Messages in this thread | | | Subject | Re: [RFC PATCH] x86/mm/fault: Allow stack access below %rsp | From | Waiman Long <> | Date | Fri, 2 Nov 2018 16:34:04 -0400 |
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On 11/02/2018 04:11 PM, Andy Lutomirski wrote: > On Fri, Nov 2, 2018 at 12:50 PM Waiman Long <longman@redhat.com> wrote: >> On 11/02/2018 03:44 PM, Dave Hansen wrote: >>> On 11/2/18 12:40 PM, Waiman Long wrote: >>>> The 64k+ limit check is kind of arbitrary. So the check is now removed >>>> to just let expand_stack() decide if a segmentation fault should happen. >>> With the 64k check removed, what's the next limit that we bump into? Is >>> it just the stack_guard_gap space above the next-lowest VMA? >> I think it is both the stack_guard_gap space above the next lowest VMA >> and the rlimit(RLIMIT_STACK). >> > Did the non-working programs ever work? Because, if not, I say let them fail.
The program was working before on older kernel. After the backport of the commit 1be7107fbe18 ("mm: larger stack guard gap, between vmas"), the program stopped working. I believe it is the removal of the check_stack_guard_page() function which did expand the stack. So the 64k check was actually not functional because of the stack expansion before hand, but now it does.
Anyway, I see the current 64k check a very crude check on an userspace error. I think there are quite a number of more powerful userspace tools out there that can do this kind of check more effectively. Kernel isn't the right place to do that.
Cheers, Longman
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