This patch also updates nxm_check_reg_move() to use nxm_src_check()
and nxm_dst_check().  A user outside of nxm-match will be added in
a future patch.
---
 lib/nx-match.c |   48 +++++++++++++++++++++++++++++++-----------------
 lib/nx-match.h |    2 ++
 2 files changed, 33 insertions(+), 17 deletions(-)

diff --git a/lib/nx-match.c b/lib/nx-match.c
index ecc284e..e94be95 100644
--- a/lib/nx-match.c
+++ b/lib/nx-match.c
@@ -1191,28 +1191,42 @@ int
 nxm_check_reg_move(const struct nx_action_reg_move *action,
                    const struct flow *flow)
 {
-    const struct nxm_field *src;
-    const struct nxm_field *dst;
+    ovs_be16 src_ofs_nbits;
+    ovs_be16 dst_ofs_nbits;
+    uint16_t n_bits;
+    int error;
 
-    if (action->n_bits == htons(0)) {
-        return BAD_ARGUMENT;
-    }
+    n_bits = ntohs(action->n_bits);
+    src_ofs_nbits = nxm_encode_ofs_nbits(ntohs(action->src_ofs), n_bits);
+    dst_ofs_nbits = nxm_encode_ofs_nbits(ntohs(action->dst_ofs), n_bits);
 
-    src = nxm_field_lookup(ntohl(action->src));
-    if (!field_ok(src, flow, ntohs(action->src_ofs) + ntohs(action->n_bits))) {
-        return BAD_ARGUMENT;
-    }
+    error = nxm_src_check(action->src, src_ofs_nbits, 1, flow);
+    error = error ? error : nxm_dst_check(action->dst, dst_ofs_nbits, 1, flow);
+    return error;
+}
 
-    dst = nxm_field_lookup(ntohl(action->dst));
-    if (!field_ok(dst, flow, ntohs(action->dst_ofs) + ntohs(action->n_bits))) {
-        return BAD_ARGUMENT;
-    }
+/* Given a flow, checks that the source field represented by 'src_header' and
+ * 'ofs_nbits' is valid, and contains at least 'min_n_bits' bits of data. */
+int
+nxm_src_check(ovs_be32 src_header, ovs_be16 ofs_nbits, size_t min_n_bits,
+              const struct flow *flow)
+{
+    const struct nxm_field *src;
+    int ofs, n_bits;
 
-    if (!dst->writable) {
-        return BAD_ARGUMENT;
+    ofs = nxm_decode_ofs(ofs_nbits);
+    n_bits = nxm_decode_n_bits(ofs_nbits);
+    src = nxm_field_lookup(ntohl(src_header));
+
+    if (!field_ok(src, flow, ofs + n_bits)) {
+        VLOG_WARN_RL(&rl, "invalid source field");
+    } else if (n_bits < min_n_bits) {
+        VLOG_WARN_RL(&rl, "insufficient bits in source field");
+    } else {
+        return 0;
     }
 
-    return 0;
+    return BAD_ARGUMENT;
 }
 
 /* Given a flow, checks that the destination field represented by 'dst_header'
diff --git a/lib/nx-match.h b/lib/nx-match.h
index 5365cca..264a65a 100644
--- a/lib/nx-match.h
+++ b/lib/nx-match.h
@@ -49,6 +49,8 @@ void nxm_format_reg_load(const struct nx_action_reg_load *, 
struct ds *);
 
 int nxm_check_reg_move(const struct nx_action_reg_move *, const struct flow *);
 int nxm_check_reg_load(const struct nx_action_reg_load *, const struct flow *);
+int nxm_src_check(ovs_be32 src, ovs_be16 ofs_nbits, size_t min_n_bits,
+                  const struct flow *);
 int nxm_dst_check(ovs_be32 dst, ovs_be16 ofs_nbits, size_t min_n_bits,
                   const struct flow *);
 
-- 
1.7.6

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