Re: [PATCH] pickles: Improvements to btf.pk

[David Faust]

On 2/14/21 9:09 AM, Jose E. Marchesi wrote:
> 
>>> +type BTF_Section =
>>> +  struct
>>> +  {
>>> +    BTF_Header header;
>>> +    BTF_Type[header.type_len] types;
>>> +    string[header.str_len] strings;
>>> +
>>> +    /* Given an offset into the BTF strings section, return the string.  */
>>> +
>>> +    method get_string = (offset<uint<32>,B> off) string:
>>> +      {
>>> +        return string @ strings'offset + off;
>>> +      }
>>> +  };
>>
>> Now that I think of it, arent the location of `types' and `strings'
>> determined by the header as an offset relative to the end of the header?
>> Something like this:
>>
>> type BTF_Section =
>>   struct
>>   {
>>     BTF_Header header;
>>     BTF_Type[header.type_len] types @ header'size + header.type_off;
>>     string[header.str_len] strings @ header'size + header.str_off;
>>     ...
>>   };
>>
>> Or, alternatively, this may be more clear but less compact:
>>
>> type BTF_Section =
>>   struct
>>   {
>>     BTF_Header header;
>>
>>     var type_off = OFFSET + header.type_off;
>>     var str_off = OFFSET + header.str_off;
>>
>>     BTF_Type[header.type_len] types @ type_off;
>>     string[header.str_len] strings @ str_off;
>>     ...
>>   };
>>
>> Note how in a struct type definition OFFSET always holds the offset of
>> the end of the last mapped field, relative to the beginning of the
>> struct.
> 
> And, if we do like above, we don't need the `btf_types' and
> `btf_strings' functions anymore.  Just accessing section.types and
> section.strings would be enough.
> 

Ahh, thank you for pointing this out! I was not aware of the OFFSET
syntax for specifying things like this. I will fix this soon.

fwiw, I've never seen any valid BTF where:
- the type_off is not 0, AND
- the string_off is not equal to type_len

and I believe some existing tools expect that both of these are true,
even though it is not a requirement.

Of course it will be better to not rely on this though. :)