[Qemu-devel] [PATCH] vmstate: Avoid seeking

[Jan Kiszka <jan.kiszka@web.de>]

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Seeking on vmstate save/load does not work if the underlying file is a
stream. We could try to make all QEMUFile* forward-seek-aware, but first
attempts in this direction indicated that it's saner to convert the few
qemu_fseek-on-vmstates users to plain reads/writes.

This fixes various subtle vmstate corruptions where unused fields were
involved.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
---

 hw/virtio-net.c |    7 ++-----
 savevm.c        |   18 ++++++++++++++++--
 2 files changed, 18 insertions(+), 7 deletions(-)

diff --git a/hw/virtio-net.c b/hw/virtio-net.c
index 2f147e5..9ccd4c8 100644
--- a/hw/virtio-net.c
+++ b/hw/virtio-net.c
@@ -745,12 +745,9 @@ static int virtio_net_load(QEMUFile *f, void *opaque, int version_id)
 
     if (version_id >= 5) {
         n->mac_table.in_use = qemu_get_be32(f);
+        qemu_get_buffer(f, n->mac_table.macs, n->mac_table.in_use * ETH_ALEN);
         /* MAC_TABLE_ENTRIES may be different from the saved image */
-        if (n->mac_table.in_use <= MAC_TABLE_ENTRIES) {
-            qemu_get_buffer(f, n->mac_table.macs,
-                            n->mac_table.in_use * ETH_ALEN);
-        } else if (n->mac_table.in_use) {
-            qemu_fseek(f, n->mac_table.in_use * ETH_ALEN, SEEK_CUR);
+        if (n->mac_table.in_use > MAC_TABLE_ENTRIES) {
             n->mac_table.multi_overflow = n->mac_table.uni_overflow = 1;
             n->mac_table.in_use = 0;
         }
diff --git a/savevm.c b/savevm.c
index 8fe9349..1e54a42 100644
--- a/savevm.c
+++ b/savevm.c
@@ -959,13 +959,27 @@ const VMStateInfo vmstate_info_buffer = {
 
 static int get_unused_buffer(QEMUFile *f, void *pv, size_t size)
 {
-    qemu_fseek(f, size, SEEK_CUR);
+    uint8_t buf[1024];
+    int block_len;
+
+    while (size > 0) {
+        block_len = MIN(sizeof(buf), size);
+        size -= block_len;
+        qemu_get_buffer(f, buf, block_len);
+    }
     return 0;
 }
 
 static void put_unused_buffer(QEMUFile *f, void *pv, size_t size)
 {
-    qemu_fseek(f, size, SEEK_CUR);
+    static const uint8_t buf[1024];
+    int block_len;
+
+    while (size > 0) {
+        block_len = MIN(sizeof(buf), size);
+        size -= block_len;
+        qemu_put_buffer(f, buf, block_len);
+    }
 }
 
 const VMStateInfo vmstate_info_unused_buffer = {


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