Re: [PATCH 03/24] perf: Add build id data in mmap2 event

[Peter Zijlstra]

On Mon, Nov 09, 2020 at 10:53:54PM +0100, Jiri Olsa wrote:
> Adding support to carry build id data in mmap2 event.
> 
> The build id data replaces maj/min/ino/ino_generation
> fields, whichc are also used to identify map's binary,
> so it's ok to replace them with build id data:
> 
>   union {
>           struct {
>                   u32       maj;
>                   u32       min;
>                   u64       ino;
>                   u64       ino_generation;
>           };
>           struct {
>                   u8        build_id[20];
>                   u8        build_id_size;

What's the purpose of a size field for a fixed size array? Also, I'd
flip the order of these fields, first have the size and then the array.

>                   u8        __reserved_1;
>                   u16       __reserved_2;
>           };
>   };
> 
> Replaced maj/min/ino/ino_generation fields give us size
> of 24 bytes. We use 20 bytes for build id data, 1 byte
> for size and rest is unused.