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X-Proofpoint-GUID: yAQxQSQ5y9oUrfCj-URVeU7YDRqSZrB8 X-Proofpoint-ORIG-GUID: yAQxQSQ5y9oUrfCj-URVeU7YDRqSZrB8 On Wed, Aug 28, 2024 at 04:23:09PM -0400, Eugene Loh via DTrace-devel wrote: > 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). I'll look around to see if I can figure out what is going on and whether it is a kernel bug (which of course may already have been fixed in some kernel version). In general, we should mention in patches like this that it seems to be a kernel bug (or first determine if it is) because otherwise we're going to implement workarounds without ever making sure the kernel bug gets fixed. Or if it is not a kernel bug, we need to look elsewhere why this goes wrong.