erofs: ensure the extra temporary copy is valid for shortened bvecs

When compressed data deduplication is enabled, multiple logical extents
may reference the same compressed physical cluster.

The previous commit 94c43de ("erofs: fix wrong primary bvec
selection on deduplicated extents") already avoids using shortened
bvecs.  However, in such cases, the extra temporary buffers also
need to be preserved for later use in z_erofs_fill_other_copies() to
to prevent data corruption.

IOWs, extra temporary buffers have to be retained not only due to
varying start relative offsets (`pageofs_out`, as indicated by
`pcl->multibases`) but also because of shortened bvecs.

android.hardware.graphics.composer@2.1.so : 270696 bytes
   0:        0..  204185 |  204185 :  628019200.. 628084736 |   65536
-> 1:   204185..  225536 |   21351 :  544063488.. 544129024 |   65536
   2:   225536..  270696 |   45160 :          0..         0 |       0

com.android.vndk.v28.apex : 93814897 bytes
...
   364: 53869896..54095257 |  225361 :  543997952.. 544063488 |   65536
-> 365: 54095257..54309344 |  214087 :  544063488.. 544129024 |   65536
   366: 54309344..54514557 |  205213 :  544129024.. 544194560 |   65536
...

Both 204185 and 54095257 have the same start relative offset of 3481,
but the logical page 55 of `android.hardware.graphics.composer@2.1.so`
ranges from 225280 to 229632, forming a shortened bvec [225280, 225536)
that cannot be used for decompressing the range from 54095257 to
54309344 of `com.android.vndk.v28.apex`.

Since `pcl->multibases` is already meaningless, just mark `be->keepxcpy`
on demand for simplicity.

Again, this issue can only lead to data corruption if `-Ededupe` is on.

Fixes: 94c43de ("erofs: fix wrong primary bvec selection on deduplicated extents")
Reviewed-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250506101850.191506-1-hsiangkao@linux.alibaba.com

Author: hsiangkao

fs/erofs/zdata.c

@@ -79,9 +79,6 @@ struct z_erofs_pcluster {
 	/* L: whether partial decompression or not */
 	bool partial;
 
-	/* L: indicate several pageofs_outs or not */
-	bool multibases;
-
 	/* L: whether extra buffer allocations are best-effort */
 	bool besteffort;
 
@@ -1046,8 +1043,6 @@ static int z_erofs_scan_folio(struct z_erofs_frontend *f,
 				break;
 
 			erofs_onlinefolio_split(folio);
-			if (f->pcl->pageofs_out != (map->m_la & ~PAGE_MASK))
-				f->pcl->multibases = true;
 			if (f->pcl->length < offset + end - map->m_la) {
 				f->pcl->length = offset + end - map->m_la;
 				f->pcl->pageofs_out = map->m_la & ~PAGE_MASK;
@@ -1093,7 +1088,6 @@ struct z_erofs_backend {
 	struct page *onstack_pages[Z_EROFS_ONSTACK_PAGES];
 	struct super_block *sb;
 	struct z_erofs_pcluster *pcl;
-
 	/* pages with the longest decompressed length for deduplication */
 	struct page **decompressed_pages;
 	/* pages to keep the compressed data */
@@ -1102,6 +1096,8 @@ struct z_erofs_backend {
 	struct list_head decompressed_secondary_bvecs;
 	struct page **pagepool;
 	unsigned int onstack_used, nr_pages;
+	/* indicate if temporary copies should be preserved for later use */
+	bool keepxcpy;
 };
 
 struct z_erofs_bvec_item {
@@ -1112,18 +1108,20 @@ struct z_erofs_bvec_item {
 static void z_erofs_do_decompressed_bvec(struct z_erofs_backend *be,
 					 struct z_erofs_bvec *bvec)
 {
+	int poff = bvec->offset + be->pcl->pageofs_out;
 	struct z_erofs_bvec_item *item;
-	unsigned int pgnr;
-
-	if (!((bvec->offset + be->pcl->pageofs_out) & ~PAGE_MASK) &&
-	    (bvec->end == PAGE_SIZE ||
-	     bvec->offset + bvec->end == be->pcl->length)) {
-		pgnr = (bvec->offset + be->pcl->pageofs_out) >> PAGE_SHIFT;
-		DBG_BUGON(pgnr >= be->nr_pages);
-		if (!be->decompressed_pages[pgnr]) {
-			be->decompressed_pages[pgnr] = bvec->page;
+	struct page **page;
+
+	if (!(poff & ~PAGE_MASK) && (bvec->end == PAGE_SIZE ||
+			bvec->offset + bvec->end == be->pcl->length)) {
+		DBG_BUGON((poff >> PAGE_SHIFT) >= be->nr_pages);
+		page = be->decompressed_pages + (poff >> PAGE_SHIFT);
+		if (!*page) {
+			*page = bvec->page;
 			return;
 		}
+	} else {
+		be->keepxcpy = true;
 	}
 
 	/* (cold path) one pcluster is requested multiple times */
@@ -1289,7 +1287,7 @@ static int z_erofs_decompress_pcluster(struct z_erofs_backend *be, int err)
 					.alg = pcl->algorithmformat,
 					.inplace_io = overlapped,
 					.partial_decoding = pcl->partial,
-					.fillgaps = pcl->multibases,
+					.fillgaps = be->keepxcpy,
 					.gfp = pcl->besteffort ? GFP_KERNEL :
 						GFP_NOWAIT | __GFP_NORETRY
 				 }, be->pagepool);
@@ -1346,7 +1344,6 @@ static int z_erofs_decompress_pcluster(struct z_erofs_backend *be, int err)
 
 	pcl->length = 0;
 	pcl->partial = true;
-	pcl->multibases = false;
 	pcl->besteffort = false;
 	pcl->bvset.nextpage = NULL;
 	pcl->vcnt = 0;