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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Christophe Leroy <christophe.leroy@csgroup.eu>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
References: <fd7938d94008711d441551c06b25a033669a0618.1629732940.git.christophe.leroy@csgroup.eu>
        <a94be61f008ab29c231b805e1a97e9dab35cb0cc.1629732940.git.christophe.leroy@csgroup.eu>
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        <2715792c-eb10-eeb8-3d49-24486abe953b@csgroup.eu>
        <877dfrrkxo.fsf@disp2133>
        <7caf5127-36fc-7c77-00f1-7be82d6f26e0@csgroup.eu>
Date:   Sat, 11 Sep 2021 10:58:07 -0500
In-Reply-To: <7caf5127-36fc-7c77-00f1-7be82d6f26e0@csgroup.eu> (Christophe
        Leroy's message of "Fri, 10 Sep 2021 10:27:03 +0000")
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Subject: Re: [PATCH v2 3/5] signal: Add unsafe_copy_siginfo_to_user()
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Christophe Leroy <christophe.leroy@csgroup.eu> writes:

> On 9/8/21 6:17 PM, Eric W. Biederman wrote:
>> Christophe Leroy <christophe.leroy@csgroup.eu> writes:
>>
>>> Le 02/09/2021 à 20:43, Eric W. Biederman a écrit :
>>>> Christophe Leroy <christophe.leroy@csgroup.eu> writes:
>>>>
>>>>> In the same spirit as commit fb05121fd6a2 ("signal: Add
>>>>> unsafe_get_compat_sigset()"), implement an 'unsafe' version of
>>>>> copy_siginfo_to_user() in order to use it within user access blocks.
>>>>>
>>>>> For that, also add an 'unsafe' version of clear_user().
>>>>
>>>> Looking at your use cases you need the 32bit compat version of this
>>>> as well.
>>>>
>>>> The 32bit compat version is too complicated to become a macro, so I
>>>> don't think you can make this work correctly for the 32bit compat case.
>>>
>>> When looking into patch 5/5 that you nacked, I think you missed the fact that we
>>> keep using copy_siginfo_to_user32() as it for the 32 bit compat case.
>>
>> I did.  My mistake.
>>
>> However that mistake was so easy I think it mirrors the comments others
>> have made that this looks like a maintenance hazard.
>>
>> Is improving the performance of 32bit kernels interesting?
>
> Yes it is, and that's what this series do.
>
>> Is improving the performance of 32bit compat support interesting?
>
> For me this is a corner case, so I left it aside for now.
>
>>
>> If performance one or either of those cases is interesting it looks like
>> we already have copy_siginfo_to_external32 the factor you would need
>> to build unsafe_copy_siginfo_to_user32.
>
> I'm not sure I understand your saying here. What do you expect me to
> do with copy_siginfo_to_external32() ?

Implement unsafe_copy_siginfo_to_user32.

> copy_siginfo_to_user32() is for compat only.
>
> Native 32 bits powerpc use copy_siginfo_to_user()

What you implemented doubles the number of test cases necessary to
compile test the 32bit ppc signal code, and makes the code noticeably
harder to follow.

Having a unsafe_copy_to_siginfo_to_user32 at least would allow the
number of test cases to remain the same as the current code.

>> So I am not going to say impossible but please make something
>> maintainable.  I unified all of the compat 32bit siginfo logic because
>> it simply did not get enough love and attention when it was implemented
>> per architecture.
>
> Yes, and ? I didn't do any modification to the compat case, so what
> you did remains.

You undid the unification between the 32bit code and the 32bit compat
code.

>> In general I think that concern applies to this case as well.  We really
>> need an implementation that shares as much burden as possible with other
>> architectures.
>
> I think yes, that's the reason why I made a generic
> unsafe_copy_siginfo_to_user() and didn't make a powerpc dedicated
> change.
>
> Once this is merged any other architecture can use
> unsafe_copy_siginfo_to_user().
>
> Did I miss something ?

Not dealing with the compat case and making the code signal stack frame
code noticeably more complicated.

If this optimization profitably applies to other architectures we need
to figure out how to implement unsafe_copy_siginfo_to_user32 or risk
making them all much worse to maintain.

Eric