Re: [RFC] perf record: missing buildid for callstack modules

[Stephane Eranian]

On Fri, Jan 15, 2016 at 1:34 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Thu, Jan 14, 2016 at 05:59:48PM -0800, Stephane Eranian wrote:
>> Peter,
>>
>> On Thu, Jan 14, 2016 at 3:36 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>> > On Thu, Jan 14, 2016 at 12:27:34PM +0100, Ingo Molnar wrote:
>> >> > +    *      u32                             filename_len;
>> >> > +    *      char                            filename[2+];
>> >
>> >> Acked-by: Ingo Molnar <mingo@xxxxxxxxxx>
>> >
>> > except of course that sizeof(u32) == 4 :/
>
>> There is no padding here. Are you concerned about running out of bits
>> in filename_len?
>
> No, I just made a mess of it :-)
>
> filename_len should have been u16 and filename should then be 6+8n in
> size.
>
why don't you make it more explicit:
   u16 filename_len
   u16 extra_len

then it would be clear what is what, no field with dual meaning.
Now, that assumes that no pathname can be longer than 65535 bytes.

>> Any extension possible because header.size - sizeof(mmap3) -
>> filename_len sizing what's after filename, right?
>
> Right, current MMAP records use the remaining size as the filename
> length, but by explicitly specifying that we can add optional fields.
>
> These fields must be after filename_len, otherwise you'd not be able to
> find filename_len and you could not compute the extra size. And given
> alignment constraints it makes sense to do it after filename[].
>
> So suppose we wanted to also add atime and ctime, we could do.
>
>   PERF_RECORD_MMAP3 {
>     ...
>     u16 filename_len;
>     char filename[6+8n];
>
>     if (extra_size >= 16) {
>       u64 stime;
>       u64 ctime;
>     };
>   }
>
> or something like that.