Re: [PATCH bpf-next v2] selftests/bpf: Fix linked_externs with bpf-gcc

[Andrii Nakryiko]

On Mon, Sep 28, 2026 at 5:01 PM Ihor Solodrai <ihor.solodrai@xxxxxxxxx> wrote:
>
> On 2026-09-28 4:14 p.m., Vineet Gupta wrote:
> > test_progs-bpf_gcc fails to build:
> >
> >    prog_tests/linked_externs.c:17:23: error: 'struct linked_arena' has no
> >        member named 'arena'
> >
> > Several problems, all from bpf-gcc not supporting address_space(1).
> >
> > First, the arena variables are declared with __arena, but that macro is
> > for pointers -- it marks the pointee address space. The one that carries
> > placement is __arena_global:
> >
> >    #if defined(__BPF_FEATURE_ADDR_SPACE_CAST) && !defined(BPF_ARENA_FORCE_ASM)
> >    #define __arena        __attribute__((address_space(1))) __attribute__((btf_type_tag("arena")))
> >    #define __arena_global __attribute__((address_space(1)))
> >    #else
> >    #define __arena        __attribute__((btf_type_tag("arena")))
> >    #define __arena_global SEC(".addr_space.1")
> >    #endif
> >
> > __BPF_FEATURE_ADDR_SPACE_CAST is a clang predefine. Under clang the two
> > are interchangeable here, since address_space(1) both places the
> > variable and is what btf_type_tag would have described. Under bpf-gcc
> > __arena is only a BTF type tag, so the definitions land in .data, the
> > arena map gets no initial value, and is_skel_data() in bpftool does not
> > emit the typed arena member -- hence the build error. Note this is
> > distinct from skel->maps.arena, which comes from SEC(".maps") and is
> > always present.
> >
> > The extern declarations need __arena_global too. Without a section, gcc
> > does not place an extern in any DATASEC, so it stays a free-floating
> > extern VAR rather than an entry of .addr_space.1. test_relink() only
> > walks that DATASEC:
> >
> >      id = btf__find_by_name_kind(btf, sec_name, BTF_KIND_DATASEC);
> >      ...
> >          ASSERT_NEQ(btf_var(t)->linkage, BTF_VAR_GLOBAL_EXTERN, "var_resolved");
> >
> > so relink_arena would report OK under bpf-gcc whether or not the linker
> > resolved anything -- it would not be testing the allocated-section path
> > this test exists for.
> >
> > Second, placement alone is not enough to run the test. Dereferencing an
> > arena global needs an addr_space_cast, which only clang emits:
> >
> >    clang:                                  bpf-gcc:
> >      r1 = 0x0 ll                             r1 = 0x0 ll
> >      r1 = addr_space_cast(r1, 0x0, 0x1)      r2 = *(u64 *)(r1 + 0x0)
> >      r1 = *(u64 *)(r1 + 0x0)
> >
> > so the verifier sees a scalar and rejects the program:
> >
> >    4: (79) r2 = *(u64 *)(r1 +0)
> >    R1 invalid mem access 'scalar'
> >
> > Guard the program bodies and skip the skeleton subtest, as
> > arena_atomics.c already does. The variables stay unconditional so the
> > arena is still populated and the skeleton still has its member. The
> > fallback bodies reference the externs without dereferencing them:
> > 'used' is ignored on an extern, so an unreferenced one is dropped and
> > .addr_space.1 loses it again.
> >
> > With this, bpf-gcc reports:
> >
> >    #217/1   linked_externs/skel_arena:SKIP
> >    #217/2   linked_externs/relink_arena:OK
> >    #217/3   linked_externs/relink_maps:OK
> >
> > with b_val present in .addr_space.1 as linkage=extern, same as clang, so
> > relink_arena covers the same path in both. clang still passes all three.
> >
> > Fixes: 34354db1b148 ("selftests/bpf: Add linked_externs test for externs in allocated sections")
> > Signed-off-by: Vineet Gupta <vineet.gupta@xxxxxxxxx>
>
> Tested-by: Ihor Solodrai <ihor.solodrai@xxxxxxxxx>
>

thanks for the fix and testing, my bad with __arena_global and GCC-BPF
build failure. applied to bpf-next, one step closer to green CI!

> This fixes the current GCC BPF selftests build failure on CI:
> https://github.com/kernel-patches/vmtest/actions/runs/36498905340/job/109187331070
>
> nit: The commit message is kinda too long. If you'd like it to be read
> by a human, try making it shorter :)
>
> Thanks!
>
> > [...]