Re: [DTrace-devel] [PATCH 2/8] Reduce stack depth if kernel returns NULL frames

Kris Van Hees
2024-08-28T20:17:27Z

On Wed, Aug 28, 2024 at 04:11:29PM -0400, Eugene Loh wrote:
> It's been a while, but if I remember correctly it's actually just the other
> way around.  That is, to make stack and depth consistent, we're forced into
> this "depth reduction" patch.  Put another way, the stack simply does not
> include NULL pointers -- there is no way to remove them.
>
> E.g., let's say we have a buffer of 8 pointers and we ask for the stack and
> get back:
>
>     0xdead 0xbeef 0xfeed 0xface NULL NULL NULL NULL
>
> Looks like 4 pointers.  But we don't count them.  We just use the return
> value from the helper function.  If it tells us "4", then everything is
> consistent.  But for some reason (I haven't looked at the code to figure out
> why), it can be 5 or even 6.  So this patch bumps that value down -- to
> attain the consistency I think you're asking about.

So you are saying that the number of values that is actually filled in is not
consistent with the return value of the bpf_get_stack() helper? That would
sound like a kernel bug.