linux-kernel - Re: [PATCH v2 3/5] signal: Add unsafe_copy_siginfo_to

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Message-ID: <672b16d1-6825-16a3-8270-33f638d42997@csgroup.eu>
Date:   Mon, 13 Sep 2021 14:56:25 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 3/5] signal: Add unsafe_copy_siginfo_to_user()



Le 11/09/2021 à 17:58, Eric W. Biederman a écrit :
> Christophe Leroy <christophe.leroy@...roup.eu> writes:
> 
>> On 9/8/21 6:17 PM, Eric W. Biederman wrote:
>>> Christophe Leroy <christophe.leroy@...roup.eu> writes:
>>>
>>>> Le 02/09/2021 à 20:43, Eric W. Biederman a écrit :
>>>>> Christophe Leroy <christophe.leroy@...roup.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.

Ok, initialy I thought it would be a too big job but finaly that's not 
so big.


> 
>> 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.

Yes and no.

We already have a different copy_siginfo_to_user() for compat and for 
native, why would anything be doubled ?

I agree it makes the code harder to follow though

> 
> 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.

Not sure I follow you here, but regardless I have sent a v3 which 
tentatively implements copy_siginfo_to_user32() for the compat case.

> 
>>> 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.
> 



Ok, let's see what you think about v3.

Thanks for you feedback
Christophe