linux-kernel - Re: [RFC 3/3] kexec: extend kexec_file

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Date:	Mon, 18 Jul 2016 11:07:05 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Dave Young <dyoung@...hat.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	AKASHI Takahiro <takahiro.akashi@...aro.org>, bhe@...hat.com,
	arnd@...db.de, linuxppc-dev@...ts.ozlabs.org,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
	ebiederm@...ssion.com, bauerman@...ux.vnet.ibm.com,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC 3/3] kexec: extend kexec_file_load system call

On Mon, Jul 18, 2016 at 10:30:24AM +0800, Dave Young wrote:
> On 07/15/16 at 02:19pm, Mark Rutland wrote:
> > On Fri, Jul 15, 2016 at 09:09:55AM -0400, Vivek Goyal wrote:
> > > On Tue, Jul 12, 2016 at 10:42:01AM +0900, AKASHI Takahiro wrote:
> > > 
> > > [..]
> > > > -SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd, int, initrd_fd,
> > > > +SYSCALL_DEFINE6(kexec_file_load, int, kernel_fd, int, initrd_fd,
> > > >  		unsigned long, cmdline_len, const char __user *, cmdline_ptr,
> > > > -		unsigned long, flags)
> > > > +		unsigned long, flags, const struct kexec_fdset __user *, ufdset)
> > > 
> > > Can one add more parameters to existing syscall. Can it break existing
> > > programs with new kernel? I was of the impression that one can't do that.
> > > But may be I am missing something.
> > 
> > I think the idea was that we would only look at the new params if a new
> > flags was set, and otherwise it would behave as the old syscall.
> > 
> > Regardless, I think it makes far more sense to add a kexec_file_load2
> > syscall if we're going to modify the prototype at all. It's a rather
> > different proposition to the existing syscall, and needs to be treated
> > as such.
> 
> I do not think it is worth to add another syscall for extra fds.
> We have open(2) as an example for different numbers of arguments
> already.

Did we change the syscall interface for that?

I was under the impression that there was always one underlying syscall,
and the C library did the right thing to pass the expected information
to the underlying syscall.

That's rather different to changing the underlying syscall.

Regardless of how this is wrapped in userspace, I do not think modifying
the existing prototype is a good idea, and I think this kind of
extension needs to be a new syscall.

Thanks,
Mark.