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Subject[RFC][PATCH] Mark AT_* path flags as deprecated and add missing RESOLVE_ flags
Date
Do we want to do this?  Or should we duplicate the RESOLVE_* flags to AT_*
flags so that existing *at() syscalls can make use of them?

David
---
commit 448731bf3b29f2b1f7c969d7efe1f0673ae13b5e
Author: David Howells <dhowells@redhat.com>
Date: Thu Mar 5 17:40:02 2020 +0000

Mark AT_* flags as deprecated and add missing RESOLVE_ flags

It has been suggested that new path-using system calls should use RESOLVE_*
flags instead of AT_* flags, but the RESOLVE_* flag functions are not a
superset of the AT_* flag functions. So formalise this by:

(1) In linux/fcntl.h, add a comment noting that the AT_* flags are
deprecated for new system calls and that RESOLVE_* flags should be
used instead.

(2) Add some missing flags:

RESOLVE_NO_TERMINAL_SYMLINKS for AT_SYMLINK_NOFOLLOW
RESOLVE_NO_TERMINAL_AUTOMOUNTS for AT_NO_AUTOMOUNT
RESOLVE_EMPTY_PATH for AT_EMPTY_PATH

(3) Make openat2() support RESOLVE_NO_TERMINAL_SYMLINKS. LOOKUP_OPEN
internally implies LOOKUP_AUTOMOUNT, and AT_EMPTY_PATH is probably not
worth supporting (maybe use dup2() instead?).

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

diff --git a/fs/open.c b/fs/open.c
index 0788b3715731..6946ad09b42b 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_TERMINAL_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..fd6ee34cce0f 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_TERMINAL_SYMLINKS | \
+ RESOLVE_NO_TERMINAL_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/fcntl.h b/include/uapi/linux/fcntl.h
index ca88b7bce553..9f5432dbd213 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -84,9 +84,20 @@
#define DN_ATTRIB 0x00000020 /* File changed attibutes */
#define DN_MULTISHOT 0x80000000 /* Don't remove notifier */

-#define AT_FDCWD -100 /* Special value used to indicate
- openat should use the current
- working directory. */
+
+/*
+ * Special dfd/dirfd file descriptor value used to indicate that *at() system
+ * calls should use the current working directory for relative paths.
+ */
+#define AT_FDCWD -100
+
+/*
+ * Pathwalk control flags, used for the *at() syscalls. These should be
+ * considered deprecated and should not be used for new system calls. The
+ * RESOLVE_* flags in <linux/openat2.h> should be used instead.
+ *
+ * There are also system call-specific flags here (REMOVEDIR, STATX, RECURSIVE).
+ */
#define AT_SYMLINK_NOFOLLOW 0x100 /* Do not follow symbolic links. */
#define AT_REMOVEDIR 0x200 /* Remove directory instead of
unlinking file. */
diff --git a/include/uapi/linux/openat2.h b/include/uapi/linux/openat2.h
index 58b1eb711360..bb8d0a7f82c2 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_TERMINAL_SYMLINKS 0x20 /* Don't follow terminal symlinks in the path */
+#define RESOLVE_NO_TERMINAL_AUTOMOUNTS 0x40 /* Don't follow terminal automounts in the path */
+#define RESOLVE_EMPTY_PATH 0x80 /* Permit a path of "" to indicate the dfd exactly */

#endif /* _UAPI_LINUX_OPENAT2_H */
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