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[Qemu-devel] [PATCH] slirp: Fix packet expiration

From: Thomas Huth
Subject: [Qemu-devel] [PATCH] slirp: Fix packet expiration
Date: Tue, 27 Sep 2011 11:20:38 +0200

The two new variables "arp_requested" and "expiration_date" in the mbuf
structure have been added after the variable-sized "m_dat_" array. The
variables have to be added before the m_dat_ array instead.
Without this patch, the expiration_date gets clobbered by code that
accesses the m_dat_ array.
I experienced this problem with the code in slirp/tftp.c: The
tftp_send_data() function created a new packet with the m_get()
function (which fills-in a default expiration_date value). Then the
TFTP code cleared the data section of the packet, which accidentially
also cleared the expiration_date. This zeroed expiration_date then
finally causes the packet to be discarded during if_start(), so that
TFTP packets were not transmitted anymore.

Signed-off-by: Thomas Huth <address@hidden>
 slirp/mbuf.h |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/slirp/mbuf.h b/slirp/mbuf.h
index 55170e5..37b9dbb 100644
--- a/slirp/mbuf.h
+++ b/slirp/mbuf.h
@@ -82,12 +82,12 @@ struct m_hdr {
 struct mbuf {
        struct  m_hdr m_hdr;
        Slirp *slirp;
+       bool    arp_requested;
+       uint64_t expiration_date;
        union M_dat {
                char    m_dat_[1]; /* ANSI don't like 0 sized arrays */
                char    *m_ext_;
        } M_dat;
-    bool     arp_requested;
-    uint64_t expiration_date;
 #define m_next         m_hdr.mh_next

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