linux-kernel - Re: [PATCHv3 1/1] Documentation: describe how to add a system call

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Message-ID: <CAHse=S_JkMZsXZxK-gWHKTxy8d+YsrO3Ehm0v27GADoV+3JHwA@mail.gmail.com>
Date:	Mon, 10 Aug 2015 08:43:21 +0100
From:	David Drysdale <drysdale@...gle.com>
To:	Pavel Machek <pavel@....cz>
Cc:	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>,
	Cyril Hrubis <chrubis@...e.cz>,
	Josh Triplett <josh@...htriplett.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>,
	Andy Lutomirski <luto@...capital.net>,
	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>, Kees Cook <keescook@...omium.org>,
	Milosz Tanski <milosz@...in.com>, Fam Zheng <famz@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-doc@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv3 1/1] Documentation: describe how to add a system call

On Wed, Aug 5, 2015 at 5:21 PM, Pavel Machek <pavel@....cz> wrote:
> Hi!
>
>> Add a document describing the process of adding a new system call,
>> including the need for a flags argument for future compatibility, and
>> covering 32-bit/64-bit concerns (albeit in an x86-centric way).
>>
>> Signed-off-by: David Drysdale <drysdale@...gle.com>
>> Reviewed-by: Michael Kerrisk <mtk.manpages@...il.com>
>> Reviewed-by: Eric B Munson <emunson@...mai.com>
>> Reviewed-by: Kees Cook <keescook@...omium.org>
>> Reviewed-by: Randy Dunlap <rdunlap@...radead.org>
>> ---
>>  Documentation/adding-syscalls.txt | 531 ++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 531 insertions(+)
>>  create mode 100644 Documentation/adding-syscalls.txt
>
> We usually align documentation to less than 80 columns, afaict.

Ah, a few re-worded paragraphs crept over the limit.  Reflowed.

>> +call.  To make sure that userspace programs can safely use flags between kernel
>> +versions, check whether the flags value holds any unknown flags, and reject the
>> +sycall (with EINVAL) if it does:
>
> syscall?

Fixed, thanks.

>> +New system call proposals, like any change to the kernel's API, should always
>> +be cc'ed to linux-api@...r.kernel.org
>
> . at and of sentence?

Fixed.

>> +System Calls Returning Elsewhere
>> +--------------------------------
>> +
>> +For most system calls, once the system call is complete the user program
>> +continues exactly where it left off -- at the next instruction, with the same
>> +stack and registers as before the system call, and with the same virtual
>> +memory space.
>
> Umm. Normally we place return value in register. And I'm not sure.. do
> syscalls preserve registers that are normally caller-clobbered in the ABI?

Reworded to:

For most system calls, once the system call is complete the user program
continues exactly where it left off -- at the next instruction, with the
stack the same and most of the registers the same as before the system call,
and with the same virtual memory space.

> Thanks,
>                                                                         Pavel

Thanks for the updates,
David

> --
> (english) http://www.livejournal.com/~pavelmachek
> (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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