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

[Eugene Loh]

On 8/28/24 16:17, Kris Van Hees wrote:

> 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.

I think there might be a kernel bug.  For a variety of practical reasons, I went with a DTrace workaround (and haven't looked at the kernel code).