On Fri, Jan 16, 2015 at 02:30:25PM +0100, Michael Kerrisk (man-pages) wrote:
[..]
>
Hi Michael,
Please find my responses below. Sorry, I got stuck in other work and
forgot about this thread.
> So, returning to the kexeec_segment structure:
>
> struct kexec_segment {
> void *buf; /* Buffer in user space */
> size_t bufsz; /* Buffer length in user space */
> void *mem; /* Physical address of kernel */
> size_t memsz; /* Physical address length */
> };
>
> Are the following statements correct:
> * buf + bufsz identify a memory region in the caller's virtual
> address space that is the source of the copy
Yes.
> * mem + memsz specify the target memory region of the copy
Yes.
> * mem is physical memory address, as seen from kernel space
Yes.
> * the number of bytes copied from userspace is min(bufsz, memsz)
Yes. bufsz can not be more than memsz. There is a check to validate
this in kernel.
result = -EINVAL;
for (i = 0; i < nr_segments; i++) {
if (image->segment[i].bufsz > image->segment[i].memsz)
return result;
}
> * if bufsz > memsz, then excess bytes in the user-space buffer
> are ignored.
You will get -EINVAL.
> * if memsz > bufsz, then excess bytes in the target kernel buffer
> are filled with zeros.
Yes.
> Also, it seems to me that 'mem' need not be page aligned.
> Is that correct? Should the man page say something about that?
> (E.g., is it generally desirable that 'mem' should be page aligned?)
mem and memsz need to be page aligned. There is a check for that too.
mstart = image->segment[i].mem;
mend = mstart + image->segment[i].memsz;
if ((mstart & ~PAGE_MASK) || (mend & ~PAGE_MASK))
return result;
>
> Likewise, 'memsz' doesn't need to beta page multiple, IIUC.
> Should the man page say anything about this? For example, should
> it note that the initialized kernel segment will be of size:
>
> (mem % PAGE_SIZE + memsz) rounded up to the next multiple of PAGE_SIZE
>
> And should it note that if 'mem' is not a multiple of the page size, then
> the initial bytes (mem % PAGE_SIZE)) in the first page of the kernel segment
> will be zeros?
>
> (Hopefully I have read kimage_load_normal_segment() correctly.)
Both mem and memsz need to be page aligned.
>
> And one further question. Other than the fact that they are used with
> different system calls, what is the difference between KEXEC_ON_CRASH
> and KEXEC_FILE_ON_CRASH?
Right now I can't think of any other difference. They both tell respective
system call that this kernel needs to be loaded in reserved memory region
for crash kernel.
Thanks
Vivek
