Re: [RFC][PATCH 2/3] perf: Fix u16 overflows

[Peter Zijlstra <peterz@infradead.org>]

On Sat, Sep 12, 2015 at 10:11:20AM +0200, Ingo Molnar wrote:
> 
> * Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > Vince reported that its possible to overflow the various size fields
> > and get weird stuff if you stick too many events in a group.
> > 
> > Put a lid on this by requiring the fixed record size not exceed 16k.
> > This is still a fair amount of events (silly amount really) and leaves
> > plenty room for callchains and stack dwarves while also avoiding
> > overflowing the u16 variables.
> 
> Does this leave a natural ABI extension route here, in case in the future it 
> becomes a problem? We should take aside a value to mean 'larger record' or such?

So this all is a result of:

struct perf_event_header {
	__u32   type;
	__u16   misc;
	__u16   size;
};

And we've not even done the 'sensible' thing of interpreting @size as
@size*8 :/ That is, because entries must be u64 aligned, the lower 3
bits of @size will always be 0.

Now there are of course ways we can 'grow' if we really have to. One
would be to set aside a MISC bit to indicate we should do that *8 thing,
which would allow up to 512 Kb records.

That said, 64k is already quite a lot of data, and I'm not sure we
want to have records bigger than that. Certainly not for samples,
copying that much data on an interrupt is just not going to be fast.

And I'm not sure there's a sensible use-case for having this many events
in a group (and there's good reasons not to do it).

In any case, the patch only pokes at internal stuff, the ABI isn't
affected beyond refusing to create humongous groups.