[PATCH v2] erofs: fix partially uninitialized misuse in z_erofs_onlinepage_fixup

[Gao Xiang]

From: Gao Xiang <hsiangkao@xxxxxxxxxx>

Hongyu reported "id != index" in z_erofs_onlinepage_fixup() with
specific aarch64 environment easily, which wasn't shown before.

After digging into that, I found that high 32 bits of page->private
was set to 0xaaaaaaaa rather than 0 (due to z_erofs_onlinepage_init
behavior with specific compiler options). Actually we only use low
32 bits to keep the page information since page->private is only 4
bytes on most 32-bit platforms. However z_erofs_onlinepage_fixup()
uses the upper 32 bits by mistake.

Let's fix it now.

Reported-by: Hongyu Jin <hongyu.jin@xxxxxxxxxx>
Fixes: 3883a79abd02 ("staging: erofs: introduce VLE decompression support")
Cc: <stable@xxxxxxxxxxxxxxx> # 4.19+
Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxx>
---
change since v1:
 move .v assignment out since it doesn't need for every loop;

 fs/erofs/zdata.h | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/fs/erofs/zdata.h b/fs/erofs/zdata.h
index 7824f5563a55..9b66c28b3ae9 100644
--- a/fs/erofs/zdata.h
+++ b/fs/erofs/zdata.h
@@ -144,22 +144,22 @@ static inline void z_erofs_onlinepage_init(struct page *page)
 static inline void z_erofs_onlinepage_fixup(struct page *page,
 	uintptr_t index, bool down)
 {
-	unsigned long *p, o, v, id;
-repeat:
-	p = &page_private(page);
-	o = READ_ONCE(*p);
+	union z_erofs_onlinepage_converter u = { .v = &page_private(page) };
+	int orig, orig_index, val;
 
-	id = o >> Z_EROFS_ONLINEPAGE_INDEX_SHIFT;
-	if (id) {
+repeat:
+	orig = atomic_read(u.o);
+	orig_index = orig >> Z_EROFS_ONLINEPAGE_INDEX_SHIFT;
+	if (orig_index) {
 		if (!index)
 			return;
 
-		DBG_BUGON(id != index);
+		DBG_BUGON(orig_index != index);
 	}
 
-	v = (index << Z_EROFS_ONLINEPAGE_INDEX_SHIFT) |
-		((o & Z_EROFS_ONLINEPAGE_COUNT_MASK) + (unsigned int)down);
-	if (cmpxchg(p, o, v) != o)
+	val = (index << Z_EROFS_ONLINEPAGE_INDEX_SHIFT) |
+		((orig & Z_EROFS_ONLINEPAGE_COUNT_MASK) + (unsigned int)down);
+	if (atomic_cmpxchg(u.o, orig, val) != orig)
 		goto repeat;
 }
 
-- 
2.24.0