Messages in this thread | | | Date | Tue, 26 Aug 2014 16:32:27 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH v2] ipc/shm: fix the historical/wrong mm->start_stack check |
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On 08/25, Hugh Dickins wrote: > > On Mon, 25 Aug 2014, Oleg Nesterov wrote: > > > As Hugh pointed out, we actually need to require the additional > > guard page, but this code was written before linux had it. > > > > 3. This wrongly assumes that the stack can only grown down. > > > > Personally I think we should simply kill this check, but I did not > > dare to do this. So the patch only fixes the 1st problem (mostly to > > avoid the usage of mm->start_stack) and ignores the VM_GROWSUP case. > > > > Signed-off-by: Oleg Nesterov <oleg@redhat.com> > > Sorry, I cannot ack this,
Hugh, I appreciate you nack even more. Thanks!
> because your comment below "at least 4 pages > plus a guard page enforced by check_stack_guard_page()" makes no sense > to me as an explanation for the 5. The guard page (gap) enforced by > check_stack_guard_page() is already at vma->vm_start ^^^^^^^^^^^^^^^^^^^^^^^^ Ahh. Yes I misunderstood this logic. And yes, you tried to explain it twice but I was too stupid.
> I did come across 2.6.34's 128k stack_expand inherited from 2.6.11's > 20 page EXTRA_STACK_VM_PAGES. With Linus's guard page enforcing a > page gap since 2.6.36.
OK, and setup_arg_pages() still does expand_stack() although stack_expand depends on RLIMIT_STACK. But I think this doesn't matter. The room was already reserved, in general mm->start_stack points into the middle of the stack.
So that check in do_shmat() can only help if the stack was not expanded due to the low RLIMIT_STACK, then this application raises RLIMIT_STACK, then does do_shmat(). But in this case it should likely crash when exec returns to usermode.
> and our difficulty > in communicating a sensible way of updating and describing the test,
I like your polite interpretation of my ignorance and inability to listen ;)
> now drives me to agree with you. Please just rip out the start_stack > test and the comment defending it.
Great. Will do.
Oleg.
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