Messages in this thread | | | Date | Sun, 24 Aug 2014 18:38:44 -0700 (PDT) | From | Hugh Dickins <> | Subject | Re: [PATCH 1/1] ipc/shm: fix the historical/wrong mm->start_stack check |
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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@redhat.com> > > Reviewed-by: Cyrill Gorcunov <gorcunov@gmail.com>
Yes, much better to use find_vma than have this strange stray use of unreliable start_stack.
Acked-by: Hugh Dickins <hughd@google.com>
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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