linux-kernel - Re: [PATCH] kexec: should use uchunk for user buffer increasing

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Date: Mon, 19 Feb 2024 10:38:22 +0800
From: Baoquan He <bhe@...hat.com>
To: "yang.zhang" <gaoshanliukou@....com>
Cc: "Eric W. Biederman" <ebiederm@...ssion.com>, kexec@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	"yang.zhang" <yang.zhang@...intek.com>, akpm@...ux-foundation.org
Subject: Re: [PATCH] kexec: should use uchunk for user buffer increasing

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@...ssion.com> wrote:
> >Baoquan He <bhe@...hat.com> writes:
> >
> >> On 01/30/24 at 06:18pm, yang.zhang wrote:
> >>> From: "yang.zhang" <yang.zhang@...intek.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