Re: [stos, again] Re: CVS commit: src/sys/arch/amd64

2020-06-03 Thread Andrew Doran
On Wed, Jun 03, 2020 at 02:03:22AM +0200, Kamil Rytarowski wrote:

> On 03.06.2020 01:49, Andrew Doran wrote:
> > On the assembly thing recall that recently you expressed a desire to remove
> > all of the amd64 assembly string functions from libc because of sanitizers -
> > I invested my time to do up a little demo to try and show you why that's not
> > a good idea:
> > 
> > http://mail-index.netbsd.org/port-amd64/2020/04/19/msg003226.html
> 
> Please note that interceptors for string functions are not just some
> extra burden, but also very useful approach to feedback a fuzzer through
> additional coverage.
>
> At least libFuzzer and honggfuzz wrap many kinds of string functions and
> use it for fuzzing. We should add a special mode in KCOV to feedback
> userland (syzkaller) with traces from string functions.
> 
> https://github.com/google/honggfuzz/blob/bbb476eec95ad927d6d7d3d367d2b3e38eed3569/libhfuzz/memorycmp.c#L24

No argument from me there at all.  I think that's a great idea and was
looking at the interceptors in TSAN recently to see how they work.

Andrew


Re: [stos, again] Re: CVS commit: src/sys/arch/amd64

2020-06-03 Thread Andrew Doran
Maxime,

I read your e-mail carefully and conclude that the best way forward here is
put this one to core@ for a technical decision.

Cheers,
Andrew

On Wed, Jun 03, 2020 at 08:25:32AM +0200, Maxime Villard wrote:
> Le 03/06/2020 ? 01:49, Andrew Doran a ?crit?:
> > On Tue, Jun 02, 2020 at 08:41:53AM +0200, Maxime Villard wrote:
> > 
> > > Le 02/06/2020 ? 00:58, Andrew Doran a ?crit?:
> > > > Module Name:src
> > > > Committed By:   ad
> > > > Date:   Mon Jun  1 22:58:06 UTC 2020
> > > > 
> > > > Modified Files:
> > > > src/sys/arch/amd64/amd64: cpufunc.S
> > > > src/sys/arch/amd64/include: frameasm.h
> > > > 
> > > > Log Message:
> > > > Reported-by: syzbot+6dd5a230d19f0cbc7...@syzkaller.appspotmail.com
> > > > 
> > > > Instrument STOS/MOVS for KMSAN to unbreak it.
> > > > 
> > > > 
> > > > To generate a diff of this commit:
> > > > cvs rdiff -u -r1.58 -r1.59 src/sys/arch/amd64/amd64/cpufunc.S
> > > > cvs rdiff -u -r1.49 -r1.50 src/sys/arch/amd64/include/frameasm.h
> > > > 
> > > > Please note that diffs are not public domain; they are subject to the
> > > > copyright notices on the relevant files.
> > > 
> > > Can you just stop ignoring the remarks that are made?
> > 
> > That's up to you Maxime.  If you habitually make it difficult for people to
> > come to a reasonable compromise with you, then you're asking to not be taken
> > seriously and will find yourself being ignored.
> 
> You are confused. I asked for KMSAN to be unbroken, and proposed an 
> alternative,
> which is atomic_load_relaxed. You were free to explain why my point was a bad
> idea or why it didn't matter, but you refused, and just added a hack on top of
> another. I'm afraid that's up to you.
> 
> But whatever, it doesn't matter. Thanks for reverting the pmap.c changes, it
> is more correct now than before.
> 
> > > I said explicitly
> > > that adding manual instrumentation here is _wrong_.
> > 
> > I don't share your assessment in the general sense.  It should be possible
> > to instrument assembly code because KMSAN is useful AND we can't get by
> > without assembly - there are some things that C just can't do (or do well
> > enough).
> > 
> > On the assembly thing recall that recently you expressed a desire to remove
> > all of the amd64 assembly string functions from libc because of sanitizers -
> > I invested my time to do up a little demo to try and show you why that's not
> > a good idea:
> > 
> > http://mail-index.netbsd.org/port-amd64/2020/04/19/msg003226.html
> 
> I saw, yes. I answered explaining that a conversation with Ryo Shimizu had
> changed my mind a bit, and seeing your results (which as far as I can tell
> do not indicate a performance improvement significant enough to not be
> considered as noise), I asked you whether it was that relevant. You didn't
> follow up though.
> 
> > The system is a balancing act.  There are lots of factors to be taken into
> > account: maintainability, tooling like KMSAN, user's varying needs, the
> > capabilites of different machines, performance, feature set and so on, and
> > dare I say it even the enjoyment of the people working on the project is
> > important too.  Myopic focus on one factor only to the detriment of others
> > is no good.
> 
> I am well aware of that.
> 
> > > The kMSan ASM fixups
> > > are limited to args/returns, and that is part of a sensical policy that
> > > _should not_ be changed without a good reason.
> > > 
> > > x86_movs/x86_stos have strictly _no reason_ to exist. Of the 18 
> > > conversions
> > > you made to them in pmap.c, not one is justified. memcpy/memset were all
> > > correct.
> > 
> > I introduced these functions as a compromise because you were unhappy with
> > use of memcpy() to copy PDEs.  See:
> > 
> > http://mail-index.netbsd.org/port-amd64/2020/05/23/msg003280.html
> > 
> > You wrote:
> > 
> > In the [XXX] window, the PTEs could be used by userland.  If you
> > copied them using memcpy(), some parts of the bytes could contain
> > stale values.
> 
> Sure, I was explicitly referring to SVS, which has an unusual way of
> accessing PTEs (contrary to pmap), which is why it needs special atomic
> care that other places do not.
> 
> > Live PDEs are also copied in pmap.c so I made a change there too.  After
> > that I decided "why not" and used the new functions everywhere PDEs/PTEs or
> > pages are block zeroed / copied.  But I'm also happy with memcpy()/memset().
> > Either way will work.  In fairness I do work too fast sometimes.
> > 
> > > The only reason you made these big unneeded changes is for SVS not to take
> > > the bus lock,
> > 
> > There is no bus lock on x86 (not since the 1990s anyway).
> > 
> > > but as was said already, atomic_load_relaxed will do what
> > > you want without the need for these functions.
> > > 
> > > Please revert _both changes now_, this one and the previous one which
> > > introduced both functions, and let's use atomic_load_relaxed.
> > 
> > You're focusing on only 

