Re: [PATCH 16/26] perf tools: Synthesize modules with mmap3

[peterz]

On Wed, Sep 16, 2020 at 12:10:21PM -0300, Arnaldo Carvalho de Melo wrote:
> Em Wed, Sep 16, 2020 at 04:17:00PM +0200, peterz@xxxxxxxxxxxxx escreveu:
> > On Wed, Sep 16, 2020 at 11:07:44AM -0300, Arnaldo Carvalho de Melo wrote:
> > > Em Wed, Sep 16, 2020 at 10:20:18AM +0200, Jiri Olsa escreveu:
> > 
> > > > > IIRC BUILD_ID_SIZE is 20 bytes which is the correct size for SHA-1. A
> > > > > build ID may be 128-bits (16 bytes) if md5 or uuid hashes are used.
> > > > > Should this test just be "> 0" ?
> > > > 
> > > > ah right, will check on that
> > > 
> > > And how do you deal with this in the kernel? I.e. to inform userspace,
> > > via the PERF_RECORD_MMAP3 (or MMAP2 with that misc bit trick) the size
> > > of the build-id?
> > 
> > The union size is 24 bytes, so there's plenty space to store a length
> > field with the buildid.
> 
> So, I think we should instead use a bit in the misc field, stating the
> kind of build-id, so that we don't waste a byte for that, I think.

There's no wastage:

	u32 min, maj;
	u64 ino;
	u64 ino_generation;

is 24 bytes, buildit is 20 bytes, that leaves us 4 bytes to encode the
buildid type without growing anything.