Re: [RFC] arm64: kexec_file_load support

On 07/05/16 at 05:03pm, AKASHI Takahiro wrote:

Hi Dave,

On Tue, Jul 05, 2016 at 09:25:56AM +0800, Dave Young wrote:

quoted

On 07/04/16 at 03:58pm, AKASHI Takahiro wrote:

quoted

Hi,

On Fri, Jul 01, 2016 at 12:46:31PM -0300, Thiago Jung Bauermann wrote:

quoted

Am Freitag, 01 Juli 2016, 14:11:12 schrieb AKASHI Takahiro:

quoted

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.

quoted

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.

quoted

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.

quoted

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.

quoted

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

quoted

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 

If we take this approach, we'd better take "flags" out of struct,
and my preference would be:

        enum kexec_file_type {
                KEXEC_FILE_TYPE_KERNEL;
                KEXEC_FILE_TYPE_INITRD;
                KEXEC_FILE_TYPE_DTB;
        }

        struct kexec_file_fd {
                enum kexec_file_type;
                int fd;
        }

        sturct kexec_file_params {
                int version;
                unsigned char *cmdline;
                unsigned long cmdline_len;
                int nr_fds;
                struct kexec_file_fd fds[0];
        }

        int kexec_file_load2(int kernel_fd, unsigned long flags,
                                sturct kexec_file_params extra);

So we don't have to retrieve extra if KEXEC_FILE_UNLOAD
(or kernel_fd < 0?),
and only once retrieve extra if extra != NULL && nr_fds == 0.

If so maybe change a bit from your precious mentioned 7 args proposal like
below?

struct kexec_file_fd {
	enum kexec_file_type;
	int fd;
}

struct kexec_fdset {
	int nr_fd;
	struct kexec_file_fd fd[0];
}

int kexec_file_load(int kernel_fd, int initrd_fd,
		    unsigned long cmdline_len, const char *cmdline_ptr,
		    unsigned long flags, struct kexec_fdset *extra_fds);

Thanks
Dave