Re: [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace

Josh Triplett
2015-09-15T05:19:49Z

On Tue, Sep 15, 2015 at 03:23:58AM +0300, Kirill A. Shutemov wrote:
> On Mon, Sep 14, 2015 at 03:50:38PM -0700, Palmer Dabbelt wrote:
> > This used to be hidden behind CONFIG_MMAP_ALLOW_UNINITIALIZED, so
> > userspace wouldn't actually ever see it be non-zero. While I could
> > have kept hiding it, the man pages seem to indicate that
> > MAP_UNINITIALIZED should be visible:
> >
> > mmap(2)
> > MAP_UNINITIALIZED (since Linux 2.6.33)
> > Don't clear anonymous pages. This flag is intended to improve
> > performance on embedded devices. This flag is honored only if the
> > kernel was configured with the CONFIG_MMAP_ALLOW_UNINITIALIZED
> > option. Because of the security implications, that option is
> > normally enabled only on embedded devices (i.e., devices where one
> > has complete control of the contents of user memory).
> >
> > and since the only time it shows up in my /usr/include is in this
> > header I believe this should have been visible to userspace (as
> > non-zero, which wouldn't do anything when or'd into the flags) all
> > along.
>
> Are you sure about "wouldn't do anything"?
> Suspiciously, 0x4000000 is also (1 << MAP_HUGE_SHIFT). I'm not sure if any
> architecture has order-1 huge pages, but still looks like we have conflict
> here.
>
> I think it's harmful to expose non-zero MAP_UNINITIALIZED to system which
> potentially can handle multiple users. Or non-trivial user space in
> general.

The flag should always exist. If it was defined to conflict with
something else, that's a serious ABI problem. But the flag
should always exist, even if the kernel ends up ignoring it.

> Should we leave it at least under '#ifndef CONFIG_MMU'? I don't think it's
> possible to have single ABI for MMU and MMU-less systems anyway. And we
> can avoid conflict with MAP_HUGE_SHIFT this way.

No; even if you have an MMU (which is useful for things like fork()), a
system without user separation (for instance, without CONFIG_MULTIUSER)
can reasonably use MAP_UNINITIALIZED.

> P.S. MAP_UNINITIALIZED itself looks very broken to me. I probably need dig
> mailing list on why it was allowed.

That's what the config option *and* explicit flag are for; there are
more than enough warnings about the implications.

- Josh Triplett