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

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Date:	Tue, 19 Jul 2016 08:55:56 +0800
From:	Dave Young <dyoung@...hat.com>
To:	Mark Rutland <mark.rutland@....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 07/18/16 at 11:07am, Mark Rutland wrote:
> 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.

I'm not sure kexec_load and kexec_file_load were included in glibc, we use
syscall directly in kexec-tools.

kexec_load man pages says there are no wrappers for both kexec_load and
kexec_file_load in glibc.

> 
> 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.

Hmm, as I replied to Vivek, there is one case about the flags, previously
the new flag will be regarded as invalid, but not we extend it it will be
valid, this maybe the only potential bad case.

Thanks
Dave