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    SubjectRe: [PATCH] mm: larger stack guard gap, between vmas
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    On Wed, 2017-07-05 at 16:23 +0200, Michal Hocko wrote:
    > On Wed 05-07-17 13:19:40, Ben Hutchings wrote:
    > > 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
    >
    > This is really worrying. This doesn't look like a gap at all. It is a
    > mapping which actually contains a code and so we should absolutely not
    > allow to scribble over it. So I am afraid the only way forward is to
    > allow per process stack gap and run this particular program to have a
    > smaller gap. We basically have two ways. Either /proc/<pid>/$file or
    > a prctl inherited on exec. The later is a smaller code. What do you
    > think?

    Distributions can do that, but what about all the other apps out there
    using JNI and private copies of the JRE?

    Soemthing I noticed is that Java doesn't immediately use MAP_FIXED.
    Look at os::pd_attempt_reserve_memory_at(). If the first, hinted,
    mmap() doesn't return the hinted address it then attempts to allocate
    huge areas (I'm not sure how intentional this is) and unmaps the
    unwanted parts. Then os::workaround_expand_exec_shield_cs_limit() re-
    mmap()s the wanted part with MAP_FIXED. If this fails at any point it
    is not a fatal error.

    So if we change vm_start_gap() to take the stack limit into account
    (when it's finite) that should neutralise
    os::workaround_expand_exec_shield_cs_limit(). I'll try this.

    Ben.

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