Re: [RFC PATCH bpf-next 2/7] bpf: Add struct argument info in btf_func_model

[Yonghong Song <yhs@fb.com>]

On 8/8/22 5:02 PM, Andrii Nakryiko wrote:
> On Tue, Jul 26, 2022 at 10:11 AM Yonghong Song <yhs@fb.com> wrote:
>>
>> Add struct argument information in btf_func_model and such information
>> will be used in arch specific function arch_prepare_bpf_trampoline()
>> to prepare argument access properly in trampoline.
>>
>> Signed-off-by: Yonghong Song <yhs@fb.com>
>> ---
>>   include/linux/bpf.h | 9 +++++++++
>>   1 file changed, 9 insertions(+)
>>
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index 20c26aed7896..173b42cf3940 100644
>> --- a/include/linux/bpf.h
>> +++ b/include/linux/bpf.h
>> @@ -726,10 +726,19 @@ enum bpf_cgroup_storage_type {
>>    */
>>   #define MAX_BPF_FUNC_REG_ARGS 5
>>
>> +/* The maximum number of struct arguments a single function may have. */
>> +#define MAX_BPF_FUNC_STRUCT_ARGS 2
>> +
>>   struct btf_func_model {
>>          u8 ret_size;
>>          u8 nr_args;
>>          u8 arg_size[MAX_BPF_FUNC_ARGS];
>> +       /* The struct_arg_idx should be in increasing order like (0, 2, ...).
>> +        * The struct_arg_bsize encodes the struct field byte size
>> +        * for the corresponding struct argument index.
>> +        */
>> +       u8 struct_arg_idx[MAX_BPF_FUNC_STRUCT_ARGS];
>> +       u8 struct_arg_bsize[MAX_BPF_FUNC_STRUCT_ARGS];
> 
> Few questions here. It might be a bad idea, but I thought I'd bring it
> up anyway.
> 
> So, is there any benefit into having these separate struct_arg_idx and
> struct_arg_bsize fields and then processing arg_size completely
> separate from struct_arg_idx/struct_arg_bsize in patch #4? Reading
> patch #4 it felt like it would be much easier to keep track of things
> if we had a single loop going over all the arguments, and then if some
> argument is a struct -- do some extra step to copy up to 16 bytes onto
> stack and store the pointer there (and skip up to one extra argument).
> And if it's not a struct arg -- do what we do right now.
> 
> What if instead we keep btf_func_mode definition as is, but for struct
> argument we add extra flag to arg_size[struct_arg_idx] value to mark
> that it is a struct argument. This limits arg_size to 128 bytes, but I
> think it's more than enough for both struct and non-struct cases,
> right? Distill function would make sure that nr_args matches number of
> logical arguments and not number of registers.
> 
> Would that work? Would that make anything harder to implement in
> arch-specific code? I don't see what, but I haven't grokked all the
> details of patch #4, so I'm sorry if I missed something obvious. The
> way I see it, it will make overall logic for saving/restoring
> registers more uniform, roughly:
> 
> for (int arg_idx = 0; arg_idx < model->arg_size; arg_idx++) {
>    if (arg & BTF_FMODEL_STRUCT_ARG) {
>      /* handle struct, calc extra registers "consumed" from
> arg_size[arg_idx] ~BTF_FMODEL_STRUCT_ARG */
>    } else {
>      /* just a normal register */
>    }
> }

Your suggestion sounds good to me. Yes, we already have
arg_size array. We can add a
	bool is_struct_arg[MAX_BPF_FUNC_ARGS];
to indicate whether the argument is a struct or not.
In this case, we can avoid duplication between
arg_size and struct_arg_bsize.

> 
> 
> If we do stick to current approach, though, let's please
> s/struct_arg_bsize/struct_arg_size/. Isn't arg_size also and already
> in bytes? It will keep naming and meaning consistent across struct and
> non-struct args.
> 
> BTW, by not having btf_func_model encode register indices in
> struct_arg_idx we keep btf_func_model pretty architecture-agnostic,
> right? It will be per each architecture specific implementation to
> perform mapping this *model* into actual registers used?
> 
> 
> 
> 
>>   };
>>
>>   /* Restore arguments before returning from trampoline to let original function
>> --
>> 2.30.2
>>