[PATCH v3] kexec_core: Remove superfluous page offset handling in segment loading

[Justinien Bouron <jbouron@amazon.com>]

During kexec_segment loading, when copying the content of the segment
(i.e. kexec_segment::kbuf or kexec_segment::buf) to its associated
pages, kimage_load_{cma,normal,crash}_segment handle the case where the
physical address of the segment is not page aligned, e.g. in
kimage_load_normal_segment:
```
	page = kimage_alloc_page(image, GFP_HIGHUSER, maddr);
	// ...
	ptr = kmap_local_page(page);
	// ...
	ptr += maddr & ~PAGE_MASK;
	mchunk = min_t(size_t, mbytes,
		PAGE_SIZE - (maddr & ~PAGE_MASK));
	// ^^^^ Non page-aligned segments handled here ^^^
	// ...
	if (image->file_mode)
		memcpy(ptr, kbuf, uchunk);
	else
		result = copy_from_user(ptr, buf, uchunk);
```
(similar logic is present in kimage_load_{cma,crash}_segment).

This is actually not needed because, prior to their loading, all
kexec_segments first go through a vetting step in
`sanity_check_segment_list`, which rejects any segment that is not
page-aligned:
```
	for (i = 0; i < nr_segments; i++) {
		unsigned long mstart, mend;
		mstart = image->segment[i].mem;
		mend   = mstart + image->segment[i].memsz;
		// ...
		if ((mstart & ~PAGE_MASK) || (mend & ~PAGE_MASK))
			return -EADDRNOTAVAIL;
		// ...
	}
```
In case `sanity_check_segment_list` finds a non-page aligned the whole
kexec load is aborted and no segment is loaded.

This means that `kimage_load_{cma,normal,crash}_segment` never actually
have to handle non page-aligned segments and `(maddr & ~PAGE_MASK) == 0`
is always true no matter if the segment is coming from a file (i.e.
`kexec_file_load` syscall), from a user-space buffer (i.e. `kexec_load`
syscall) or created by the kernel through `kexec_add_buffer`. In the
latter case, `kexec_add_buffer` actually enforces the page alignment:
```
	/* Ensure minimum alignment needed for segments. */
	kbuf->memsz = ALIGN(kbuf->memsz, PAGE_SIZE);
	kbuf->buf_align = max(kbuf->buf_align, PAGE_SIZE);
```

Signed-off-by: Justinien Bouron <jbouron@amazon.com>
Reviewed-by: Gunnar Kudrjavets <gunnarku@amazon.com>
---
Changes since v1:
	- Reworked commit message as requested by Baoquan He
	  <bhe@redhat.com>
	- Removed accidental whitespace change
	- v1 Link: https://lore.kernel.org/lkml/20250910163116.49148-1-jbouron@amazon.com/

Changes since v2:
	- Removed unused variable in kimage_load_cma_segment() which was
	  causing a warning and failing build with `make W=1`. Thanks
	  Andy Shevchenko for finding this issue
	- v2 Link: https://lore.kernel.org/lkml/20250929160220.47616-1-jbouron@amazon.com/
---
 kernel/kexec_core.c | 15 +++------------
 1 file changed, 3 insertions(+), 12 deletions(-)

diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index fa00b239c5d9..5ed7a2383d5d 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -742,7 +742,6 @@ static int kimage_load_cma_segment(struct kimage *image, int idx)
 	struct kexec_segment *segment = &image->segment[idx];
 	struct page *cma = image->segment_cma[idx];
 	char *ptr = page_address(cma);
-	unsigned long maddr;
 	size_t ubytes, mbytes;
 	int result = 0;
 	unsigned char __user *buf = NULL;
@@ -754,15 +753,12 @@ static int kimage_load_cma_segment(struct kimage *image, int idx)
 		buf = segment->buf;
 	ubytes = segment->bufsz;
 	mbytes = segment->memsz;
-	maddr = segment->mem;
 
 	/* Then copy from source buffer to the CMA one */
 	while (mbytes) {
 		size_t uchunk, mchunk;
 
-		ptr += maddr & ~PAGE_MASK;
-		mchunk = min_t(size_t, mbytes,
-				PAGE_SIZE - (maddr & ~PAGE_MASK));
+		mchunk = min_t(size_t, mbytes, PAGE_SIZE);
 		uchunk = min(ubytes, mchunk);
 
 		if (uchunk) {
@@ -784,7 +780,6 @@ static int kimage_load_cma_segment(struct kimage *image, int idx)
 		}
 
 		ptr    += mchunk;
-		maddr  += mchunk;
 		mbytes -= mchunk;
 
 		cond_resched();
@@ -839,9 +834,7 @@ static int kimage_load_normal_segment(struct kimage *image, int idx)
 		ptr = kmap_local_page(page);
 		/* Start with a clear page */
 		clear_page(ptr);
-		ptr += maddr & ~PAGE_MASK;
-		mchunk = min_t(size_t, mbytes,
-				PAGE_SIZE - (maddr & ~PAGE_MASK));
+		mchunk = min_t(size_t, mbytes, PAGE_SIZE);
 		uchunk = min(ubytes, mchunk);
 
 		if (uchunk) {
@@ -904,9 +897,7 @@ static int kimage_load_crash_segment(struct kimage *image, int idx)
 		}
 		arch_kexec_post_alloc_pages(page_address(page), 1, 0);
 		ptr = kmap_local_page(page);
-		ptr += maddr & ~PAGE_MASK;
-		mchunk = min_t(size_t, mbytes,
-				PAGE_SIZE - (maddr & ~PAGE_MASK));
+		mchunk = min_t(size_t, mbytes, PAGE_SIZE);
 		uchunk = min(ubytes, mchunk);
 		if (mchunk > uchunk) {
 			/* Zero the trailing part of the page */
-- 
2.43.0