linux-kernel - Re: [RFC] arm64: kexec_file

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Message-ID: <20160705012556.GC8190@dhcp-128-65.nay.redhat.com>
Date:	Tue, 5 Jul 2016 09:25:56 +0800
From:	Dave Young <dyoung@...hat.com>
To:	AKASHI Takahiro <takahiro.akashi@...aro.org>,
	Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com>,
	kexec@...ts.infradead.org, ebiederm@...ssion.com, bhe@...hat.com,
	vgoyal@...hat.com, will.deacon@....com, catalin.marinas@....com,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC] arm64: kexec_file_load support

On 07/04/16 at 03:58pm, AKASHI Takahiro wrote:
> Hi,
> 
> On Fri, Jul 01, 2016 at 12:46:31PM -0300, Thiago Jung Bauermann wrote:
> > Am Freitag, 01 Juli 2016, 14:11:12 schrieb AKASHI Takahiro:
> > > I'm not sure whether there is any demand for kexec_file_load
> > > support on arm64, but anyhow I'm working on this and now
> > > my early prototype code does work fine.
> > 
> > It is necessary if you want to support loading only signed kernels, and also 
> > if you want IMA to measure the kernel in its event log.
> > 
> > > There is, however, one essential issue:
> > > While arm64 kernel requires a device tree blob to be set up
> > > correctly at boot time, the current system call API doesn't
> > > have this parameter.
> > >     int kexec_file_load(int kernel_fd, int initrd_fd,
> > >                         unsigned long cmdline_len, const char
> > > *cmdline_ptr, unsigned long flags);
> > > 
> > > Should we invent a new system call, like kexec_file_load2,
> > > and, if so, what kind of interface would be desired?
> > 
> > I'm facing the same issue on powerpc. What I'm doing is taking the device 
> > tree that was used to boot the current kernel and modifying it as necessary 
> > to pass it to the next kernel.
> 
> That is exactly what I do.
> 
> > I agree that it would be better if we could have a system call where a 
> > custom device tree could be passed. One suggestion is:
> 
> For powerpc, you might be able to use dtbImage instead of Image
> without changing the kernel interfaces.
> > 
> > kexec_file_load2(int fds[], int fd_types[], int nr_fds,
> > 		 unsigned long cmdline_len, const char *cmdline_ptr,
> > 		unsigned long flags);
> 
> You don't want to simply add one more argument, i.e. dtb_fd, don't you.
> 
> I prefer a slightly-simpler interface:
>         struct kexec_file_fd {
>                 enum kexec_file_type;
>                 int fd;
>         }
> 
>         int kexec_file_load2(struct kexec_file_fd[], int nr_fds, int flags);
> 
> Or if you want to keep the compatibility with the existing system call,
> 
>         int kexec_file_load(int kernel_fd, int initrd_fd,
>                         unsigned long cmdline_len, const char *cmdline_ptr,
>                         unsigned long flags,
>                         int struct kexec_file_fd[], int nr_fds);
> 
> Here SYSCALL_DEFINE7() have to be defined, and I'm not sure that we will not
> have a problem in adding a system call with more than 6 arguments.
> 
> > Where fds is an array with nr_fds file descriptors and fd_types is an array 
> > specifying what each fd in fds is. So for example, if fds[i] is the kernel, 
> > then fd_types[i] would have the value KEXEC_FILE_KERNEL_FD. If fds[i] is the 
> > device tree blob, fd_types[i], would have the value KEXEC_FILE_DTB and so 
> > on. That way, the syscall can be extended for an arbitrary number and types 
> > of segments that have to be loaded, just like kexec_load.
> > 
> > Another option is to have a struct:
> > 
> > kexec_file_load2(struct kexec_file_params *params, unsigned long params_sz);
> 
> Wow, we can add any number of new parameters with this interface.
> 
> Thanks,
> -Takahiro AKASHI
> 
> > Where:
> > 
> > struct kexec_file_params {
> > 	int version;	/* allows struct to be extended in the future */
> > 	int fds[];
> > 	int fd_types[];
> > 	int nr_fds;
> > 	unsigned long cmdline_len;
> > 	const char *cmdline_ptr;
> > 	unsigned long flags;
> > };
> > 
> > This is even more flexible.

I would like to vote for this one, and use kexec_file_fd fds[] in the struct 
 
Thanks
Dave