Re: [PATCH] arch: x86: power: cpu: init %gs before __restore_processor_state (clang)

From: Pavel Machek <pavel@ucw.cz>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Pavel Machek <pavel@denx.de>, Roman Kiryanov <rkir@google.com>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>,
	linux-pm@vger.kernel.org, Greg KH <gregkh@linuxfoundation.org>,
	Alistair Delva <adelva@google.com>,
	Haitao Shan <hshan@google.com>
Subject: Re: [PATCH] arch: x86: power: cpu: init %gs before __restore_processor_state (clang)
Date: Sat, 19 Sep 2020 00:20:06 +0200	[thread overview]
Message-ID: <20200918222006.GA9613@duo.ucw.cz> (raw)
In-Reply-To: <CAKwvOdnuR9GF2tZuyS7Fe8i-S0W2YZ45qP8xJRm_gf+g+oF4=Q@mail.gmail.com>

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Hi!

> > > When load_TR_desc and load_mm_ldt are inlined into
> > > fix_processor_context due to LTO, they cause
> > > fix_processor_context (or in this case __restore_processor_state,
> > > as fix_processor_context was inlined into __restore_processor_state)
> > > to access the stack canary through %gs, but before
> > > __restore_processor_state has restored the previous value
> > > of %gs properly. LLVM appears to be inlining functions with stack
> > > protectors into functions compiled with -fno-stack-protector,
> > > which is likely a bug in LLVM's inliner that needs to be fixed.
> >
> > That's rather ugly.
> >
> > Would it be easier to simply mark those functions as noinline or
> > something like that?
> 
> It is possible, and a possible solution here.  We discussed that
> internally, and found it to not be great.  You only want to prevent
> inlining across TUs for LTO; currently there's no great way to
> express

You should really be doing such discussion publicly.

I believe this is not performance critical so just disabling inlining
(maybe conditional on clang) is best solution.

> compiler.  You could emulate that somewhat by wrapping the noinline
> attribute in CONFIG_LTO_CLANG, but that doesn't solve allowing
> inlining within the same TU.  If you've been following the thread,

Yep, just do that.

BR,
								Pavel

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