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

[Segher Boessenkool <segher@kernel.crashing.org>]

On Wed, Nov 25, 2020 at 11:56:40AM -0800, Kees Cook wrote:
> On Sat, Nov 14, 2020 at 11:20:17AM +0100, Ard Biesheuvel wrote:
> > 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;

This is because it isn't clear at all what it *should* do, for some
options.  For others it is obvious, and it works just fine for those.

(And we do not rigorously define the behaviour of almost *anything*, not
in the user manual anyway!)

The interface has huge usability problems.  We want to wean people off
of using this attribute.  But claiming all kinds of FUD about it is a
disservice to users: it works fine for where it does work, there is no
reason for people to hurriedly change their code (or change it at all).

> > 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.
> 
> Perhaps it's worth adding this to CodingStyle or deprecated.rst?

Sure!  But please don't claim things that aren't broken are.


Segher