Re: [stos, again] Re: CVS commit: src/sys/arch/amd64

2020-06-03 Thread Kamil Rytarowski
On 04.06.2020 00:42, Andrew Doran wrote:
> On Wed, Jun 03, 2020 at 02:03:22AM +0200, Kamil Rytarowski wrote:
> 
>> On 03.06.2020 01:49, Andrew Doran wrote:
>>> On the assembly thing recall that recently you expressed a desire to remove
>>> all of the amd64 assembly string functions from libc because of sanitizers -
>>> I invested my time to do up a little demo to try and show you why that's not
>>> a good idea:
>>>
>>> http://mail-index.netbsd.org/port-amd64/2020/04/19/msg003226.html
>>
>> Please note that interceptors for string functions are not just some
>> extra burden, but also very useful approach to feedback a fuzzer through
>> additional coverage.
>>
>> At least libFuzzer and honggfuzz wrap many kinds of string functions and
>> use it for fuzzing. We should add a special mode in KCOV to feedback
>> userland (syzkaller) with traces from string functions.
>>
>> https://github.com/google/honggfuzz/blob/bbb476eec95ad927d6d7d3d367d2b3e38eed3569/libhfuzz/memorycmp.c#L24
> 
> No argument from me there at all.  I think that's a great idea and was
> looking at the interceptors in TSAN recently to see how they work.
> 
> Andrew
> 

My note was not about switching away from ASM functions for certain
functions, but rather giving an option to disable them under a sanitizer
and adding an interceptor that can be useful for feedbacking a fuzzer.
It's still not clear whether we will create such interface in KCOV as it
has to be coordinated with Google+Linux as we would like to have a
compatible interface for syzkaller.

TSAN - do you mean the userland ones?

BTW. There is a work-in-progress MKSANITIZER support for TSan, but it
used to create unkillable processes (kernel bug). Basically when using a
TSanitized userland applications, you will quickly end up with such
processes (from AFAIR calling ls(1) and other simple applications are
enough).

If you are interested, I can share a reproducer.


Re: [stos, again] Re: CVS commit: src/sys/arch/amd64

2020-06-03 Thread Maxime Villard

Le 03/06/2020 à 01:49, Andrew Doran a écrit :

On Tue, Jun 02, 2020 at 08:41:53AM +0200, Maxime Villard wrote:


Le 02/06/2020 ? 00:58, Andrew Doran a ?crit?:

Module Name:src
Committed By:   ad
Date:   Mon Jun  1 22:58:06 UTC 2020

Modified Files:
src/sys/arch/amd64/amd64: cpufunc.S
src/sys/arch/amd64/include: frameasm.h

Log Message:
Reported-by: syzbot+6dd5a230d19f0cbc7...@syzkaller.appspotmail.com

Instrument STOS/MOVS for KMSAN to unbreak it.


To generate a diff of this commit:
cvs rdiff -u -r1.58 -r1.59 src/sys/arch/amd64/amd64/cpufunc.S
cvs rdiff -u -r1.49 -r1.50 src/sys/arch/amd64/include/frameasm.h

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.


Can you just stop ignoring the remarks that are made?


That's up to you Maxime.  If you habitually make it difficult for people to
come to a reasonable compromise with you, then you're asking to not be taken
seriously and will find yourself being ignored.


