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Re: Discussion at Linux Foundation Japan Symposium


Hi -

On Fri, Dec 19, 2008 at 03:50:58PM +0000, Richard J Moore wrote:

> [...]  I still think there's a case to make ST only weakly dependent
> on debuginfo.  The original dprobes would use the public names in
> the ELF headers to resolve symbols and only resort to debuginfo if
> needed.

It turns it's needed rather a lot. :-( Without it, we have no typing
information with which pointer/struct values can be intelligently
processed.  We also don't have location information to *find* data
such as parameters and locals.  It is also our current solution for
prologue analysis, so we know where to insert probes on plain
(non-regparms) functions compiled by current gcc (where location list
data can be simply wrong).

Some of this has been predicated on the belief that debuginfo quality
will improve enough that we do not need to resort to hard-coded
per-architecture per-compiler hacks like some in gdb.


> [...] Would could build some scripts with the binaries where debug
> info is available, interpret that info as relative offsets to public
> symbols then dispense with the info on the shipped production
> script.  See where I'm going here?

It sounds like an interesting sort of shortcut - one that we have not
seriously considered.  It may be far too optimistic, considering the
degree of configuration/optimization change possible between two
similar kernel builds, and the prevalent hostility toward reusable
kernel modules.

> And even for one-off scripts, this is still useful. I have on many
> occasions started with a dump, found the information I need
> extracting with a dynamic tracepoint and developed the script in
> terms relative to public symbols. No debug info needed..

Could you elaborate with an example?

- FChE


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