Re: SMP races in proc with thread_struct

[Brian Gerst <bgerst@didntduck.org>]

Andreas Schwab wrote:
> 
> Keith Owens <kaos@ocs.com.au> writes:
> 
> |> On Fri, 04 May 2001 07:34:20 -0500,
> |> Todd Inglett <tinglett@vnet.ibm.com> wrote:
> |> >But this is where hell breaks loose.  Every process has a valid parent
> |> >-- unless it is dead and nobody cares.  Process N has already exited and
> |> >released from the tasklist while its parent was still alive.  There was
> |> >no reason to reparent it.  It just got released.  So N's task_struct has
> |> >a dangling ptr to its parent.  Nobody is holding the parent task_struct,
> |> >either.  When the parent died memory for its task_struct was released.
> |> >This is ungood.
> |>
> |> Wrap the reference to the parent task structure with exception table
> |> recovery code, like copy_from_user().
> 
> Exception tables only protect accesses to user virtual memory.  Kernel
> memory references must always be valid in the first place.
> 
> Andreas.

The virtual address being accessed is irrelevant.  It's the address of
the faulting instruction that determines what the kernel will do if it
can't deal with a page fault.  If the access was made from kernel mode
the exception handler (if there is one) always gets invoked, otherwise
it oopses.

--

				Brian Gerst