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

From Palmer Dabbelt <palmer@dabbelt.com>
Newsgroups linux.kernel
Subject Re: [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace
Date 2015-09-15 03:00 +0200
Message-ID <q8MLf-5WC-5@gated-at.bofh.it> (permalink)
References <q8Mie-5pB-11@gated-at.bofh.it>
Organization linux.* mail to news gateway

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On Mon, 14 Sep 2015 17:23:58 PDT (-0700), kirill@shutemov.name 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"?

That was bad writing for me.  I'd originally written something like "I
believe this should have been visible to userspace all along", but
then added the ()'s.  I meant to say:

 * I think MAP_UNINITIALIZED should have been non-zero in userspace.
 * MAP_UNINITAILIZED was zero in userspace.
 * A zero MAP_UNINITIALIZED does nothing when OR'd in.

> 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.

This doesn't have MAP_UNINITIALIZED do anything by default, it just
defines the flag the same way on all systems.  I was under the
impression that this just happened if I set MAP_UNINITIALIZED.
Looking at MAP_HUGE_SHIFT it mmap.c, that's definitely why my mmap()
test case ignored the set MAP_UNINITIALIZED on my PC.

I'm going to make this

 #ifndef MAP_UNINITAILIZED
 #define MAP_UNINITAILIZED 0
 #endif

and then leave Xtensa's port alone.  This is what Arnd suggested
originally, sorry for the extra work!

> 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.

The whole goal here was to eliminate "#ifndef CONFIG_*" from the
user-visible headers.  This all started because I got bit by a very
similar-looking bug (see patch #1), so I'd prefer not to go down that
route.

> P.S. MAP_UNINITIALIZED itself looks very broken to me. I probably need dig
> mailing list on why it was allowed.
> But that's other topic.
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Re: [PATCH] Remove #ifdef CONFIG_64BIT from all asm-generic/fcntl.h Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-09 23:10 +0200
  [PATCH 06/13] Hide ep_take_care_of_epollwakeup() behind #ifdef __KERNEL__ Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-09 23:20 +0200
  [PATCH 11/13] Hide MAX_RAW_MINORS behind #ifdef __KERNEL__ Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-09 23:20 +0200
    Re: [PATCH 11/13] Hide MAX_RAW_MINORS behind #ifdef __KERNEL__ David Howells <dhowells@redhat.com> - 2015-09-10 13:20 +0200
  [PATCH 02/13] Always expose __SYSCALL(... fork ...) Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-09 23:20 +0200
  [PATCH 13/13] Re-enable and clean up "check_config()" in headers_check.pl Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-09 23:20 +0200
  [PATCH 05/13] Hide some of "struct elf_prstatus" behind #ifdef __KERNEL__ Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-09 23:20 +0200
  [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-09 23:20 +0200
  [PATCH 03/13] Hide COMPAT_ATM_ADDPARTY behind #ifdef __KERNEL__ Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-09 23:20 +0200
  [PATCH 08/13] Hide MAX_SHARED_LIBS behind #ifdef __KERNEL__ Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-09 23:20 +0200
  Re: [PATCH] Remove #ifdef CONFIG_64BIT from all asm-generic/fcntl.h David Howells <dhowells@redhat.com> - 2015-09-10 13:20 +0200
    Re: [PATCH] Remove #ifdef CONFIG_64BIT from all asm-generic/fcntl.h David Howells <dhowells@redhat.com> - 2015-09-10 13:20 +0200
      Re: [PATCH] Remove #ifdef CONFIG_64BIT from all asm-generic/fcntl.h Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
  [PATCH 06/13] Move ep_take_care_of_epollwakeup() to fs/eventpoll.c Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
  [PATCH 02/13] Use sys_ni.c instead of #ifdef to disable fork on CONFIG_NOMMU Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
  [PATCH 01/13] Remove #ifdef CONFIG_64BIT from all asm-generic/fcntl.h Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
  [PATCH 05/13] Split FDPIC "struct elf_prstatus" to "struct elf_fdpic_prstatus" Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
    Re: [PATCH 05/13] Split FDPIC "struct elf_prstatus" to "struct elf_fdpic_prstatus" David Howells <dhowells@redhat.com> - 2015-09-17 12:20 +0200
  [PATCH v3] Remove #ifdef CONFIG_* from all userspace headers Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
    [PATCH 09/13] Move bp_type_idx to kernel/event/hw_breakpoint.c Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
      Re: [PATCH 09/13] Move bp_type_idx to kernel/event/hw_breakpoint.c Peter Zijlstra <peterz@infradead.org> - 2015-09-15 10:10 +0200
        Re: [PATCH 09/13] Move bp_type_idx to kernel/event/hw_breakpoint.c Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 20:50 +0200
          Re: [PATCH 09/13] Move bp_type_idx to kernel/event/hw_breakpoint.c Peter Zijlstra <peterz@infradead.org> - 2015-09-15 21:40 +0200
            Re: [PATCH 09/13] Move bp_type_idx to kernel/event/hw_breakpoint.c Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-17 06:00 +0200
        Re: [PATCH 09/13] Move bp_type_idx to kernel/event/hw_breakpoint.c Arnd Bergmann <arnd@arndb.de> - 2015-09-15 23:20 +0200
          Re: [PATCH 09/13] Move bp_type_idx to kernel/event/hw_breakpoint.c David Howells <dhowells@redhat.com> - 2015-09-17 12:30 +0200
    [PATCH 13/13] Re-enable and clean up "check_config()" in headers_check.pl Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
    [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
      Re: [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-09-15 02:30 +0200
        Re: [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 03:00 +0200
        Re: [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace Josh Triplett <josh@joshtriplett.org> - 2015-09-15 07:20 +0200
          Re: [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-09-15 11:50 +0200
            Re: [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace Josh Triplett <josh@joshtriplett.org> - 2015-09-15 16:20 +0200
              Re: [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace David Howells <dhowells@redhat.com> - 2015-09-17 12:20 +0200
    [PATCH 08/13] Move MAX_SHARED_LIBS to fs/binfmt_flat.c Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
    [PATCH 07/13] Make FB_BACKLIGHT_{LEVELS,MAX} always visible Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
    [PATCH 12/13] Remove AT_VECTOR_SIZE_ARCH on x86 Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
    [PATCH 11/13] Always define MAX_RAW_MINORS as 65535 in userspace Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
      Re: [PATCH 11/13] Always define MAX_RAW_MINORS as 65535 in userspace "H. Peter Anvin" <hpa@zytor.com> - 2015-09-15 22:50 +0200
        Re: [PATCH 11/13] Always define MAX_RAW_MINORS as 65535 in userspace Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-17 05:10 +0200
    [PATCH 10/13] Move USE_WCACHING to drivers/block/pktcdvd.c Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
    [PATCH 03/13] Move COMPAT_ATM_ADDPARTY to net/atm/svc.c Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
      Re: [PATCH 03/13] Move COMPAT_ATM_ADDPARTY to net/atm/svc.c David Howells <dhowells@redhat.com> - 2015-09-17 12:00 +0200
        Re: [PATCH 03/13] Move COMPAT_ATM_ADDPARTY to net/atm/svc.c Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-17 23:00 +0200

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