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Date: Tue, 18 Jul 2017 12:27:26 -0500
From: ebiederm@...ssion.com (Eric W. Biederman)
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Andy Lutomirski <luto@...nel.org>,
Al Viro <viro@...iv.linux.org.uk>,
Oleg Nesterov <oleg@...hat.com>,
Andrei Vagin <avagin@...tuozzo.com>,
Thomas Gleixner <tglx@...utronix.de>, Greg KH <greg@...ah.com>,
Andrey Vagin <avagin@...nvz.org>,
Serge Hallyn <serge@...lyn.com>,
Pavel Emelyanov <xemul@...tuozzo.com>,
Cyrill Gorcunov <gorcunov@...nvz.org>,
Peter Zijlstra <peterz@...radead.org>,
Willy Tarreau <w@....eu>,
"linux-arch\@vger.kernel.org" <linux-arch@...r.kernel.org>,
Linux API <linux-api@...r.kernel.org>,
Linux Containers <containers@...ts.linux-foundation.org>,
Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [PATCH 7/7] signal: Remove kernel interal si_code magic
Linus Torvalds <torvalds@...ux-foundation.org> writes:
> On Tue, Jul 18, 2017 at 7:06 AM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>> struct siginfo is a union and the kernel since 2.4 has been hiding a union
>> tag in the high 16bits of si_code using the values:
>> __SI_KILL
>> __SI_TIMER
>> __SI_POLL
>> __SI_FAULT
>> __SI_CHLD
>> __SI_RT
>> __SI_MESGQ
>> __SI_SYS
>>
>> While this looks plausible on the surface, in practice this situation has
>> not worked well.
>
> So on the whole I think we just need to do this, but the part I really
> hate about this series is still this the siginfo_layout() part.
>
> I can well believe that it is needed for the compat case. siginfo is a
> piece of crap crazy type, and re-ordering fields for compat is
> something we are always going to have to do.
>
> But for the native case, the *only* reason we do not just copy the
> siginfo as-is seems to be that it's just too big, due to other bad
> design decisions in siginfo ("let's make sure it's big enough by
> allocating 512 bytes for it).
>
> And afaik, absolutely nobody uses more than about 36 bytes of that
> 512-byte _sifields union (and that one use is SIGILL with three
> pointers and three integers and some padding.
>
> So why don't we just say "screw this idiotic layout crap, and just
> unconditionally copy that much smaller maximum of bytes"?
>
> Leave that layout thing purely for compat handling.
I completely agree.
> Yes, yes, there's a couple of small gotchas's:
>
> - "_sys_private" for posix timers, and it would have to be moved to
> the end of the structure so that it doesn't get copied.
I don't think we actually need _sys_private at all.
I think the best solution would involve embedding struct siginfo
into struct k_itimer (as we always allocate one). Then we can just
perform container_of on the siginfo and look at the k_itimer instead.
> - make sure those 36 bytes are cleared when allocating the siginfo
> (this should be trivial) so that we don't leak any other memory.
>
> But on the whole, it looks pretty straightforward to just get rid of
> those stupid layout things, and make them purely about compat stuff.
>
> Please?
>
> The si_code stuff clearly needs to be done regardless, so much of this
> patch series looks good to me. But if we're doign this cleanup, can't
> we please go that one extra step and get rid of the crazy "let's treat
> the union as different types", and just treat it as a largely opaque
> thing.
>
> Pretty please?
That is my next step.
I have started on it but it is a big additional patch. I have to insert
a bunch of memsets to ensure we are not copying unitialized stack
contents to userspace.
I have been convinced not to expect any performance issues:
- Worst case two reads from memory 60ns*2 = 120ns.
- 650ns time to send a signal.
- 350ns time to receive a signal.
So that is maybe a 10% change, and more likely lost completely
in the noise.
I intend to measure the performance change just copying it all to see if
I even need to optimize to just copy the needed 36 bytes.
The diffstat for introducing a clear_siginfo to ensure we have made
those memsets is huge so I am worried about introducing bugs along
the way or missing something.
