Re: [1/3] kprobes-func-args-268-rc3.patch

[Suparna Bhattacharya <suparna@in.ibm.com>]

On Thu, Aug 05, 2004 at 02:54:23PM +0200, Andi Kleen wrote:
> On Thu, Aug 05, 2004 at 05:54:31PM +0530, Suparna Bhattacharya wrote:
> > > I think you misunderstood Linus' suggestion.  The problem with
> > > modifying arguments on the stack frame is always there because the C
> > > ABI allows it. One suggested solution was to use a second function
> > 
> > I did realise that it is the ABI which allows this, but I thought
> > that the only situation in which we know gcc to actually clobber
> > arguments from the callee in practice is for tailcall optimization. 
> 
> It just breaks the most common workaround. 

Just curious, do you know if other cases/optimizations where the
callee clobbers arguments on stack ?

> 
> > I'm not sure if that can be guaranteed and yes saving bytes from
> > stack would avoid the problem totally (hence the comment) and make
> > it less tied to expected innards of the compiler. The only issue 
> > with that is deciding the maximum number of arguments so it is 
> > generic enough. 
> 
> 64bytes, aka 16 arguments seem far enough.

OK, is there is consensus on this ? 
We'd have to make the code check for stack boundary etc and probably 
compare and copy back only if there has been a change.

> 
> > > call that passes the arguments again to get a private copy. But the
> > > compiler's tail call optimization could sabotate that when you a
> > > are not careful.
> > > 
> > > That's all quite hackish and compiler dependent. I would suggest an 
> > > assembly wrapper that copies the arguments when !CONFIG_REGPARM.
> > > Just assume the function doesn't have more than a fixed number
> > > of arguments, that should be good enough.
> > > 
> > > This way you avoid any subtle compiler dependencies.
> > > With CONFIG_REGPARM it's enough to just save/restore pt_regs,
> > > which kprobes will do anyways.
> > > >  
> > 
> > Even with CONFIG_REGPARM, if you have a large 
> > number of arguments for example, is spill over into stack 
> > a possibility ?
> 
> Yes. For more than three (Linux uses -mregparm=3) 
> Also varargs arguments will be always on the stack I think.

Right, so making the copy dependent on !CONFIG_REGPARM wouldn't
make sense would it ?

Regards
Suparna

-- 
Suparna Bhattacharya (suparna@in.ibm.com)
Linux Technology Center
IBM Software Lab, India