linux-kernel - Re: [PATCHv2 1/1] Documentation: describe how to add a system call

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Message-ID: <CALCETrXwrf--FG+KdbG-oBixhG+odB7ukW_F7CkBGb0KQ7ZSVA@mail.gmail.com>
Date:	Fri, 31 Jul 2015 15:54:56 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	Kees Cook <keescook@...omium.org>,
	David Drysdale <drysdale@...gle.com>,
	Ingo Molnar <mingo@...nel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	Shuah Khan <shuahkh@....samsung.com>,
	Jonathan Corbet <corbet@....net>,
	Eric B Munson <emunson@...mai.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Peter Zijlstra <peterz@...radead.org>,
	Vivek Goyal <vgoyal@...hat.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	David Herrmann <dh.herrmann@...il.com>,
	"Theodore Ts'o" <tytso@....edu>, Milosz Tanski <milosz@...in.com>,
	Fam Zheng <famz@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCHv2 1/1] Documentation: describe how to add a system call

On Fri, Jul 31, 2015 at 3:08 PM,  <josh@...htriplett.org> wrote:
> On Fri, Jul 31, 2015 at 02:19:29PM -0700, Andy Lutomirski wrote:
>> On Fri, Jul 31, 2015 at 1:59 PM,  <josh@...htriplett.org> wrote:
>> > Agreed.  I think the proposal above would be a net improvement, but
>> > ideally you'd want something that's annotated and generates automatic
>> > marshalling code.
>> >
>>
>> I assume this is idle musing.  If, however, we were to actually do
>> this, I'd suggest we seriously consider speaking the Cap'n Proto
>> serialization format.  It's quite nice, it encodes and decodes *very*
>> quickly and, unlike TLV schemes, you don't have to read it in order,
>> making the read-side code less awkward.
>
> That seems like *massive* overkill for a kernel<->userspace syscall
> interface.  I was more thinking about having a few standardized marshal
> types, and incrementally adding more when more patterns show up.  For a
> first pass, just automatically running copy_from_user and
> copy_param_struct on appropriate sets of __user parameters identified as
> such in a structured text file seems quite sufficient.  (Plus
> automatically generating syscalls.h from that.)

If a param struct does the trick, then I agree.  It's when you start
having lists and other variable-size stuff that it gets messier.

For reference, my Cap'n Proto parser (just the basic structure of
messages, not the actual schema bits, although the latter is more or
less just some structs) is about 300 lines of code.  It's arguably
simpler than nlattr, once you throw nesting in.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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