[PATCH] kexec: should use uchunk for user buffer increasing

[yang.zhang]

Thanks, i would post v2 patch.
Could you please provide the email address for andrew.









At 2024-02-19 10:38:22, "Baoquan He" <bhe at redhat.com> wrote:
>On 02/19/24 at 10:00am, yang.zhang wrote:
>> 
>> 
>> 
>> Thanks for your replies.
>> Do you have plans to merge the improving code for clarity, or just keep them unchanged.
>
>You need post v2 to change those two places as Eric has demonstrated.
>Please CC Andrew when you post.
>
>> 
>> At 2024-02-05 20:27:33, "Eric W. Biederman" <ebiederm at xmission.com> wrote:
>> >Baoquan He <bhe at redhat.com> writes:
>> >
>> >> On 01/30/24 at 06:18pm, yang.zhang wrote:
>> >>> From: "yang.zhang" <yang.zhang at hexintek.com>
>> >>> 
>> >>> 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