bpf: Add the __arena and __nullable BTF type tags

Kernel structures increasingly want to publish pointers into a BPF arena
to the BPF programs they are shared with, and some pointer members that
reach programs as trusted are legitimately NULL at times. Neither can be
expressed today: a pointer member reaches the program as an ordinary
kernel pointer, and sched_ext resorts to a plain u64 holding a
pre-translated arena address that programs cast before use.

Define __arena and __nullable as BTF type tags, the way __user, __rcu
and __percpu already are, so that tagged members survive into vmlinux
and module BTF and the verifier can act on them. To kernel code and to
sparse the qualifiers are empty: the pointers are ordinary kernel
pointers. The verifier side follows in the next patches.

The tags live in a header of their own, bpf_compiler_types.h, rather
than in compiler_types.h. They mean something to the BPF verifier only,
and only the few structures shared with BPF programs that want to
publish such a pointer carry them, so the code that declares such a
structure includes the header and the rest of the kernel never sees the
qualifiers.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261019132203.3034698-6-memxor@gmail.com

include/linux/bpf_compiler_types.h

@@ -0,0 +1,21 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __LINUX_BPF_COMPILER_TYPES_H
+#define __LINUX_BPF_COMPILER_TYPES_H
+
+#include <linux/compiler_types.h>
+
+/*
+ * Qualifiers for the BPF verifier rather than address spaces: __arena marks a
+ * pointer into a BPF arena, __nullable a pointer that may be NULL. They reach
+ * BPF programs as BTF type tags on the members of kernel structs shared with
+ * them; to kernel code and to sparse the pointers are ordinary.
+ */
+#ifdef __CHECKER__
+# define __arena
+# define __nullable
+#else
+# define __arena	BTF_TYPE_TAG(arena)
+# define __nullable	BTF_TYPE_TAG(nullable)
+#endif
+
+#endif /* __LINUX_BPF_COMPILER_TYPES_H */