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 <b...@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? -- Michal Hocko SUSE Labs