[PATCH 01/14] VFS: Add additional RESOLVE_* flags [ver #18]

[David Howells <dhowells@redhat.com>]

Add additional RESOLVE_* flags to correspond to AT_* flags that aren't
currently implemented:

	RESOLVE_NO_TRAILING_SYMLINKS    for AT_SYMLINK_NOFOLLOW
	RESOLVE_NO_TRAILING_AUTOMOUNTS  for AT_NO_AUTOMOUNT
	RESOLVE_EMPTY_PATH              for AT_EMPTY_PATH

This is necessary for fsinfo() to use RESOLVE_* flags instead of AT_* flags
if the latter are to be considered deprecated for new system calls.

Also make openat2() handle RESOLVE_NO_TRAILING_SYMLINKS.

Automounting is currently forced by doing an open(), so adding support to
openat2() for RESOLVE_NO_TRAILING_AUTOMOUNTS is not trivial.

Reported-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Aleksa Sarai <cyphar@cyphar.com>
---

 fs/open.c                    |    8 +++++---
 include/linux/fcntl.h        |    3 ++-
 include/uapi/linux/openat2.h |    8 +++++++-
 3 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/fs/open.c b/fs/open.c
index 0788b3715731..7c38a7605c21 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -977,7 +977,7 @@ inline struct open_how build_open_how(int flags, umode_t mode)
 inline int build_open_flags(const struct open_how *how, struct open_flags *op)
 {
 	int flags = how->flags;
-	int lookup_flags = 0;
+	int lookup_flags = LOOKUP_FOLLOW | LOOKUP_AUTOMOUNT;
 	int acc_mode = ACC_MODE(flags);
 
 	/* Must never be set by userspace */
@@ -1055,8 +1055,8 @@ inline int build_open_flags(const struct open_how *how, struct open_flags *op)
 
 	if (flags & O_DIRECTORY)
 		lookup_flags |= LOOKUP_DIRECTORY;
-	if (!(flags & O_NOFOLLOW))
-		lookup_flags |= LOOKUP_FOLLOW;
+	if (flags & O_NOFOLLOW)
+		lookup_flags &= ~LOOKUP_FOLLOW;
 
 	if (how->resolve & RESOLVE_NO_XDEV)
 		lookup_flags |= LOOKUP_NO_XDEV;
@@ -1068,6 +1068,8 @@ inline int build_open_flags(const struct open_how *how, struct open_flags *op)
 		lookup_flags |= LOOKUP_BENEATH;
 	if (how->resolve & RESOLVE_IN_ROOT)
 		lookup_flags |= LOOKUP_IN_ROOT;
+	if (how->resolve & RESOLVE_NO_TRAILING_SYMLINKS)
+		lookup_flags &= ~LOOKUP_FOLLOW;
 
 	op->lookup_flags = lookup_flags;
 	return 0;
diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h
index 7bcdcf4f6ab2..eacf17a8ca34 100644
--- a/include/linux/fcntl.h
+++ b/include/linux/fcntl.h
@@ -19,7 +19,8 @@
 /* List of all valid flags for the how->resolve argument: */
 #define VALID_RESOLVE_FLAGS \
 	(RESOLVE_NO_XDEV | RESOLVE_NO_MAGICLINKS | RESOLVE_NO_SYMLINKS | \
-	 RESOLVE_BENEATH | RESOLVE_IN_ROOT)
+	 RESOLVE_BENEATH | RESOLVE_IN_ROOT | RESOLVE_NO_TRAILING_SYMLINKS | \
+	 RESOLVE_NO_TRAILING_AUTOMOUNTS | RESOLVE_EMPTY_PATH)
 
 /* List of all open_how "versions". */
 #define OPEN_HOW_SIZE_VER0	24 /* sizeof first published struct */
diff --git a/include/uapi/linux/openat2.h b/include/uapi/linux/openat2.h
index 58b1eb711360..2647a108f116 100644
--- a/include/uapi/linux/openat2.h
+++ b/include/uapi/linux/openat2.h
@@ -22,7 +22,10 @@ struct open_how {
 	__u64 resolve;
 };
 
-/* how->resolve flags for openat2(2). */
+/*
+ * Path resolution paths to replace AT_* paths in all new syscalls that would
+ * use them.
+ */
 #define RESOLVE_NO_XDEV		0x01 /* Block mount-point crossings
 					(includes bind-mounts). */
 #define RESOLVE_NO_MAGICLINKS	0x02 /* Block traversal through procfs-style
@@ -35,5 +38,8 @@ struct open_how {
 #define RESOLVE_IN_ROOT		0x10 /* Make all jumps to "/" and ".."
 					be scoped inside the dirfd
 					(similar to chroot(2)). */
+#define RESOLVE_NO_TRAILING_SYMLINKS	0x20 /* Don't follow trailing symlinks in the path */
+#define RESOLVE_NO_TRAILING_AUTOMOUNTS	0x40 /* Don't follow trailing automounts in the path */
+#define RESOLVE_EMPTY_PATH	0x80	/* Permit a path of "" to indicate the dfd exactly */
 
 #endif /* _UAPI_LINUX_OPENAT2_H */