linux-kernel - RE: [PATCH v2 seccomp 2/6] asm/syscall.h: Add syscall

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Date:   Thu, 24 Sep 2020 13:47:54 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'YiFei Zhu' <zhuyifei1999@...il.com>,
        "containers@...ts.linux-foundation.org" 
        <containers@...ts.linux-foundation.org>
CC:     YiFei Zhu <yifeifz2@...inois.edu>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Aleksa Sarai <cyphar@...har.com>,
        "Andrea Arcangeli" <aarcange@...hat.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dimitrios Skarlatos <dskarlat@...cmu.edu>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Hubertus Franke <frankeh@...ibm.com>,
        Jack Chen <jianyan2@...inois.edu>,
        Jann Horn <jannh@...gle.com>,
        Josep Torrellas <torrella@...inois.edu>,
        Kees Cook <keescook@...omium.org>,
        Tianyin Xu <tyxu@...inois.edu>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Tycho Andersen <tycho@...ho.pizza>,
        Valentin Rothberg <vrothber@...hat.com>,
        Will Drewry <wad@...omium.org>
Subject: RE: [PATCH v2 seccomp 2/6] asm/syscall.h: Add syscall_arches[] array

From: YiFei Zhu 
> Sent: 24 September 2020 13:44
> 
> Seccomp cache emulator needs to know all the architecture numbers
> that syscall_get_arch() could return for the kernel build in order
> to generate a cache for all of them.
> 
> The array is declared in header as static __maybe_unused const
> to maximize compiler optimiation opportunities such as loop
> unrolling.

I doubt the compiler will do what you want.
Looking at it, in most cases there are one or two entries.
I think only MIPS has three.

So a static inline function that contains a list of
conditionals will generate better code that any kind of
array lookup.
For x86-64 you end up with something like:

#ifdef CONFIG_IA32_EMULATION
	if (sd->arch == AUDIT_ARCH_I386) return xxx;
#endif
	return yyy;

Probably saves you having multiple arrays that need to be
kept carefully in step.

	David

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