On 08/24, Hugh Dickins wrote: > > 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 <o...@redhat.com> > > > Reviewed-by: Cyrill Gorcunov <gorcu...@gmail.com> > > Yes, much better to use find_vma than have this strange stray use > of unreliable start_stack. > > Acked-by: Hugh Dickins <hu...@google.com>
Thanks! > though like Manfred I didn't quite see how overflow was impossible > on unfamiliar architectures. And you can't see, because the comment is simply wrong, I'll send v2. > > > 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: And it is not clear to me why the kernel should care at all, > okay that we're all more interested in just removing that start_stack. so perhaps v2 should simply remove it? Or do you think it would be safer to not do this? Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
