Patchwork [4/8] block: fix sector comparism in multiwrite_req_compare

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Submitter Kevin Wolf
Date May 20, 2010, 1:10 p.m.
Message ID <1274361043-25862-5-git-send-email-kwolf@redhat.com>
Download mbox | patch
Permalink /patch/53063/
State New
Headers show

Comments

Kevin Wolf - May 20, 2010, 1:10 p.m.
From: Christoph Hellwig <hch@lst.de>

The difference between the start sectors of two requests can be larger
than the size of the "int" type, which can lead to a not correctly
sorted multiwrite array and thus spurious I/O errors and filesystem
corruption due to incorrect request merges.

So instead of doing the cute sector arithmetics trick spell out the
exact comparisms.

Spotted by Kevin Wolf based on a testcase from Michael Tokarev.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block.c |   14 +++++++++++++-
 1 files changed, 13 insertions(+), 1 deletions(-)

Patch

diff --git a/block.c b/block.c
index bfe46e3..89eece7 100644
--- a/block.c
+++ b/block.c
@@ -1929,7 +1929,19 @@  static void multiwrite_cb(void *opaque, int ret)
 
 static int multiwrite_req_compare(const void *a, const void *b)
 {
-    return (((BlockRequest*) a)->sector - ((BlockRequest*) b)->sector);
+    const BlockRequest *req1 = a, *req2 = b;
+
+    /*
+     * Note that we can't simply subtract req2->sector from req1->sector
+     * here as that could overflow the return value.
+     */
+    if (req1->sector > req2->sector) {
+        return 1;
+    } else if (req1->sector < req2->sector) {
+        return -1;
+    } else {
+        return 0;
+    }
 }
 
 /*