[PATCH bpf-next v4 14/16] libbpf: Render the arena type tag with its address space

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

A kernel struct member tagged __arena reaches a program built against
vmlinux.h as a pointer in the kernel address space with the tag as an
attribute, struct scx_cmask __attribute__((btf_type_tag("arena"))) *cmask,
so the program casts it to the arena address space before dereferencing
it, although the verifier already hands it an arena pointer.

The tag denotes a pointer into the BPF arena, address_space(1) to the
compiler, so render it as such: __attribute__((address_space(1)))
followed by the tag attribute as before, which keeps the dump faithful
to the BTF. A program then reads the member as an arena pointer and
hands it to anything declared with its own __arena without a cast.

Nothing else changes for consumers of the C dump. Skeletons strip the
modifiers of variable types and never carried the tag. Clang without
casts between address spaces still accepts the declaration and only a
program touching such a member would need a cast there, as it did for
every arena pointer before. GCC has no address spaces for BPF and warns,
under -Wattributes, that it ignores the attribute, which the selftests
build already silences.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 tools/lib/bpf/btf_dump.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
index 996e61b5f11f..7dedd3975c39 100644
--- a/tools/lib/bpf/btf_dump.c
+++ b/tools/lib/bpf/btf_dump.c
@@ -1595,6 +1595,9 @@ static void btf_dump_emit_type_chain(struct btf_dump *d,
 			name = btf_name_of(d, t->name_off);
 			if (btf_kflag(t))
 				btf_dump_printf(d, " __attribute__((%s))", name);
+			else if (strcmp(name, "arena") == 0)
+				/* the tag denotes a pointer into the BPF arena address space */
+				btf_dump_printf(d, " __attribute__((address_space(1))) __attribute__((btf_type_tag(\"arena\")))");
 			else
 				btf_dump_printf(d, " __attribute__((btf_type_tag(\"%s\")))", name);
 			break;
-- 
2.53.0-Meta