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

[Gao Xiang]

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

The previous commit 94c43de73521 ("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 at 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 at 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 `keepxcpy`
on demand for simplicity.

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

Fixes: 94c43de73521 ("erofs: fix wrong primary bvec selection on deduplicated extents")
Signed-off-by: Gao Xiang <hsiangkao at linux.alibaba.com>
---
 fs/erofs/zdata.c | 31 ++++++++++++++-----------------
 1 file changed, 14 insertions(+), 17 deletions(-)

diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
index 5c061aaeeb45..b8e6b76c23d5 100644
--- a/fs/erofs/zdata.c
+++ b/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;
-- 
2.43.5