Add mtod_check()

Recent security issues demonstrate the lack of safety care when casting
a mbuf to a particular structure type. At least, it should check that
the buffer is large enough. The following patches will make use of this
function.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

2 files changed, 12 insertions, 0 deletions

diff --git a/src/mbuf.c b/src/mbuf.c
index b47f64e..b4152c3 100644
--- a/src/mbuf.c
+++ b/src/mbuf.c
@@ -263,3 +263,14 @@ struct mbuf *m_dup(Slirp *slirp, struct mbuf *m,
 
     return n;
 }
+
+void *mtod_check(struct mbuf *m, size_t len)
+{
+    if (m->m_len >= len) {
+        return m->m_data;
+    }
+
+    DEBUG_ERROR("mtod failed");
+
+    return NULL;
+}
diff --git a/src/mbuf.h b/src/mbuf.h
index 47b3b14..23c5458 100644
--- a/src/mbuf.h
+++ b/src/mbuf.h
@@ -126,6 +126,7 @@ void m_adj(struct mbuf *, int);
 int m_copy(struct mbuf *, struct mbuf *, int, int);
 struct mbuf *m_dup(Slirp *slirp, struct mbuf *m, bool copy_header, size_t header_size);
 struct mbuf *dtom(Slirp *, void *);
+void *mtod_check(struct mbuf *, size_t len);
 
 static inline void ifs_init(struct mbuf *ifm)
 {