slirp: Prevent recursion of if_start

if_start can be called recursively via if_encap. Avoid this as our
scheme of dequeuing packets is not compatible with this.

CC: Fabien Chouteau <chouteau@adacore.com>
CC: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
CC: Stefan Weil <sw@weilnetz.de>
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>

slirp/if.c

@@ -163,10 +163,17 @@ void if_start(Slirp *slirp)
 
     DEBUG_CALL("if_start");
 
+    if (slirp->if_start_busy) {
+        return;
+    }
+    slirp->if_start_busy = true;
+
     while (slirp->if_queued) {
         /* check if we can really output */
-        if (!slirp_can_output(slirp->opaque))
+        if (!slirp_can_output(slirp->opaque)) {
+            slirp->if_start_busy = false;
             return;
+        }
 
         /*
          * See which queue to get next packet from
@@ -221,4 +228,6 @@ void if_start(Slirp *slirp)
     }
 
     slirp->if_queued = requeued;
+
+    slirp->if_start_busy = false;
 }

slirp/slirp.h

@@ -239,6 +239,7 @@ struct Slirp {
     struct mbuf if_fastq;   /* fast queue (for interactive data) */
     struct mbuf if_batchq;  /* queue for non-interactive data */
     struct mbuf *next_m;    /* pointer to next mbuf to output */
+    bool if_start_busy;     /* avoid if_start recursion */
 
     /* ip states */
     struct ipq ipq;         /* ip reass. queue */