bpf: define KF_ARENA_* flags for bpf_arena kfuncs

bpf_arena_alloc_pages() and bpf_arena_free_pages() work with the
bpf_arena pointers [1], which is indicated by the __arena macro in the
kernel source code:

    #define __arena __attribute__((address_space(1)))

However currently this information is absent from the debug data in
the vmlinux binary. As a consequence, bpf_arena_* kfuncs declarations
in vmlinux.h (produced by bpftool) do not match prototypes expected by
the BPF programs attempting to use these functions.

Introduce a set of kfunc flags to mark relevant types as bpf_arena
pointers. The flags then can be detected by pahole when generating BTF
from vmlinux's DWARF, allowing it to emit corresponding BTF type tags
for the marked kfuncs.

With recently proposed BTF extension [2], these type tags will be
processed by bpftool when dumping vmlinux.h, and corresponding
compiler attributes will be added to the declarations.

[1] https://lwn.net/Articles/961594/
[2] https://lore.kernel.org/bpf/20250130201239.1429648-1-ihor.solodrai@linux.dev/

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20250206003148.2308659-1-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/include/linux/btf.h b/include/linux/btf.h
index 2a08a2b55592eedc09188052c1b06dfa6688f241..ebc0c0c9b9446581685e20b0ea6a608e6468b42d 100644 (file)
--- a/include/linux/btf.h
+++ b/include/linux/btf.h
@@ -76,6 +76,9 @@
 #define KF_ITER_DESTROY (1 << 10) /* kfunc implements BPF iter destructor */
 #define KF_RCU_PROTECTED (1 << 11) /* kfunc should be protected by rcu cs when they are invoked */
 #define KF_FASTCALL     (1 << 12) /* kfunc supports bpf_fastcall protocol */
+#define KF_ARENA_RET    (1 << 13) /* kfunc returns an arena pointer */
+#define KF_ARENA_ARG1   (1 << 14) /* kfunc takes an arena pointer as its first argument */
+#define KF_ARENA_ARG2   (1 << 15) /* kfunc takes an arena pointer as its second argument */
 
 /*
  * Tag marking a kernel function as a kfunc. This is meant to minimize the

diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
index 870aeb51d70add0b1fb238a0d91acd7baea171af..0975d7f225449b65f7f9b32456224c353b8f5cde 100644 (file)
--- a/kernel/bpf/arena.c
+++ b/kernel/bpf/arena.c
@@ -577,8 +577,8 @@ __bpf_kfunc void bpf_arena_free_pages(void *p__map, void *ptr__ign, u32 page_cnt
 __bpf_kfunc_end_defs();
 
 BTF_KFUNCS_START(arena_kfuncs)
-BTF_ID_FLAGS(func, bpf_arena_alloc_pages, KF_TRUSTED_ARGS | KF_SLEEPABLE)
-BTF_ID_FLAGS(func, bpf_arena_free_pages, KF_TRUSTED_ARGS | KF_SLEEPABLE)
+BTF_ID_FLAGS(func, bpf_arena_alloc_pages, KF_TRUSTED_ARGS | KF_SLEEPABLE | KF_ARENA_RET | KF_ARENA_ARG2)
+BTF_ID_FLAGS(func, bpf_arena_free_pages, KF_TRUSTED_ARGS | KF_SLEEPABLE | KF_ARENA_ARG2)
 BTF_KFUNCS_END(arena_kfuncs)
 
 static const struct btf_kfunc_id_set common_kfunc_set = {