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

[Andi Kleen]

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. 

> 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.

> > 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.

-Andi
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