Re: [PATCH v2 seccomp 2/6] asm/syscall.h: Add syscall_arches[] array

[YiFei Zhu]

On Thu, Sep 24, 2020 at 10:09 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> Right, sorry, I may not have been clear. When building my RFC I noticed
> that I couldn't use NR_syscall very "early" in the header file include
> stack on arm64, which complicated things. So I guess what I mean is
> something like "it's probably better to do all these seccomp-specific
> macros/etc in asm/include/seccomp.h rather than in syscall.h because I
> know at least one architecture that might cause trouble."

Ah. Makes sense.

> Ironicailly, that's the only place I actually know for sure where people
> using x32 because it shows measurable (10%) speed-up for builders:
> https://lore.kernel.org/lkml/CAOesGMgu1i3p7XMZuCEtj63T-ST_jh+BfaHy-K6LhgqNriKHAA@xxxxxxxxxxxxxx

Wow. 10% is significant. Makes you wonder why x32 hasn't conquered the world.

> So, yes, as you and Jann both point out, it wouldn't be terrible to just
> ignore x32, it seems a shame to penalize it. That said, if the masking
> step from my v1 is actually noticable on a native workload, then yeah,
> probably x32 should be ignored. My instinct (not measured) is that it's
> faster than walking a small array.[citation needed]

My instinct: should be pretty similar, with the loop unrolled.

You convince me that penalizing supporting x32 would be a pity :( The
10% is so nice I want it.

> It's easier to do a per-arch revert (i.e. all the -stable tree
> machinery, etc) with a single SHA instead of having to write a partial
> revert, etc.

I see. Thanks for clarifying.

How about this? Rather than specifically designing names for bitmasks
(native, compat, multiplex), just have SECCOMP_ARCH_{1,2,3}? Each arch
number would provide the size of the bitmap and a static inline
function to check the given seccomp_data belongs to the arch and if
so, the order of the bit in the bitmap. There is no need for the
shifts and madness in seccomp.c; it's arch-dependent code in their own
seccomp.h. We let the preprocessor and compiler to make things
optimized.

YiFei Zhu