linux-kernel - [PATCH 4.4 24/86] md/bitmap: md_bitmap_get

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Message-Id: <20201109125022.023292577@linuxfoundation.org>
Date:   Mon,  9 Nov 2020 13:54:31 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Zhao Heming <heming.zhao@...e.com>,
        Song Liu <songliubraving@...com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 4.4 24/86] md/bitmap: md_bitmap_get_counter returns wrong blocks

From: Zhao Heming <heming.zhao@...e.com>

[ Upstream commit d837f7277f56e70d82b3a4a037d744854e62f387 ]

md_bitmap_get_counter() has code:

```
    if (bitmap->bp[page].hijacked ||
        bitmap->bp[page].map == NULL)
        csize = ((sector_t)1) << (bitmap->chunkshift +
                      PAGE_COUNTER_SHIFT - 1);
```

The minus 1 is wrong, this branch should report 2048 bits of space.
With "-1" action, this only report 1024 bit of space.

This bug code returns wrong blocks, but it doesn't inflence bitmap logic:
1. Most callers focus this function return value (the counter of offset),
   not the parameter blocks.
2. The bug is only triggered when hijacked is true or map is NULL.
   the hijacked true condition is very rare.
   the "map == null" only true when array is creating or resizing.
3. Even the caller gets wrong blocks, current code makes caller just to
   call md_bitmap_get_counter() one more time.

Signed-off-by: Zhao Heming <heming.zhao@...e.com>
Signed-off-by: Song Liu <songliubraving@...com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/md/bitmap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
index 391090c455cea..65281f168c6fb 100644
--- a/drivers/md/bitmap.c
+++ b/drivers/md/bitmap.c
@@ -1332,7 +1332,7 @@ __acquires(bitmap->lock)
 	if (bitmap->bp[page].hijacked ||
 	    bitmap->bp[page].map == NULL)
 		csize = ((sector_t)1) << (bitmap->chunkshift +
-					  PAGE_COUNTER_SHIFT - 1);
+					  PAGE_COUNTER_SHIFT);
 	else
 		csize = ((sector_t)1) << bitmap->chunkshift;
 	*blocks = csize - (offset & (csize - 1));
-- 
2.27.0