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SubjectRe: [RFC] arm64: kexec_file_load support

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.

-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.
> []'s
> Thiago Jung Bauermann
> IBM Linux Technology Center

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