Re: [PATCH 1/1] ipc/shm: fix the historical/wrong mm->start_stack check

[Hugh Dickins]

On Sat, 23 Aug 2014, Oleg Nesterov wrote:
> On 08/23, Cyrill Gorcunov wrote:
> >
> > On Sat, Aug 23, 2014 at 04:43:27PM +0200, Oleg Nesterov wrote:
> > > The ->start_stack check in do_shmat() looks ugly and simply wrong.
> > >
> > > 1. ->start_stack is only valid right after exec(), the application
> > >    can switch to another stack and even unmap this area.
> > >
> > > 2. The reason for this check is not clear at all. The application
> > >    should know what it does. And why 4 pages? And why in fact it
> > >    requires 5 pages?
> > >
> > > 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 VM_GROWSUP.
> > >
> > > Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> > Reviewed-by: Cyrill Gorcunov <gorcunov@xxxxxxxxx>

Yes, much better to use find_vma than have this strange stray use
of unreliable start_stack.

Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>

though like Manfred I didn't quite see how overflow was impossible
on unfamiliar architectures.

> 
> Thanks!
> 
> > I don't understand this check either, the comment above it says nothing
> > but only commits what code is doing not explaining why.
> 
> Yes, and this check predates the git history. I even looked into
> git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git but this
> change was added by the huge "v2.5.0.7 -> v2.5.0.8" update in 2002,
> and obviously without any explanation (apart from "fix up proper shmat
> semantics", but this connects SHM_REMAP itself).

I'd say it comes earlier, from Christoph Rohland's 2.4.17-pre7's
"Add missing checks on shmat()", though I didn't find more than that.

We can all understand wanting to leave a gap below the growsdown stack,
but of course could argue about growsup and 1 or 4 or 5 or whatever:
okay that we're all more interested in just removing that start_stack.

Hugh
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