Re: [PATCH 19/21] i386 Kprobes semaphore fix

[Zachary Amsden]

Prasanna S Panchamukhi wrote:

> On Wed, Nov 09, 2005 at 09:52:40AM -0800, Zachary Amsden wrote:
>  
>
> > Ingo Molnar wrote:
> >
> >    
> >
> > > * Zachary Amsden <zach@xxxxxxxxxx> wrote:
> > >
> > >
> > >
> > >      
> > >
> > > > > I believe user space kprobes are being worked on by some IBM India folks 
> > > > > yes.
> > > > >    
> > > > >
> > > > >          
> > > > I'm convinced this is pointless.  What does it buy you over a ptrace 
> > > > based debugger?  Why would you want extra code running in the kernel 
> > > > that can be done perfectly well in userspace?
> > > >  
> > > >
> > > >        
> > > kprobes are not just for 'debuggers', they are also used for tracing and 
> > > other dynamic instrumentation in projects like systemtap. Ptrace is way 
> > > too slow and limited for things like that.
> > >
> > >
> > >      
> > Well, if there is a justification for it, that means we really should 
> > handle all the nasty EIP conversion cases due to segmentation and v8086 
> > mode in the kprobes code.  I was hoping that might not be the case.
> >
> >    
>
> As Ingo mentioned above, Systemtap uses kprobes infrastructure to provide
> dynamic kernel instrumentation. Using which user can add lots of probes 
> easily, so we need to take care of this fast path.  
>
> Instead of calling convert_eip_to_linear() for all cases, you can
> just check if it is in kernel mode and calculate the address directly
>
> 	if (kernel mode)
>                addr = regs->eip - sizeof(kprobe_opcode_t);
>        else
>                addr = convert_eip_to_linear(..);
>
> there by avoiding call to convert_eip_to_linear () for every kernel probes.
>
> As Andi mentioned user space probes support is in progress and 
> this address conversion will help in case of user space probes as well.

I like this better.  I have to rework that patch anyways, since it no 
longer applies cleanly.

Zach

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