[OmniOS-discuss] Good way to debug DTrace invalid address errors?

[Chris Siebenmann]

 I have a relatively complicated chunk of dtrace code that reads kernel
data structures and chases pointers through them. Some of the time it
spits out 'invalid address' errors during execution, for example:

	dtrace: error on enabled probe ID 8 (ID 75313: fbt:nfssrv:nfs3_fhtovp:return): invalid address (0x2e8) in action #6 at DIF offset 40
	dtrace: error on enabled probe ID 8 (ID 75313: fbt:nfssrv:nfs3_fhtovp:return): invalid address (0xbaddcafebaddcc7e) in action #6 at DIF offset 60

I'd like to find out exactly what pointer dereferences or other
operations are failing here, so that I can figure out how to work
around the issue. However, I have no solid idea how to map things
like 'probe ID 8' and 'DIF offset 60' to particular lines in my
DTrace source code.

 I assume that the answer to this involves reading DIF (the DTrace
intermediate form). I've looked at 'dtrace -Se' output from this
DTrace script, but I can't identify the spot I need to look at.
In particular, as far as I can see nothing in the output has
instructions with an offset as high as 40 or 60.

 I can flail around sticking guards in and varying how I do stuff
to make the errors go away, but I'd like to understand how to debug
this sort of stuff so I can have more confidence in my changes.

 Thanks in advance for any suggestions, and if people want to see
the actual code involved it is this DTrace script:

	https://github.com/siebenmann/cks-dtrace/blob/master/nfs3-long.d

(Look at line 144 for the specific dtrace probe that is probably
failing, since it's the only probe on fbt:nfssrv:nfs3_fhtovp:return.)

	- cks
PS: it's entirely possible that there's a better way to do what I'm
    trying here, too. These DTrace scripts were originally written
    on Solaris 10 update 8 and haven't been drastically revised
    since.