Re: komd issues - was [Re: [openMosix-general] openmosix 2.4.26 in 2.4.31?]

On Thu, Sep 21, 2006 at 01:27:05PM -0600, Matt Dew wrote:
> From what I understand so far:
> One of LD_ASSUME_KERNEL's functions is to tell glibc how to handle
> system calls.  In linux 2.4, system calls use int 0x80 so we could
> modify arch/i386/kernel/entry.S to check for migrated processes and then
> handle the system call differently if need be. 

It's doing the same on 2.6.

> As of 2.6, system calls can be called other ways as well such as jumping
> straight to the syscall handler address.  Trapping 0x80 won't work all
> the time. We have to change the interrupt table, rather than trying to
> conditionally use our own.

We *never* trapped interrupt 0x80. neither do we need to change the
interrupt table, which we never did anyway.

> Apparently 2.6.18 also changes the way it handles system calls.

No, it doesn't change the way it handle system calls, it's just
randomizing where the VDSO is. which, the ABI is perfectly entitled to.
So far we relied on the fact that the VDSO was constant across kernels,
which is not going to be the case anymore...

There's 2 solutions:
- the easy one consist in just desactivating the randomization, which
  is not useful to anything anyway. It seems the security protection
  that it gives are absolutely extremely tiny and just security by
  obscurity.
- the hard one could be done in multiple way. not totally sure, since I
  didn't look the code, but gs segment fixup could probably solve the
  problem. it doesn't seems to be worst it at all.

-- 
Vincent Hanquez

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