On 2019-12-16, Florian Weimer [off-list ref] wrote:

quoted

diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index 1d338357df8a..58c3a0e543c6 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h

@@ -93,5 +93,40 @@
 
 #define AT_RECURSIVE		0x8000	/* Apply to the entire subtree */
 
+/*
+ * Arguments for how openat2(2) should open the target path. If @resolve is
+ * zero, then openat2(2) operates very similarly to openat(2).
+ *
+ * However, unlike openat(2), unknown bits in @flags result in -EINVAL rather
+ * than being silently ignored. @mode must be zero unless one of {O_CREAT,
+ * O_TMPFILE} are set.
+ *
+ * @flags: O_* flags.
+ * @mode: O_CREAT/O_TMPFILE file mode.
+ * @resolve: RESOLVE_* flags.
+ */
+struct open_how {
+	__aligned_u64 flags;
+	__u16 mode;
+	__u16 __padding[3]; /* must be zeroed */
+	__aligned_u64 resolve;
+};
+
+#define OPEN_HOW_SIZE_VER0	24 /* sizeof first published struct */
+#define OPEN_HOW_SIZE_LATEST	OPEN_HOW_SIZE_VER0
+
+/* how->resolve flags for openat2(2). */
+#define RESOLVE_NO_XDEV		0x01 /* Block mount-point crossings
+					(includes bind-mounts). */
+#define RESOLVE_NO_MAGICLINKS	0x02 /* Block traversal through procfs-style
+					"magic-links". */
+#define RESOLVE_NO_SYMLINKS	0x04 /* Block traversal through all symlinks
+					(implies OEXT_NO_MAGICLINKS) */
+#define RESOLVE_BENEATH		0x08 /* Block "lexical" trickery like
+					"..", symlinks, and absolute
+					paths which escape the dirfd. */
+#define RESOLVE_IN_ROOT		0x10 /* Make all jumps to "/" and ".."
+					be scoped inside the dirfd
+					(similar to chroot(2)). */
 
 #endif /* _UAPI_LINUX_FCNTL_H */

Would it be possible to move these to a new UAPI header?

In glibc, we currently do not #include <linux/fcntl.h>.  We need some of
the AT_* constants in POSIX mode, and the header is not necessarily
namespace-clean.  If there was a separate header for openat2 support, we
could use that easily, and we would only have to maintain the baseline
definitions (which never change).

Sure, (assuming nobody objects) I can move it to "linux/openat2.h".

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>