libbpf: Render the arena type tag with its address space

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>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261019132203.3034698-15-memxor@gmail.com

0 files changed, 0 insertions, 0 deletions