linux-kernel - RE: [PATCH v2 24/39] x86/cet/shstk: Add user-mode shadow stack support

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Message-ID: <b1d59744f54743a4a8131787580d181a@AcuMS.aculab.com>
Date:   Wed, 5 Oct 2022 13:32:53 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Kees Cook' <keescook@...omium.org>
CC:     'Dave Hansen' <dave.hansen@...el.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
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        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
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        Florian Weimer <fweimer@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
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        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
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        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: RE: [PATCH v2 24/39] x86/cet/shstk: Add user-mode shadow stack
 support

From: Kees Cook
> Sent: 04 October 2022 20:32
...
> Oh, yes! I do this all the time with FORTIFY shenanigans. Right, so,
> instead of a macro, the "cannot be un-inlined" could be enforced with
> this (untested):
> 
> static __always_inline void set_clr_bits_msrl(u32 msr, u64 set, u64 clear)
> {
> 	u64 val, new_val;
> 
> 	BUILD_BUG_ON(!__builtin_constant_p(msr) ||
> 		     !__builtin_constant_p(set) ||
> 		     !__builtin_constant_p(clear));

You can reduce the amount of text the brain has to parse
by using:

	BUILD_BUG_ON(!__builtin_constant_p(msr + set + clear));

Just requires the brain to do a quick 'oh yes'...

	David

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