You are confused. I asked for KMSAN to be unbroken, and proposed an alternative,
which is atomic_load_relaxed. You were free to explain why my point was a bad
idea or why it didn't matter, but you refused, and just added a hack on top of
another. I'm afraid that's up to you.

But whatever, it doesn't matter. Thanks for reverting the pmap.c changes, it
is more correct now than before.


I said explicitly
that adding manual instrumentation here is _wrong_.


I don't share your assessment in the general sense.  It should be possible
to instrument assembly code because KMSAN is useful AND we can't get by
without assembly - there are some things that C just can't do (or do well
enough).

On the assembly thing recall that recently you expressed a desire to remove
all of the amd64 assembly string functions from libc because of sanitizers -
I invested my time to do up a little demo to try and show you why that's not
a good idea:

http://mail-index.netbsd.org/port-amd64/2020/04/19/msg003226.html


I saw, yes. I answered explaining that a conversation with Ryo Shimizu had
changed my mind a bit, and seeing your results (which as far as I can tell
do not indicate a performance improvement significant enough to not be
considered as noise), I asked you whether it was that relevant. You didn't
follow up though.


The system is a balancing act.  There are lots of factors to be taken into
account: maintainability, tooling like KMSAN, user's varying needs, the
capabilites of different machines, performance, feature set and so on, and
dare I say it even the enjoyment of the people working on the project is
important too.  Myopic focus on one factor only to the detriment of others
is no good.


I am well aware of that.


The kMSan ASM fixups
are limited to args/returns, and that is part of a sensical policy that
_should not_ be changed without a good reason.

x86_movs/x86_stos have strictly _no reason_ to exist. Of the 18 conversions
you made to them in pmap.c, not one is justified. memcpy/memset were all
correct.


I introduced these functions as a compromise because you were unhappy with
use of memcpy() to copy PDEs.  See:

http://mail-index.netbsd.org/port-amd64/2020/05/23/msg003280.html

You wrote:

In the [XXX] window, the PTEs could be used by userland.  If you
copied them using memcpy(), some parts of the bytes could contain
stale values.


Sure, I was explicitly referring to SVS, which has an unusual way of
accessing PTEs (contrary to pmap), which is why it needs special atomic
care that other places do not.


Live PDEs are also copied in pmap.c so I made a change there too.  After
that I decided "why not" and used the new functions everywhere PDEs/PTEs or
pages are block zeroed / copied.  But I'm also happy with memcpy()/memset().
Either way will work.  In fairness I do work too fast sometimes.


The only reason you made these big unneeded changes is for SVS not to take
the bus lock,


There is no bus lock on x86 (not since the 1990s anyway).


but as was said already, atomic_load_relaxed will do what
you want without the need for these functions.

Please revert _both changes now_, this one and the previous one which
introduced both functions, and let's use atomic_load_relaxed.


You're focusing on only one factor.  I'll explain in detail.  Here is the
original code I replaced:

 685 static inline pt_entry_t
 686 svs_pte_atomic_read(struct pmap *pmap, size_t idx)
 687 {
 688/*
 689 * XXX: We don't have a basic atomic_fetch_64 function?
 690 */
 691return atomic_cas_64(>pm_pdir[idx], 666, 666);
 692 }
...
 717/* User slots. */
 718for (i = 0; i < PDIR_SLOT_USERLIM; i++) {
 719pte = svs_pte_atomic_read(pmap, i);
 720ci->ci_svs_updir[i] = pte;
 721}

There's no need for an atomic op there because fetches on x86 are by
definition atomic, and it 

Re: [stos, again] Re: CVS commit: src/sys/arch/amd64

2020-06-03 Thread Maxime Villard

Le 03/06/2020 à 02:03, Kamil Rytarowski a écrit :

On 03.06.2020 01:49, Andrew Doran wrote:

On the assembly thing recall that recently you expressed a desire to remove
all of the amd64 assembly string functions from libc because of sanitizers -
I invested my time to do up a little demo to try and show you why that's not
a good idea:

http://mail-index.netbsd.org/port-amd64/2020/04/19/msg003226.html


Please note that interceptors for string functions are not just some
extra burden, but also very useful approach to feedback a fuzzer through
additional coverage.

At least libFuzzer and honggfuzz wrap many kinds of string functions and
use it for fuzzing. We should add a special mode in KCOV to feedback
userland (syzkaller) with traces from string functions.

https://github.com/google/honggfuzz/blob/bbb476eec95ad927d6d7d3d367d2b3e38eed3569/libhfuzz/memorycmp.c#L24


Yes, and not just that either.

When you use ASM instead of C, you basically prevent _any kind_ of useful
transformation the compiler could make.

It includes sanitizers, but also coverage as you said; and also retpoline,
PAC, BTI, CET, SafeStack, and in short, a very big bunch of modern
features.

Favoring C rather than ASM in the general sense offers much bigger
benefits than just "it accomodates kMSan".

Maxime