Re: [PATCH] libbpf: extending BTF_KIND_INIT to accommodate some unusual types

[Alan Maguire]

On 23/04/2024 14:15, Xin Liu wrote:
> On Mon, 22 Apr 2024 10:43:38 -0700 Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote:
> 
>> On Mon, Apr 22, 2024 at 7:46 AM Xin Liu <liuxin350@xxxxxxxxxx> wrote:
>>>
>>> In btf__add_int, the size of the new btf_kind_int type is limited.
>>> When the size is greater than 16, btf__add_int fails to be added
>>> and -EINVAL is returned. This is usually effective.
>>>
>>> However, when the built-in type __builtin_aarch64_simd_xi in the
>>> NEON instruction is used in the code in the arm64 system, the value
>>> of DW_AT_byte_size is 64. This causes btf__add_int to fail to
>>> properly add btf information to it.
>>>
>>> like this:
>>>   ...
>>>    <1><cf>: Abbrev Number: 2 (DW_TAG_base_type)
>>>     <d0>   DW_AT_byte_size   : 64              // over max size 16
>>>     <d1>   DW_AT_encoding    : 5        (signed)
>>>     <d2>   DW_AT_name        : (indirect string, offset: 0x53): __builtin_aarch64_simd_xi
>>>    <1><d6>: Abbrev Number: 0
>>>   ...
>>>
>>> An easier way to solve this problem is to treat it as a base type
>>> and set byte_size to 64. This patch is modified along these lines.
>>>
>>> Fixes: 4a3b33f8579a ("libbpf: Add BTF writing APIs")
>>> Signed-off-by: Xin Liu <liuxin350@xxxxxxxxxx>
>>> ---
>>>  tools/lib/bpf/btf.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
>>> index 2d0840ef599a..0af121293b65 100644
>>> --- a/tools/lib/bpf/btf.c
>>> +++ b/tools/lib/bpf/btf.c
>>> @@ -1934,7 +1934,7 @@ int btf__add_int(struct btf *btf, const char *name, size_t byte_sz, int encoding
>>>         if (!name || !name[0])
>>>                 return libbpf_err(-EINVAL);
>>>         /* byte_sz must be power of 2 */
>>> -       if (!byte_sz || (byte_sz & (byte_sz - 1)) || byte_sz > 16)
>>> +       if (!byte_sz || (byte_sz & (byte_sz - 1)) || byte_sz > 64)
>>
>>
>> maybe we should just remove byte_sz upper limit? We can probably
>> imagine 256-byte integers at some point, so why bother artificially
>> restricting it?
>>
>> pw-bot: cr
> 
> In the current definition of btf_kind_int, bits has only 8 bits, followed
> by 8 bits of unused interval. When we expand, we should only use 16 bits
> at most, so the maximum value should be 8192(1 << 16 / 8), directly removing
> the limit of byte_sz. It may not fit the current design. For INT type btfs
> greater than 255, how to dump is still a challenge.
> 
> Does the current version support a maximum of 8192 bytes?
> 

Presuming we expanded BTF_INT_BITS() as per

-#define BTF_INT_BITS(VAL)       ((VAL)  & 0x000000ff)
+#define BTF_INT_BITS(VAL)       ((VAL)  & 0x0000ffff)

..as you say we'd be able to represent a 65535-bit value. So if we
preserve the power-of-two restriction on byte sizes, we'd have to choose
between either having ints which

- have a byte_sz maximum of <= 4096 bytes, with all 32768 bits usable; or
- have a byte_sz maximum of <= 8192 bytes, with 65535 out of 65536 bits
usable

The first option seems more intuitive to me.

In terms of dumping, we could probably just dump a hex representation of
the relevant bytes.

>>
>>>                 return libbpf_err(-EINVAL);
>>>         if (encoding & ~(BTF_INT_SIGNED | BTF_INT_CHAR | BTF_INT_BOOL))
>>>                 return libbpf_err(-EINVAL);
>>> --
>>> 2.33.0
>>>
>>
>