Re: [PATCH] mm: larger stack guard gap, between vmas

Ben Hutchings
2017-07-05T12:20:23Z

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On Tue, 2017-07-04 at 16:31 -0700, Linus Torvalds wrote:
> On Tue, Jul 4, 2017 at 4:01 PM, Ben Hutchings <ben@decadent.org.uk>
> wrote:
> >
> > We have:
> >
> > bottom = 0xff803fff
> > sp =     0xffffb178
> >
> > The relevant mappings are:
> >
> > ff7fc000-ff7fd000 rwxp 00000000 00:00 0
> > fffdd000-ffffe000 rw-p 00000000 00:00
> > 0                                  [stack]
>
> Ugh. So that stack is actually 8MB in size, but the alloca() is about
> to use up almost all of it, and there's only about 28kB left between
> "bottom" and that 'rwx' mapping.
>
> Still, that rwx mapping is interesting: it is a single page, and it
> really is almost exactly 8MB below the stack.
>
> In fact, the top of stack (at 0xffffe000) is *exactly* 8MB+4kB from
> the top of that odd one-page allocation (0xff7fd000).
>
> Can you find out where that is allocated? Perhaps a breakpoint on
> mmap, with a condition to catch that particular one?
[...]

Found it, and it's now clear why only i386 is affected:
http://hg.openjdk.java.net/jdk8/jdk8/hotspot/file/tip/src/os/linux/vm/os_linux.cpp#l4852
http://hg.openjdk.java.net/jdk8/jdk8/hotspot/file/tip/src/os_cpu/linux_x86/vm/os_linux_x86.cpp#l881

Ben.

--
Ben Hutchings
Anthony's Law of Force: Don't force it, get a larger hammer.

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