Systemtap Upcoming Feature: Address to File:Line Mapping

[Abe Jakop]

----- Original Message -----
> From: "Frank Ch. Eigler" <fche@redhat.com>

> > [...]  As for using this new feature, there are two new tapset
> > functions, usymfileline() and symfileline(). The former requires a
> > user-space address, and a kernel address for the the latter.
> 
> Could we also have [u]symfile and/or [u]symline?  That way, script
> code wouldn't need to parse [u]symfileline if they only wanted one
> or the other.

Yes. I will add those functions into the tapset library.

> > Since this feature stores line data for runtime access,
> > using this feature may result in large probe modules.
> 
> Looking forward to advice as to whether further data
> compression may significantly help.

That's definitely on the list things to do for this feature.

> > [...]
> > linenumbers.stp
> > ===============
> >  1 probe process.statement("*") {
> >  2   printf("addr: %#x, file and line: %s\n",
> >  3          addr(), usymfileline(addr()) )
> >  4 }
> 
> (uaddr() in this case.)

Oops. Good catch. 

> Glancing at that pf3.stp sample, maybe [u]sym* family of functions
> should more regularly throw catchable errors in case of errors,
> instead of hard-coding a policy of returning a hexified parameter.
> That way the caller can more easily omit the info in its report.  So,
> 
> before:
> 
>     try { // modname() can throw
>       fn = "k:".modname(addr()).":".symname(addr()).":".symfileline(addr())
>     } catch {
>       fn = "k:<unknown>:".symname(addr()).":".symfileline(addr())
>     }
> 
> after:
> 
>   try { part_1 = "k:".modname(addr()) }
>   catch { part_1 = sprintf("k:%p",addr()) }
>   try { part_2 = ":".symname(addr()) }
>   catch { part_2 = "" }
>   try { part_3 = ":".symfileline(addr()) }
>   catch { part3 = "" }
>   fn = part_1.part_2.part_3

Would this not cause backwards compatibility issues? Or were you thinking
this change would be effective for SystemTap versions >=2.7?

Should I open a PR for this change and another for the feature below?

> Hm, what if we had an expression-context try/catch?  (Maybe a
> generalization of ? : ... sort of like @choose_defined()?)
> 
>   part_1 = "k:" . try_catch(modname(addr()), sprintf("%p",addr()))
>   part_2 = try_catch(":".symname(addr()), "")
>   part_3 = try_catch(":".symfilename(addr()), "")
>   fn = part_1.part_2.part_3
> 
> - FChE

Thanks for the feedback,

Abegail