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Re: CVS commit: src/sys/kern

On 07.11.2019 16:26, Martin Husemann wrote:
> On Thu, Nov 07, 2019 at 02:53:08PM +0100, Kamil Rytarowski wrote:
>> On 07.11.2019 14:25, Valery Ushakov wrote:
>>> If the sanitizer does complain about other uses, there is little point
>>> in fixing one instance and not the others.
>> We already agreed with Christos that this is appeasing of GCC. If you
>> want to scan the whole kernel (or whole C) file for more occurrences of
>> violations - please go for it.
> No. The commit needs to be reverted, and then
>  a) either the root cause for the unaligned address be fixed or
>  b) some other means be found to make the sanitizer shut up
> As uwe said: papering over a tiny detail that *never* hits in the real
> world but potentialy hiding a real issue is not the way to go.

I don't have a readily available reproducer locally but it was breaking
syzbot from booting after the switch to gcc8. I will fix it differently
aligning the whole struct (so the same approach as we use in userland)
and backout this change.

> Martin
> P.S.: Independend of this I would still like an official C standard
> clarification; in my reading a simple address calculation is not
> accessing an object through a pointer (which would be the undefined
> behaviour). If the C standard is not clear on this, it needs to be
> improved.

Unfortunately the C committee went into the opposite direction here and
specified a potential dereference. All I can do now is to add another
exception, &p[x] is allowed and it is not far from &p->x. It is also a
tricky part as some things are unequally documented for p->x and for
(*p).x... so not sure if it is worth trying out really... especially
that offsetof() as defined for this purpose.

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