erofs: fix partially uninitialized misuse in z_erofs_onlinepage_fixup

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-and-tested-by: Hongyu Jin <hongyu.jin@unisoc.com>
Fixes: 3883a79abd02 ("staging: erofs: introduce VLE decompression support")
Cc: <stable@vger.kernel.org> # 4.19+
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Link: https://lore.kernel.org/r/20200618234349.22553-1-hsiangkao@aol.com
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>

diff --git a/fs/erofs/zdata.h b/fs/erofs/zdata.h
index 7824f55..9b66c28 100644 (file)
--- 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;
 }