Baoquan He <bhe@xxxxxxxxxx> writes:
> On 01/30/24 at 06:18pm, yang.zhang wrote:
>> From: "yang.zhang" <yang.zhang@xxxxxxxxxxxx>
>>
>> Because of alignment requirement in kexec-tools, there is
>> no problem for user buffer increasing when loading segments.
>> But when coping, the step is uchunk, so we should use uchunk
>> not mchunk.
>
> In theory, ubytes is <= mbytes. So uchunk is always <= mchunk. If ubytes
> is exhausted, while there's still remaining mbytes, then uchunk is 0,
> there's still mchunk stepping forward. If I understand it correctly,
> this is a good catch. Not sure if Eric has comment on this to confirm.
As far as I can read the code the proposed change is a noop.
I agree it is more correct to not advance the pointers we read from,
but since we never read from them after that point it does not
matter.
>
> static int kimage_load_normal_segment(struct kimage *image,
> struct kexec_segment *segment)
> {
> ......
>
> ptr += maddr & ~PAGE_MASK;
> mchunk = min_t(size_t, mbytes,
> PAGE_SIZE - (maddr & ~PAGE_MASK));
> uchunk = min(ubytes, mchunk);
> ......}
If we are going to improve the code for clarity. We probably
want to do something like:
diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index d08fc7b5db97..1a8b8ce6bf15 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -800,22 +800,24 @@ static int kimage_load_normal_segment(struct kimage *image,
PAGE_SIZE - (maddr & ~PAGE_MASK));
uchunk = min(ubytes, mchunk);
- /* For file based kexec, source pages are in kernel memory */
- if (image->file_mode)
- memcpy(ptr, kbuf, uchunk);
- else
- result = copy_from_user(ptr, buf, uchunk);
+ if (uchunk) {
+ /* For file based kexec, source pages are in kernel memory */
+ if (image->file_mode)
+ memcpy(ptr, kbuf, uchunk);
+ else
+ result = copy_from_user(ptr, buf, uchunk);
+ ubytes -= uchunk;
+ if (image->file_mode)
+ kbuf += uchunk;
+ else
+ buf += uchunk;
+ }
kunmap_local(ptr);
if (result) {
result = -EFAULT;
goto out;
}
- ubytes -= uchunk;
maddr += mchunk;
- if (image->file_mode)
- kbuf += mchunk;
- else
- buf += mchunk;
mbytes -= mchunk;
cond_resched();
And make it exceedingly clear that all of the copying and the rest
only happens before uchunk goes to zero. Otherwise we are relying
on a lot of operations becoming noops when uchunk goes to zero.
Eric
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