Re: [PATCH bpf-next v3 2/9] selftests/bpf: Add test for scalar/arena pointer addition

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Fri Apr 3, 2026 at 4:25 PM EDT, Alexei Starovoitov wrote:
> On Thu, Apr 2, 2026 at 9:27 PM Emil Tsalapatis <emil@etsalapatis.com> wrote:
>>
>> From: Emil Tsalapatis <etsal@meta.com>
>>
>> Add a selftest that ensures scalar += ptr_to_arena instructions
>> are accepted by the verifier.
>>
>> Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
>> ---
>>  .../selftests/bpf/progs/verifier_arena.c      | 29 +++++++++++++++++++
>>  1 file changed, 29 insertions(+)
>>
>> diff --git a/tools/testing/selftests/bpf/progs/verifier_arena.c b/tools/testing/selftests/bpf/progs/verifier_arena.c
>> index c4b8daac4388..450e8f1181cd 100644
>> --- a/tools/testing/selftests/bpf/progs/verifier_arena.c
>> +++ b/tools/testing/selftests/bpf/progs/verifier_arena.c
>> @@ -477,4 +477,33 @@ int arena_kfuncs_under_bpf_lock(void *ctx)
>>
>>         return 0;
>>  }
>> +/*
>> + * Test that scalar += PTR_TO_ARENA correctly upgrades the
>> + * destination register to a PTR_TO_ARENA.
>> + */
>> +SEC("syscall")
>> +__success __retval(0)
>> +int scalar_add_arena_ptr(void *ctx)
>> +{
>> +#if defined(__BPF_FEATURE_ADDR_SPACE_CAST)
>
> I think the #if can be removed, since the whole test is in asm?

With Clang 18 the assembly fails to load for me with "Invalid operand for
instruction". AFAICT this is because addr_space_cast is only recognized as a
valid instruction from the BPF backend starting Clang >= 19, which is
also the first version that defines the feature flag.

>
>> +       int __arena *result, *arena_ptr;
>> +
>> +       /* Needed for the verifier to link the arena to the subprog. */
>> +       volatile char __arena *base = arena_base(&arena);
>> +
>> +       /* Passing base here to avoid optimizing it out during compilation. */
>> +       asm volatile (
>> +               "%[arena_ptr] = 8192;"
>> +               "%[arena_ptr] = addr_space_cast(%[arena_ptr], 0x0, 0x1);"
>> +               "%[result] = 12;"
>> +               "%[result] += %[arena_ptr];"
>> +               : [result] "=r"(result),
>> +                 [arena_ptr] "=&r"(arena_ptr)
>> +               : "r"(base)
>> +               :
>> +       );
>> +#endif
>> +       return 0;
>> +}
>> +
>>  char _license[] SEC("license") = "GPL";
>> --
>> 2.53.0
>>