From: ebiederm@xmission.com (Eric W. Biederman)
To: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Andy Lutomirski <luto@kernel.org>,
Oleg Nesterov <oleg@redhat.com>,
Andrei Vagin <avagin@virtuozzo.com>,
Thomas Gleixner <tglx@linutronix.de>, Greg KH <greg@kroah.com>,
Andrey Vagin <avagin@openvz.org>, Serge Hallyn <serge@hallyn.com>,
Pavel Emelyanov <xemul@virtuozzo.com>,
Cyrill Gorcunov <gorcunov@openvz.org>,
Peter Zijlstra <peterz@infradead.org>, Willy Tarreau <w@1wt.eu>,
"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
Linux API <linux-api@vger.kernel.org>,
Linux Containers <containers@lists.linux-foundation.org>,
Michael Kerrisk <mtk.manpages@gmail.com>
Subject: Re: Simplfying copy_siginfo_to_user
Date: Mon, 31 Jul 2017 11:37:41 -0500 [thread overview]
Message-ID: <87mv7k1s16.fsf@xmission.com> (raw)
In-Reply-To: <20170725013756.GH2063@ZenIV.linux.org.uk> (Al Viro's message of "Tue, 25 Jul 2017 02:37:57 +0100")
Al Viro <viro@ZenIV.linux.org.uk> writes:
2> On Mon, Jul 24, 2017 at 10:43:34AM -0700, Linus Torvalds wrote:
>> On Sat, Jul 22, 2017 at 1:25 PM, Eric W. Biederman
>> <ebiederm@xmission.com> wrote:
>> > I played with some clever changes such as limiting the copy to 48 bytes,
>> > disabling the memset and the like but I could not get a strong enough
>> > signal to say that any one change removed the extra or a clear part of
>> > it 20ns.
>>
>> What CPU did you use? Because the SMAP bit in particular matters.
>>
>> The field-by-field copies are extremely slow on modern CPU's that
>> implement SMAP, unless you also use the special "unsafe_put_user()"
>> code (or the nasty old put_user_ex() code that some of the x86 signal
>> code uses).
>>
>> So one of the advantages of just copy_to_user() ends up being visible
>> only on Broadwell+ (or whatever the SMAP cutoff is).
>
> Guys, could you take a look at vfs.git#work.siginfo? I'd been pretty
> much buried lately (and probably will for several more weeks - long-distance
> moves *suck*), so that thing got stalled, but it might be worth a
> look.
There is some good stuff in there. If you don't mind I am going to
cherry pick out your unification of struct siginfo and struct compat_siginfo.
> The code generated in copy_siginfo_to_user() in it looks reasonably good,
> we don't copy more than we need and all copying to userland is done
> by copy_to_user() - one call per call of copy_siginfo_to_user(), so
> SMAP crap is not an issue.
There is actually a core problem with doing things that way. You rely
on having the siginfo union member stored in the high bits of si_code.
I have just fixed that in my tree and replaced using the high bits
with calling the function siginfo_layout.
It has been a significant problem storing the union member differently
in the kernel than in userspace. It has allowed for some pretty
horrendous gaffs in the archictecures changing the meaning of SI_USER
when specific signals are delivered over. It has also meant that ptrace
siginfo injection and tg_sigqueueinfo have been broken for some signals
almost since the interface was added.
Without any optimization and just changing the code to be copy_to_user
I am seeing a maybe 2% slowdown. Given that no one has seemed to care
overly for the performance of signal delivery I suspect an almost
unmeasurable slowdown is a reasonable tradeoff for simpler code.
> The next thing I hope to do is converting compat side of that thing to
> the same; that got stalled.
All of that said your precise copying code appears reasonable and quite
nice so I may adopt it on the compat side.
> Al "Buried in boxes" Viro...
Eric "Also Buried in boxes" Biederman
prev parent reply other threads:[~2017-07-31 16:37 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <87lgot2loq.fsf@xmission.com>
[not found] ` <87zid90vye.fsf_-_@xmission.com>
[not found] ` <20170615225426.GP31671@ZenIV.linux.org.uk>
[not found] ` <87poe4zrs1.fsf@xmission.com>
[not found] ` <CA+55aFxpv+gchzs7AYgSC8feAOV=B6mjFgBVm4Kx+83J2CNE-w@mail.gmail.com>
[not found] ` <87poe3vsa9.fsf@xmission.com>
[not found] ` <CALCETrX=SquyR8JZqHDNx=_FQKQo-0u9AxfdUwJs_hujVO2A-g@mail.gmail.com>
[not found] ` <87h8zfua59.fsf@xmission.com>
[not found] ` <CALCETrWPBn31Dye=81r2ZMainNOnDy5c_QxbU2uRjnJs0ie=Zg@mail.gmail.com>
[not found] ` <87r2yjsuwl.fsf@xmission.com>
[not found] ` <20170616191602.GA10675@1wt.eu>
2017-06-30 12:36 ` [PATCH 0/8] signal: Fix sending signals with siginfo Eric W. Biederman
2017-07-18 14:04 ` [PATCH v2 0/7] " Eric W. Biederman
2017-07-18 14:06 ` [PATCH 1/7] signal/alpha: Document a conflict with SI_USER for SIGTRAP Eric W. Biederman
[not found] ` <20170718140651.15973-1-ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
2017-07-18 18:22 ` Richard Henderson
2017-07-18 14:06 ` [PATCH 4/7] signal/mips: Document a conflict with SI_USER with SIGFPE Eric W. Biederman
2017-08-07 16:18 ` Maciej W. Rozycki
2017-08-07 17:41 ` Linus Torvalds
2017-08-07 19:55 ` Ralf Baechle
2017-08-08 15:29 ` Eric W. Biederman
2017-08-08 23:19 ` Maciej W. Rozycki
[not found] ` <87o9shg7t7.fsf_-_-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
2017-07-18 14:06 ` [PATCH 2/7] signal/ia64: " Eric W. Biederman
2017-07-18 14:06 ` [PATCH 3/7] signal/sparc: " Eric W. Biederman
2017-07-18 14:06 ` [PATCH 5/7] signal/testing: Don't look for __SI_FAULT in userspace Eric W. Biederman
2017-07-18 14:06 ` [PATCH 6/7] fcntl: Don't use ambiguous SIG_POLL si_codes Eric W. Biederman
[not found] ` <20170718140651.15973-6-ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
2017-07-20 16:16 ` Oleg Nesterov
2017-07-21 2:33 ` Eric W. Biederman
2017-07-18 14:06 ` [PATCH 7/7] signal: Remove kernel interal si_code magic Eric W. Biederman
[not found] ` <20170718140651.15973-7-ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
2017-07-18 16:57 ` Linus Torvalds
[not found] ` <CA+55aFyKsmf+BpYjcH30MGpHTDJ=zgYPx6kwyEB9CXXFxj_xsw-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-07-18 17:27 ` Eric W. Biederman
[not found] ` <878tjlbqpt.fsf-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
2017-07-22 20:25 ` Simplfying copy_siginfo_to_user Eric W. Biederman
[not found] ` <8760ek5ics.fsf_-_-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
2017-07-24 17:43 ` Linus Torvalds
[not found] ` <CA+55aFyH5W2doo9vxXta_-pXfNXqQ19d7z48k1hmfAot+aJvMw-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-07-24 19:01 ` Eric W. Biederman
2017-07-25 1:37 ` Al Viro
2017-07-31 16:37 ` Eric W. Biederman [this message]
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