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

On Fri, Jul 31, 2015 at 03:54:56PM -0700, Andy Lutomirski wrote:

On Fri, Jul 31, 2015 at 3:08 PM,  [off-list ref] wrote:

quoted

On Fri, Jul 31, 2015 at 02:19:29PM -0700, Andy Lutomirski wrote:

quoted

On Fri, Jul 31, 2015 at 1:59 PM,  [off-list ref] wrote:

quoted

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.

Sure, agreed.  But I really hope we don't create new kernel ABIs that
involve constructs like that.

- Josh Triplett