linux-kernel - Re: [PATCH v2] x86: fix early boot crash on gcc-10

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Date:   Fri, 17 Apr 2020 21:06:07 +0200
From:   Jakub Jelinek <jakub@...hat.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Borislav Petkov <bp@...en8.de>,
        Sergei Trofimovich <slyfox@...too.org>,
        Michael Matz <matz@...e.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH v2] x86: fix early boot crash on gcc-10

On Fri, Apr 17, 2020 at 11:22:25AM -0700, Nick Desaulniers wrote:
> > Sorry, I don't quite follow.  The idea is that an empty asm statement
> > in foo() should prevent foo() from being inlined into bar()?
> 
> s/inlined/tail called/

Yeah.  The thing is, the caller changes the stack protector guard base
value, so at the start of the function it saves a different value then
it compares at the end.  But, the function that it calls at the end
actually doesn't return, so this isn't a problem.
If it is tail called though, the stack protector guard checking is done
before the tail call and it crashes.
If the called function is marked with noreturn attribute or _Noreturn,
at least GCC will also not tail call it and all is fine, but not sure
what LLVM does in that case.

	Jakub