[PATCH v4 9/9] scsi/scsi_bus: fix races in REPORT LUNS

[Maxim Levitsky]

Currently scsi_target_emulate_report_luns iterates
over child devices list twice, and there is guarantee, that
it will not be changed meanwhile.

This reason for two loops is that it needs to know how much memory
to allocate.

Avoid this by iterating once, and allocating the memory for the output
dynamically with reserving enought memory so that in practice it won't
be reallocated often.

Bugzilla for reference: https://bugzilla.redhat.com/show_bug.cgi?id=1866707

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
 hw/scsi/scsi-bus.c | 62 ++++++++++++++++++++++------------------------
 1 file changed, 29 insertions(+), 33 deletions(-)

diff --git a/hw/scsi/scsi-bus.c b/hw/scsi/scsi-bus.c
index feab20b76d..150dee2e6a 100644
--- a/hw/scsi/scsi-bus.c
+++ b/hw/scsi/scsi-bus.c
@@ -438,19 +438,25 @@ struct SCSITargetReq {
 static void store_lun(uint8_t *outbuf, int lun)
 {
     if (lun < 256) {
+        /* Simple logical unit addressing method*/
+        outbuf[0] = 0;
         outbuf[1] = lun;
-        return;
+    } else {
+        /* Flat space addressing method */
+        outbuf[0] = 0x40 | (lun >> 8);
+        outbuf[1] = (lun & 255);
     }
-    outbuf[1] = (lun & 255);
-    outbuf[0] = (lun >> 8) | 0x40;
 }
 
 static bool scsi_target_emulate_report_luns(SCSITargetReq *r)
 {
     BusChild *kid;
-    int i, len, n;
     int channel, id;
-    bool found_lun0;
+    uint8_t tmp[8] = {0};
+    int len = 0;
+
+    /* reserve space for 63 LUNs*/
+    GByteArray *buf = g_byte_array_sized_new(512);
 
     if (r->req.cmd.xfer < 16) {
         return false;
@@ -460,46 +466,36 @@ static bool scsi_target_emulate_report_luns(SCSITargetReq 
*r)
     }
     channel = r->req.dev->channel;
     id = r->req.dev->id;
-    found_lun0 = false;
-    n = 0;
 
-    rcu_read_lock();
 
-    QTAILQ_FOREACH_RCU(kid, &r->req.bus->qbus.children, sibling) {
-        DeviceState *qdev = kid->child;
-        SCSIDevice *dev = SCSI_DEVICE(qdev);
+    /* add size (will be updated later to correct value */
+    g_byte_array_append(buf, tmp, 8);
+    len += 8;
 
-        if (dev->channel == channel && dev->id == id) {
-            if (dev->lun == 0) {
-                found_lun0 = true;
-            }
-            n += 8;
-        }
-    }
-    if (!found_lun0) {
-        n += 8;
-    }
-
-    scsi_target_alloc_buf(&r->req, n + 8);
+    /* add LUN0 */
+    g_byte_array_append(buf, tmp, 8);
+    len += 8;
 
-    len = MIN(n + 8, r->req.cmd.xfer & ~7);
-    memset(r->buf, 0, len);
-    stl_be_p(&r->buf[0], n);
-    i = found_lun0 ? 8 : 16;
+    rcu_read_lock();
     QTAILQ_FOREACH_RCU(kid, &r->req.bus->qbus.children, sibling) {
         DeviceState *qdev = kid->child;
         SCSIDevice *dev = SCSI_DEVICE(qdev);
 
-        if (dev->channel == channel && dev->id == id) {
-            store_lun(&r->buf[i], dev->lun);
-            i += 8;
+        if (dev->channel == channel && dev->id == id && dev->lun != 0) {
+            store_lun(tmp, dev->lun);
+            g_byte_array_append(buf, tmp, 8);
+            len += 8;
         }
     }
-
     rcu_read_unlock();
 
-    assert(i == n + 8);
-    r->len = len;
+    r->buf_len = len;
+    r->buf = g_byte_array_free(buf, FALSE);
+    r->len = MIN(len, r->req.cmd.xfer & ~7);
+
+    /* store the LUN list length */
+    stl_be_p(&r->buf[0], len - 8);
+
     return true;
 }
 
-- 
2.26.2