Re: [PATCH] mm: larger stack guard gap, between vmas
From: Ben Hutchings
Date: Wed Jul 05 2017 - 08:20:27 EST
On Tue, 2017-07-04 at 16:31 -0700, Linus Torvalds wrote:
> On Tue, Jul 4, 2017 at 4:01 PM, Ben Hutchings <ben@xxxxxxxxxxxxxxx>
> > We have:
> > bottom = 0xff803fff
> > sp =ÂÂÂÂÂ0xffffb178
> > The relevant mappings are:
> > ff7fc000-ff7fd000 rwxp 00000000 00:00 0
> > fffdd000-ffffe000 rw-p 00000000 00:00
> > 0ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ[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:
Anthony's Law of Force: Don't force it, get a larger hammer.
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