Re: [RFC][PATCH 21/24] x86/entry: Disable stack-protector for IST entry C handlers

[Ard Biesheuvel <ardb@kernel.org>]

On Sat, 14 Nov 2020 at 02:59, Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:
>
> On Sat, Nov 14, 2020 at 1:27 AM Segher Boessenkool
> <segher@kernel.crashing.org> wrote:
> >
> > But you cannot limit yourself to the greatest common denominator: the
> > kernel wants to use many features only available on newer compiler
> > versions, too; this is no different.
>
> What we do is conditionally enable both new and differentiating
> features. We don't break GCC or LLVM just because the other introduces
> a new shiny feature, nor we break builds with old compilers (except
> when raising the minimum supported version).
>
> That is why we need a level of indirection in things like attributes
> nowadays, and why patches like the above are not ideal anymore
> (regardless of which attribute we are talking about).
>

In spite of the apparent difference of opinion here, there are two
irrefutable facts about __attribute__((optimize)) on GCC that can only
lead to the conclusion that we must never use it in Linux:
- the GCC developers refuse to rigorously define its behavior, so we
don't know what it actually does;
- it has been observed to nullify unrelated command line arguments in
unexpected and undocumented ways.

So it does not matter whether you call that quirky or something else,
there is no way we can make meaningful use of it, and therefore,
abstracting or parameterizing any of its uses should be avoided as
well.