RE: [RESEND PATCH v4 8/8] arm64: Allow 64-bit tasks to invoke compat syscalls

From: David Laight <David.Laight@ACULAB.COM>
To: 'Arnd Bergmann' <arnd@kernel.org>, Amanieu d'Antras <amanieu@gmail.com>
Cc: Ryan Houdek <Houdek.Ryan@fex-emu.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Steven Price <steven.price@arm.com>,
	Mark Brown <broonie@kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: RE: [RESEND PATCH v4 8/8] arm64: Allow 64-bit tasks to invoke compat syscalls
Date: Tue, 18 May 2021 20:26:12 +0000	[thread overview]
Message-ID: <3cb1d369e5e8431284e527e3e74fa6f2@AcuMS.aculab.com> (raw)
In-Reply-To: <CAK8P3a0=iSUBu5GnuWoxEjB0Hpd3iHeVwe2Njfj6x64hoJA5oA@mail.gmail.com>

From: Arnd Bergmann
> Sent: 18 May 2021 14:02
...
> 
> I'm still undecided about this approach. It is an easy way to expose the 32-bit
> ABIs, it mostly copies what x86-64 already does with 32-bit syscalls and
> it doesn't expose a lot of attack surface that isn't already exposed to normal
> 32-bit tasks running compat mode.
> 
> On the other hand, exposing the entire aarch32 syscall set seems both
> too broad and not broad enough: Half of the system calls behave the
> exact same way in native and compat mode, so they wouldn't need to
> be exposed like this, a lot of others are trivially emulated in user space
> by calling the native versions. The syscalls that are actually hard to do
> such as ioctl() or the signal handling will work for aarch32 emulation, but
> they are still insufficient to correctly emulate other 32-bit architectures
> that have a slightly different ABI. This means the interface is a fairly good
> fit for Tango, but much less so for FEX.

To my mind because the kernel already contains the emulation code
there isn't much point trying to replicate it in userspace.

OTOH I think they are trying to emulate x86 system calls not arm ones.
So the structure layouts don't always match.
However it is probably a lot nearer than the 64bit arm.

Whether including some of the 'x32' code in an arm kernel will
help is another matter - it might be a useful source of differences.

Am I also right in thinking that this isn't actually needed as part
of a 'generic' ARM kernel? Just ones for some specific platforms?

	David

(Oh - I'm not involved in the project and will probably never use it.)

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