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Date:   Fri, 15 Jul 2022 16:03:47 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Guenter Roeck <linux@roeck-us.net>,
        Peter Zijlstra <peterz@infradead.org>,
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Subject: Re: [PATCH 5.15 00/78] 5.15.55-rc1 review
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On Thu, Jul 14, 2022 at 01:39:25PM -0700, Linus Torvalds wrote:
> On Thu, Jul 14, 2022 at 10:02 AM Borislav Petkov <bp@alien8.de> wrote:
> >
> > On Thu, Jul 14, 2022 at 09:51:40AM -0700, Linus Torvalds wrote:
> > > Oh, absolutely. Doing an -rc7 is normal.
> >
> > Good. I'm gathering all the fallout fixes and will send them to you on
> > Sunday, if nothing unexpected happens.
> 
> Btw, I assume that includes the clang fix for the
> x86_spec_ctrl_current section attribute.

Yap. Here's the current lineup:

https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/log/?h=x86/urgent

> That's kind of personally embarrassing that it slipped through: I do
> all my normal test builds that I actually *boot* with clang.
> 
> But since I kept all of the embargoed stuff outside my normal trees,
> it also meant that the test builds I did didn't have my "this is my
> clang tree" stuff in it.
> 
> And so I - like apparently everybody else - only did those builds with gcc.
> 
> And gcc for some reason doesn't care about this whole "you redeclared
> that variable with a different attribute" thing.

... so why does clang care? Or, why doesn't gcc care?

I guess I need to talk to gcc folks again.

> In the 'x86_spec_ctrl_current' case, that nonsensical code _worked_
> (with gcc), because despite the declaration being for a regular
> variable, the actual definition was in the proper segment.

I'm guessing this is the reason why gcc doesn't fail - it probably looks
at the declaration but doesn't care too much about it. And it is the
definition that matters.

While clang goes, uh, ah, declaration and definition mismatch, I better
warn.

> But that 'myvariable' thing above does end up being another example of
> how we are clearly missing some type checkng in this area.
> 
> I'm not sure if there's any way to get that section mismatch at
> compile-time at all.

Well, apparently, clang can:

arch/x86/kernel/cpu/bugs.c:58:21: error: section attribute is specified on redeclared variable [-Werror,-Wsection]

so there's a -Wsection warning which gcc could implement too.

> For the static declarations, we could just make DECLARE_PER_CPU() add
> some prefix/postfix to the name (and obviously then do it at use time
> too).
>
> We have that '__pcpu_scope_##name' thing to make sure of globally
> unique naming due to the whole weak type thing. I wonder if we could
> do something similar to verify that "yes, this has been declared as a
> percpu variable" at use time?

But how?

We need to save the info how a var has been declared and then use that
info at access time.

Yeah, lemme bother compiler guys a bit...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette