[PATCH bpf-next v2 03/13] compiler_types: Add __arena and __nullable BTF type tag qualifiers

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

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.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 include/linux/compiler_types.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index c5921f139007..e97ee2d4d1df 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -51,6 +51,8 @@
 # define __iomem	__attribute__((noderef, address_space(__iomem)))
 # define __percpu	__attribute__((noderef, address_space(__percpu)))
 # define __rcu		__attribute__((noderef, address_space(__rcu)))
+# define __arena
+# define __nullable
 static inline void __chk_user_ptr(const volatile void __user *ptr) { }
 static inline void __chk_io_ptr(const volatile void __iomem *ptr) { }
 /* other */
@@ -70,6 +72,14 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { }
 # define __iomem
 # define __percpu	__percpu_qual BTF_TYPE_TAG(percpu)
 # define __rcu		BTF_TYPE_TAG(rcu)
+/*
+ * 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 kernel struct members; to kernel
+ * code the pointers are ordinary.
+ */
+# define __arena	BTF_TYPE_TAG(arena)
+# define __nullable	BTF_TYPE_TAG(nullable)
 
 # define __chk_user_ptr(x)	(void)0
 # define __chk_io_ptr(x)	(void)0
-- 
2.53.0-Meta