arch/alpha/kernel/osf_sys.c | 1 +
arch/alpha/kernel/signal.c | 2 ++
arch/alpha/kernel/traps.c | 5 ++++
arch/alpha/mm/fault.c | 2 ++
arch/arc/kernel/traps.c | 14 ++++++----
arch/arc/mm/fault.c | 1 +
arch/arm/kernel/ptrace.c | 2 ++
arch/arm/kernel/swp_emulate.c | 1 +
arch/arm/kernel/traps.c | 5 ++++
arch/arm/mm/alignment.c | 1 +
arch/arm/mm/fault.c | 3 ++
arch/arm/vfp/vfpmodule.c | 2 +-
arch/arm64/kernel/debug-monitors.c | 13 +++++----
arch/arm64/kernel/fpsimd.c | 2 +-
arch/arm64/kernel/ptrace.c | 13 +++++----
arch/arm64/kernel/traps.c | 2 ++
arch/arm64/mm/fault.c | 4 +++
arch/blackfin/kernel/traps.c | 1 +
arch/c6x/kernel/traps.c | 1 +
arch/cris/mm/fault.c | 1 +
arch/frv/kernel/traps.c | 7 +++++
arch/frv/mm/fault.c | 1 +
arch/hexagon/kernel/traps.c | 1 +
arch/hexagon/mm/vm_fault.c | 2 ++
arch/ia64/kernel/brl_emu.c | 3 ++
arch/ia64/kernel/signal.c | 2 ++
arch/ia64/kernel/traps.c | 3 +-
arch/ia64/kernel/unaligned.c | 1 +
arch/ia64/mm/fault.c | 1 +
arch/m32r/kernel/traps.c | 1 +
arch/m32r/mm/fault.c | 1 +
arch/m68k/kernel/traps.c | 2 ++
arch/m68k/mm/fault.c | 3 +-
arch/metag/kernel/traps.c | 2 ++
arch/metag/mm/fault.c | 2 ++
arch/microblaze/kernel/exceptions.c | 1 +
arch/microblaze/mm/fault.c | 1 +
arch/mips/kernel/traps.c | 29 +++++++++++++------
arch/mips/mm/fault.c | 1 +
arch/mn10300/kernel/fpu.c | 1 +
arch/mn10300/kernel/traps.c | 1 +
arch/mn10300/mm/fault.c | 1 +
arch/mn10300/mm/misalignment.c | 2 ++
arch/nios2/kernel/traps.c | 1 +
arch/openrisc/kernel/traps.c | 5 +++-
arch/openrisc/mm/fault.c | 1 +
arch/parisc/kernel/ptrace.c | 1 +
arch/parisc/kernel/traps.c | 2 ++
arch/parisc/kernel/unaligned.c | 2 ++
arch/parisc/math-emu/driver.c | 1 +
arch/parisc/mm/fault.c | 1 +
arch/powerpc/kernel/process.c | 2 ++
arch/powerpc/kernel/traps.c | 4 +--
arch/powerpc/mm/fault.c | 1 +
arch/powerpc/platforms/cell/spufs/fault.c | 2 +-
arch/s390/kernel/traps.c | 3 ++
arch/s390/mm/fault.c | 2 ++
arch/score/kernel/traps.c | 1 +
arch/score/mm/fault.c | 1 +
arch/sh/kernel/hw_breakpoint.c | 1 +
arch/sh/kernel/traps_32.c | 4 +++
arch/sh/math-emu/math.c | 1 +
arch/sh/mm/fault.c | 1 +
arch/sparc/kernel/process_64.c | 1 +
arch/sparc/kernel/sys_sparc_32.c | 1 +
arch/sparc/kernel/sys_sparc_64.c | 1 +
arch/sparc/kernel/traps_32.c | 10 +++++++
arch/sparc/kernel/traps_64.c | 15 ++++++++++
arch/sparc/kernel/unaligned_32.c | 1 +
arch/sparc/mm/fault_32.c | 1 +
arch/sparc/mm/fault_64.c | 1 +
arch/tile/kernel/hardwall.c | 1 +
arch/tile/kernel/ptrace.c | 2 +-
arch/tile/kernel/single_step.c | 24 +++++++++-------
arch/tile/kernel/traps.c | 4 ++-
arch/tile/kernel/unaligned.c | 46 +++++++++++++++++--------------
arch/tile/mm/fault.c | 1 +
arch/um/kernel/ptrace.c | 2 +-
arch/um/kernel/trap.c | 4 ++-
arch/unicore32/kernel/fpu-ucf64.c | 3 +-
arch/unicore32/mm/fault.c | 3 ++
arch/x86/entry/vsyscall/vsyscall_64.c | 2 +-
arch/x86/kernel/ptrace.c | 2 +-
arch/x86/kernel/traps.c | 3 ++
arch/x86/kvm/mmu.c | 1 +
arch/x86/mm/fault.c | 1 +
arch/xtensa/kernel/ptrace.c | 1 +
arch/xtensa/kernel/traps.c | 1 +
arch/xtensa/mm/fault.c | 1 +
drivers/usb/core/devio.c | 4 +--
fs/fcntl.c | 1 +
include/linux/ptrace.h | 2 +-
include/linux/signal.h | 5 ++++
ipc/mqueue.c | 1 +
kernel/debug/kdb/kdb_main.c | 1 +
kernel/ptrace.c | 2 +-
kernel/seccomp.c | 2 +-
kernel/signal.c | 21 ++++++++++----
kernel/time/posix-timers.c | 2 +-
mm/memory-failure.c | 1 +
100 files changed, 272 insertions(+), 85 deletions(-)
Eric
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