Hi Vivek,
On 01/27/2015 03:24 PM, Vivek Goyal wrote:
> 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.
Okay.
>> * mem + memsz specify the target memory region of the copy
>
> Yes.
Okay.
>> * mem is physical memory address, as seen from kernel space
>
> Yes.
Okay.
>> * 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;
> }
Okay. So it's more precise to leave discussion of min(bufz, memsz)
out of the man page just to say: bufsz bytes are transferred;
if bufsz < memsz, then the excess bytes in the target region are
filled with zeros. Right?
>> * if bufsz > memsz, then excess bytes in the user-space buffer
>> are ignored.
>
> You will get -EINVAL.
Okay.
>> * if memsz > bufsz, then excess bytes in the target kernel buffer
>> are filled with zeros.
>
> Yes.
Okay.
>> 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 the error if not is EADDRNOTAVAIL, right?
>> 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.
Okay.
I've made various adjustments to the page in the light of your comments
above. Thanks!
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
