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

[Vineet Gupta <vineet.gupta@linux.dev>]

On 9/28/26 3:42 PM, bot+bpf-ci@kernel.org wrote:
>> diff --git a/tools/testing/selftests/bpf/progs/linked_arena1.c b/tools/testing/selftests/bpf/progs/linked_arena1.c
>> index 3e8788dec1d9..dd9c5fa424dd 100644
>> --- a/tools/testing/selftests/bpf/progs/linked_arena1.c
>> +++ b/tools/testing/selftests/bpf/progs/linked_arena1.c
>> @@ -10,13 +10,33 @@ struct {
>>   	__uint(max_entries, 1); /* number of pages */
>>   } arena SEC(".maps");
>>
>> -long __arena a_val = 1;
>> +/*
>> + * Dereferencing an arena global needs the compiler to emit an
>> + * addr_space_cast, which only clang does. Keep the variables so the arena
>> + * is still populated and the skeleton still has its arena member, but skip
>> + * the test elsewhere.
>> + */
>> +#ifdef __BPF_FEATURE_ADDR_SPACE_CAST
>> +bool skip_tests __attribute((__section__(".data"))) = false;
>> +#else
>> +bool skip_tests = true;
>> +#endif
>> +
>> +long __arena_global a_val = 1;
>>   extern long __arena b_val; /* defined in linked_arena2.c */
>>
>>   SEC("syscall")
>>   int sum1(void *ctx)
>>   {
>> +#ifdef __BPF_FEATURE_ADDR_SPACE_CAST
>>   	return a_val + b_val;
>> +#else
>> +	/*
>> +	 * Reference the extern without dereferencing it, so that the relink
>> +	 * test still has an extern to resolve.
>> +	 */
>> +	return (long)&b_val;
>> +#endif
>>   }
> Does the relink_arena test exercise externs in allocated sections under
> bpf-gcc as intended?
>
> The commit message explains that __arena is a pointee annotation and
> that placement comes from __arena_global. The definition a_val was
> switched to __arena_global, but the extern declaration b_val still uses
> __arena.
>
> Under gcc, __arena expands to __attribute__((btf_type_tag("arena")))
> with no section, so b_val has no explicit section. GCC does not put an
> extern that has no explicit section into any BTF DATASEC (GCC
> PR112849), which means under bpf-gcc b_val is a free-floating extern
> VAR, not an entry of the .addr_space.1 DATASEC.
>
> In tools/lib/bpf/linker.c, find_glob_sym_btf() returns btf_sec_id = 0
> for it. linker_append_elf_sym() never takes the branch for externs in a
> real allocated section:
>
>      if (sym_is_extern && btf_sec_id) {
>          ...
>          if (strcmp(sec_name, BTF_EXTERN_SEC) != 0) {
>              src_sec = find_src_sec_by_name(obj, sec_name);
>
> That branch is what d07d3efd0141 ("libbpf: Fix static linking of
> externs placed in allocated sections") fixed and what 34354db1b148 is
> meant to cover. The check in test_relink() only walks the .addr_space.1
> 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");
>
> Under gcc that DATASEC never holds an extern, so the check cannot fail.
> The reported "linked_externs/relink_arena:OK" for bpf-gcc would hold
> with or without the linker fix.
>
> By contrast, linked_maps1.c declares its extern with an explicit section
> (extern map2_t map2 SEC(".maps");), so relink_maps covers the
> allocated-section path under both compilers.
>
> Declaring b_val with __arena_global would do the same here: under gcc
> that is SEC(".addr_space.1"), and the bpf_gcc flavor builds with
> -Wno-attributes, so the ignored 'used' on an extern is silent; under
> clang it is address_space(1), same as today, because clang ignores
> btf_type_tag on non-pointer types.

Thanks. Fixed in v2.

I had only checked that b_val survived as an ELF UND symbol and missed
that it was not in the DATASEC test_relink() walks:

   [23] DATASEC '.addr_space.1' size=0 vlen=1
           type_id=18 offset=0 size=8 (VAR 'a_val')
   [18] VAR 'b_val' type_id=11, linkage=extern     <- not in the datasec

With __arena_global on the extern declarations it matches clang:

   [24] DATASEC '.addr_space.1' size=0 vlen=2
           type_id=18 offset=0 size=8 (VAR 'b_val')
           type_id=19 offset=0 size=8 (VAR 'a_val')
   [18] VAR 'b_val' type_id=13, linkage=extern

One correction: the 'used' that SEC() adds does not keep the extern
alive. With __arena_global on the extern but nothing referencing it,
bpf-gcc still drops it and .addr_space.1 goes back to vlen=1 -- 'used'
is ignored on a declaration, which is what -Wno-attributes is hiding
here. So v2 needs both halves: __arena_global for the placement, and the

     return (long)&b_val;

in the !__BPF_FEATURE_ADDR_SPACE_CAST body so the extern is still
referenced. Dropping either one and relink_arena goes back to passing
pointlessly under bpf-gcc.

Thanks,
-Vineet