Re: access pointer and global variables

[Jim Keniston <jkenisto at us dot ibm dot com>]

On Thu, 2007-06-07 at 13:39 -0700, Stone, Joshua I wrote:
> David Smith wrote:
> > I agree with you that this might be implementable, but is it what you 
> > would really want?  In a return probe would you want the original value 
> > of $prev->pid or the current value (assuming the function changed the 
> > data pointed to by $prev)?
> > 
> > I'd guess that most times you'd want the current value.
> 
> That's a hard question to answer, because it can easily go either way. 
> For strace-like scripts, you generally want to say "this function with 
> these arguments produced this return value" -- so you want the original 
> values.  But there are certainly times when you want to see how the 
> arguments were updated.

I agree: it depends.

In my current application of stap, the value I'm interested in is
regs->eip.  regs is the function arg, and its value doesn't change, but
the value of regs->eip does change.  When I refer to $regs->eip in the
return probe, what I want is entry_value($regs)->eip, but what stap
gives me is entry_value($regs->eip).

So I had to cache the value of regs, and write an embedded-C function
(to be called from the return probe) to cast regs[tid()] back to a
struct pt_regs* and get the (updated) eip member.  (Cuz stap doesn't
support casts, right?)

> 
> For a lot of cases, I think the arguments aren't modified, so it 
> wouldn't matter when you read them.
> 
> I think we can either find a way to do both, for the best flexibility, 
> or else pick the one that's the most sane (implementable? matches most 
> users' expectation?) and document how it works.

I'd much rather see the former.  Make stap require the script author to
specify what he/she wants -- e.g., via a built-in function such as
entry_value($arg) -- at the very least if it's a dereferenced pointer.
Even if it's just an int or something, the current practice of allowing
plain old $arg in a return probe could be misleading.
 
> 
> 
> Josh

Jim