[PATCH bpf-next v4 16/17] bpftool: Render the arena type tag as __bpf_arena

[Ihor Solodrai <ihor.solodrai@linux.dev>]

Kernel structs shared with BPF programs mark pointers into a BPF arena
with __arena, which kernel BTF carries as a plain "arena" type tag
that the verifier relies on. btf_dump prints the tag as
__attribute__((btf_type_tag("arena"))) on a pointer in the default
address space, so a program built against vmlinux.h has to cast such a
pointer to the arena address space before using it.

Attach a kflagged decl tag to each "arena" type tag in the dump split,
so that btf_dump prints __bpf_arena instead. In the generated header:

  #define __bpf_arena __attribute__((address_space(1))) __attribute__((btf_type_tag("arena")))

So the program gets an arena pointer and its BTF keeps the tag the
kernel expects. Programs can pre-define the macro.

Use __bpf_arena to avoid conflicts with unguarded __arena definitions
in libarena and sched_ext headers.

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
 tools/bpf/bpftool/Documentation/bpftool-btf.rst            | 6 ++++++
 tools/bpf/bpftool/btf.c                                    | 5 +++++
 .../testing/selftests/bpf/bpftool_btf_dump_sorted.expected | 4 ++++
 .../selftests/bpf/bpftool_btf_dump_unsorted.expected       | 4 ++++
 .../selftests/bpf/prog_tests/test_struct_ops_arena.c       | 2 +-
 .../selftests/bpf/progs/struct_ops_arena_member_render.c   | 7 +++----
 6 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/tools/bpf/bpftool/Documentation/bpftool-btf.rst b/tools/bpf/bpftool/Documentation/bpftool-btf.rst
index a3778f449d73..9fe035b80103 100644
--- a/tools/bpf/bpftool/Documentation/bpftool-btf.rst
+++ b/tools/bpf/bpftool/Documentation/bpftool-btf.rst
@@ -123,6 +123,12 @@ a type marked ``__bpf_static_offset``, including through ``bpf_core_field_*``
 and the CO-RE bitfield helpers. Compile with ``-D__bpf_static_offset=`` to
 use these helpers with an affected compiler.
 
+Pointers carrying the ``arena`` type tag in kernel BTF are rendered with
+``__bpf_arena``. This macro expands to
+``__attribute__((address_space(1))) __attribute__((btf_type_tag("arena")))``,
+so programs can use the pointers without explicit casts. Define
+``__bpf_arena`` before including the header to override it.
+
 EXAMPLES
 ========
 **# bpftool btf dump id 1226**
diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
index 1329745c5854..c06b01f2c5b3 100644
--- a/tools/bpf/bpftool/btf.c
+++ b/tools/bpf/bpftool/btf.c
@@ -1103,6 +1103,8 @@ static const struct {
 } btf_c_type_tags[] = {
 	/* the arena attribute in older kernel BTF */
 	{ true, "address_space(1)", "__attribute__((address_space(1)))" },
+	/* __arena in include/linux/bpf_compiler_types.h */
+	{ false, "arena", "__bpf_arena" },
 };
 
 static int btf_c_annotate_type_tags(struct btf *btf)
@@ -1276,6 +1278,9 @@ static int dump_btf_c(struct btf *btf,
 	printf("#define __bpf_static_offset\n");
 	printf("#endif\n");
 	printf("#endif\n\n");
+	printf("#ifndef __bpf_arena\n");
+	printf("#define __bpf_arena __attribute__((address_space(1))) __attribute__((btf_type_tag(\"arena\")))\n");
+	printf("#endif\n\n");
 	printf("#ifndef __ksym\n");
 	printf("#define __ksym __attribute__((section(\".ksyms\")))\n");
 	printf("#endif\n\n");
diff --git a/tools/testing/selftests/bpf/bpftool_btf_dump_sorted.expected b/tools/testing/selftests/bpf/bpftool_btf_dump_sorted.expected
index 190cf16e49a2..0ecf53b29042 100644
--- a/tools/testing/selftests/bpf/bpftool_btf_dump_sorted.expected
+++ b/tools/testing/selftests/bpf/bpftool_btf_dump_sorted.expected
@@ -17,6 +17,10 @@
 #endif
 #endif
 
+#ifndef __bpf_arena
+#define __bpf_arena __attribute__((address_space(1))) __attribute__((btf_type_tag("arena")))
+#endif
+
 #ifndef __ksym
 #define __ksym __attribute__((section(".ksyms")))
 #endif
diff --git a/tools/testing/selftests/bpf/bpftool_btf_dump_unsorted.expected b/tools/testing/selftests/bpf/bpftool_btf_dump_unsorted.expected
index 1a0a7b016c50..24a8db96eab0 100644
--- a/tools/testing/selftests/bpf/bpftool_btf_dump_unsorted.expected
+++ b/tools/testing/selftests/bpf/bpftool_btf_dump_unsorted.expected
@@ -17,6 +17,10 @@
 #endif
 #endif
 
+#ifndef __bpf_arena
+#define __bpf_arena __attribute__((address_space(1))) __attribute__((btf_type_tag("arena")))
+#endif
+
 #ifndef __ksym
 #define __ksym __attribute__((section(".ksyms")))
 #endif
diff --git a/tools/testing/selftests/bpf/prog_tests/test_struct_ops_arena.c b/tools/testing/selftests/bpf/prog_tests/test_struct_ops_arena.c
index e7580b9f0fc2..2e915b350cd2 100644
--- a/tools/testing/selftests/bpf/prog_tests/test_struct_ops_arena.c
+++ b/tools/testing/selftests/bpf/prog_tests/test_struct_ops_arena.c
@@ -259,7 +259,7 @@ static void arena_autobind_excl(void)
 	struct_ops_arena_autobind__destroy(skel);
 }
 
-/* The same through the vmlinux.h rendering of the members, which takes casts. */
+/* The same through an older bpftool's vmlinux.h rendering of the members, which takes casts. */
 static void arena_member_render(void)
 {
 	struct struct_ops_arena_member_render *skel;
diff --git a/tools/testing/selftests/bpf/progs/struct_ops_arena_member_render.c b/tools/testing/selftests/bpf/progs/struct_ops_arena_member_render.c
index 253ffdd86d32..b6b66934266b 100644
--- a/tools/testing/selftests/bpf/progs/struct_ops_arena_member_render.c
+++ b/tools/testing/selftests/bpf/progs/struct_ops_arena_member_render.c
@@ -11,10 +11,9 @@
 char _license[] SEC("license") = "GPL";
 
 /*
- * The module types as vmlinux.h renders them: the tag is an attribute on a
- * pointer in the kernel address space, so the program has to cast the member
- * to the arena address space before using it. That is what a program built
- * against vmlinux.h sees for a kernel struct with an __arena member.
+ * The module types as an older bpftool renders them in vmlinux.h: the tag is
+ * an attribute on a pointer in the kernel address space, so the program has
+ * to cast the member to the arena address space before using it.
  */
 #define __arena_tag	__attribute__((btf_type_tag("arena")))
 #define __nullable_tag	__attribute__((btf_type_tag("nullable")))
-- 
2.56.0