEC));
 
diff --git a/fs/open.c b/fs/open.c
index 91f1139591ab..e019ddecc73c 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -1160,12 +1160,12 @@ 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 | OPENAT2_EMPTY_PATH)
 
 inline struct open_how build_open_how(int flags, umode_t mode)
 {
 	struct open_how how = {
-		.flags = flags & VALID_OPEN_FLAGS,
+		.flags = ((unsigned int) flags) & VALID_OPEN_FLAGS,
 		.mode = mode & S_IALLUGO,
 	};
 
@@ -1185,9 +1185,6 @@ inline int build_open_flags(const struct open_how *how, struct open_flags *op)
 	int lookup_flags = 0;
 	int acc_mode = ACC_MODE(flags);
 
-	BUILD_BUG_ON_MSG(upper_32_bits(VALID_OPEN_FLAGS),
-			 "struct open_flags doesn't yet handle flags > 32 bits");
-
 	/*
 	 * Strip flags that aren't relevant in determining struct open_flags.
 	 */
@@ -1281,6 +1278,8 @@ inline int build_open_flags(const struct open_how *how, struct open_flags *op)
 		lookup_flags |= LOOKUP_DIRECTORY;
 	if (!(flags & O_NOFOLLOW))
 		lookup_flags |= LOOKUP_FOLLOW;
+	if (flags & OPENAT2_EMPTY_PATH)
+		lookup_flags |= LOOKUP_EMPTY;
 
 	if (how->resolve & RESOLVE_NO_XDEV)
 		lookup_flags |= LOOKUP_NO_XDEV;
@@ -1362,7 +1361,7 @@ 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..d1bb87ff70e3 100644
--- a/include/linux/fcntl.h
+++ b/include/linux/fcntl.h
@@ -7,10 +7,13 @@
 
 /* List of all valid flags for the open/openat flags argument: */
 #define VALID_OPEN_FLAGS \
+	 /* lower 32-bit flags */ \
 	(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 | \
+	 /* upper 32-bit flags (openat2(2) only) */ \
+	 OPENAT2_EMPTY_PATH)
 
 /* List of all valid flags for the how->resolve argument: */
 #define VALID_RESOLVE_FLAGS \
diff --git a/include/uapi/linux/openat2.h b/include/uapi/linux/openat2.h
index a5feb7604948..c34f32e6fa96 100644
--- a/include/uapi/linux/openat2.h
+++ b/include/uapi/linux/openat2.h
@@ -40,4 +40,8 @@ struct open_how {
 					return -EAGAIN if that's not
 					possible. */
 
+/* openat2(2) exclusive flags are defined in the upper 32 bits of
+   open_how->flags  */
+#define OPENAT2_EMPTY_PATH	0x100000000 /* (1ULL << 32) */
+
 #endif /* _UAPI_LINUX_OPENAT2_H */
-- 
2.53.0