Re: [patch] kprobes: dont steal interrupts from vm86

[Prasanna S Panchamukhi]

Hi,
On Thu, Dec 02, 2004 at 10:28:32PM +0300, Stas Sergeev wrote:
> Hello.
> 
> Prasanna S Panchamukhi wrote:
> >Yes, there is a small bug in kprobes. Kprobes int3 handler
> >was returning wrong value. Please check out if the patch
> >attached with this mail fixes your problem.
> >Please let me know if you have any issues.
> Yes. After several days of debugging,
> I am pointing to this problem again.
> Unfortunately your patch appeared not
> to work. It only masks the problem.
> I was surprised that you check VM_MASK
> after you already used "addr" a couple
> of times - this "addr" is completely
> bogus and should not be used. Now this
> turned out more important. The problem
> is that the "addr" calculated only from
> the value of EIP, is bogus not only when
> VM flag is set. It is also bogus if the
> program uses segmentation and the
> CS_base!=0. I have many of the like
> programs here and they all are broken
> because kprobes still steal the int3 from
> them. They do not use V86, but they use
> segments instead of the flat layout, so
> the address cannot be calculated by the
> EIP value.

Well, a test program is always better. I 
would appreciate if you can sent me the
test program.

> I would suggest something like the attached
> patch. I know nothing about kprobes (sorry)
> so I don't know what CS you need. If you
> need not only __KERNEL_CS, you probably
> want the (regs->xcs & 4) check to see if
> the CS is not from LDT at least. Does this
> make sense?
> Anyway, would be nice to get this fixed.
> This can cause Oopses because you deref
> the completely bogus pointer later in the
> code.
> Writing a test-case for this problem is
> not a several-minutes work, but if you
> really need one, I may try to hack it out.
> 
> Thanks.
> 



Thanks
Prasanna

-- 

Prasanna S Panchamukhi
Linux Technology Center
India Software Labs, IBM Bangalore
Ph: 91-80-25044636
<prasanna@xxxxxxxxxx>
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