[PATCH] kexec_core: fix the assignment to kimage->control_page

[PATCH] kexec_core: fix the assignment to kimage->control_page

[Yuntao Wang]

image->control_page represents the starting address for allocating the next
control page, while hole_end represents the address of the last valid byte
of the currently allocated control page.

Therefore, after successfully allocating a control page, image->control_page
should be updated to `hole_end + 1`, rather than hole_end.

Signed-off-by: Yuntao Wang <ytcoode@gmail.com>
---
 kernel/kexec_core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index 2f039a7d9af9..a08031b57a61 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -457,7 +457,7 @@ static struct page *kimage_alloc_crash_control_pages(struct kimage *image,
 		/* If I don't overlap any segments I have found my hole! */
 		if (i == image->nr_segments) {
 			pages = pfn_to_page(hole_start >> PAGE_SHIFT);
-			image->control_page = hole_end;
+			image->control_page = hole_end + 1;
 			break;
 		}
 	}
-- 
2.43.0


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Re: [PATCH] kexec_core: fix the assignment to kimage->control_page

[Andrew Morton]

On Thu, 21 Dec 2023 12:23:08 +0800 Yuntao Wang <ytcoode@gmail.com> wrote:

> image->control_page represents the starting address for allocating the next
> control page, while hole_end represents the address of the last valid byte
> of the currently allocated control page.
> 
> Therefore, after successfully allocating a control page, image->control_page
> should be updated to `hole_end + 1`, rather than hole_end.

Thanks.  Again, please include a description of the userspace-visible effects of the bug.

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Re: [PATCH] kexec_core: fix the assignment to kimage->control_page

[Yuntao Wang]

On Thu, 21 Dec 2023 14:12:58 -0800, Andrew Morton <akpm@linux-foundation.org> wrote:

> On Thu, 21 Dec 2023 12:23:08 +0800 Yuntao Wang <ytcoode@gmail.com> wrote:
> 
> > image->control_page represents the starting address for allocating the next
> > control page, while hole_end represents the address of the last valid byte
> > of the currently allocated control page.
> > 
> > Therefore, after successfully allocating a control page, image->control_page
> > should be updated to `hole_end + 1`, rather than hole_end.
> 
> Thanks.  Again, please include a description of the userspace-visible effects of the bug.

This bug actually does not affect the correctness of allocating control
pages, because image->control_page is currently only used in
kimage_alloc_crash_control_pages(), and this function, when allocating
control pages, will first align image->control_page up to the nearest
`(1 << order) << PAGE_SHIFT` boundary, then use this value as the starting
address of the next control page. This ensures that the newly allocated
control page will use the correct starting address and not overlap with
previously allocated control pages.

Although it does not affect the correctness of the final result, it is
better for us to set image->control_page to the correct value, in case it
might be used elsewhere in the future, potentially causing errors.

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