[Qemu-devel] [PATCH] blkverify: Handle overlapping I/O vector buffers

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

When blkverify clones an I/O vector in order to perform mirrored reads
and then compare their contents, it does not take into account the
layout of individual buffers.  It turns out this is important because
guests may issue requests with overlapping buffers and the results
differ depending on how buffers are overlapped.

This patch introduces logic to honor overlap relationships when cloning
I/O vectors.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
---
 block/blkverify.c |   62 +++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 files changed, 60 insertions(+), 2 deletions(-)

diff --git a/block/blkverify.c b/block/blkverify.c
index 97717a6..524b777 100644
--- a/block/blkverify.c
+++ b/block/blkverify.c
@@ -166,17 +166,75 @@ static ssize_t blkverify_iovec_compare(QEMUIOVector *a, QEMUIOVector *b)
     return -1;
 }
 
+typedef struct {
+    int src_index;
+    struct iovec *src_iov;
+    void *dest_base;
+} IOVectorSortElem;
+
+static int sortelem_cmp_src_base(const void *a, const void *b)
+{
+    const IOVectorSortElem *elem_a = a;
+    const IOVectorSortElem *elem_b = b;
+
+    /* Don't overflow */
+    if (elem_a->src_iov->iov_base < elem_b->src_iov->iov_base) {
+        return -1;
+    } else if (elem_a->src_iov->iov_base > elem_b->src_iov->iov_base) {
+        return 1;
+    } else {
+        return 0;
+    }
+}
+
+static int sortelem_cmp_src_index(const void *a, const void *b)
+{
+    const IOVectorSortElem *elem_a = a;
+    const IOVectorSortElem *elem_b = b;
+
+    return elem_a->src_index - elem_b->src_index;
+}
+
 /**
  * Copy contents of I/O vector
+ *
+ * The relative relationships of overlapping iovecs are preserved.  This is
+ * necessary to ensure identical semantics in the cloned I/O vector.
  */
 static void blkverify_iovec_clone(QEMUIOVector *dest, const QEMUIOVector *src,
                                   void *buf)
 {
+    IOVectorSortElem sortelems[src->niov];
+    void *last_end;
     int i;
 
+    /* Sort by source iovecs by base address */
+    for (i = 0; i < src->niov; i++) {
+        sortelems[i].src_index = i;
+        sortelems[i].src_iov = &src->iov[i];
+    }
+    qsort(sortelems, src->niov, sizeof(sortelems[0]), sortelem_cmp_src_base);
+
+    /* Allocate buffer space taking into account overlapping iovecs */
+    last_end = NULL;
+    for (i = 0; i < src->niov; i++) {
+        struct iovec *cur = sortelems[i].src_iov;
+        ptrdiff_t rewind = 0;
+
+        /* Detect overlap */
+        if (last_end && last_end > cur->iov_base) {
+            rewind = last_end - cur->iov_base;
+        }
+
+        sortelems[i].dest_base = buf - rewind;
+        buf += cur->iov_len - MIN(rewind, cur->iov_len);
+        last_end = MAX(cur->iov_base + cur->iov_len, last_end);
+    }
+
+    /* Sort by source iovec index and build destination iovec */
+    qsort(sortelems, src->niov, sizeof(sortelems[0]), sortelem_cmp_src_index);
     for (i = 0; i < src->niov; i++) {
-        qemu_iovec_add(dest, buf, src->iov[i].iov_len);
-        buf += src->iov[i].iov_len;
+        qemu_iovec_add(dest, sortelems[i].dest_base, src->iov[i].iov_len);
     }
 }
 
-- 
1.7.1