netlink_delinearize: zero shift removal

Remove shifts-by-0.  These can occur after binop postprocessing
has adjusted the RHS value to account for a mask operation.

Example: frag frag-off @s4

Is internally represented via:

  [ exthdr load ipv6 2b @ 44 + 2 => reg 1 ]
  [ bitwise reg 1 = ( reg 1 & 0x0000f8ff ) ^ 0x00000000 ]
  [ bitwise reg 1 = ( reg 1 >> 0x00000003 ) ]
  [ lookup reg 1 set s ]

First binop masks out unwanted parts of the 16-bit field.
Second binop needs to left-shift so that lookups in the set will work.

When decoding, the first binop is removed after the exthdr load
has been adjusted accordingly.  Constant propagation adjusts the
shift-value to 0 on removal.  This change then gets rid of the
shift-by-0 entirely.

After this change, 'frag frag-off @s4' input is shown as-is.

Signed-off-by: Florian Westphal <fw@strlen.de>

1 files changed, 21 insertions, 0 deletions

diff --git a/src/netlink_delinearize.c b/src/netlink_delinearize.c
index e37a34f3..323e9150 100644
--- a/src/netlink_delinearize.c
+++ b/src/netlink_delinearize.c
@@ -2322,6 +2322,20 @@ static void map_binop_postprocess(struct rule_pp_ctx *ctx, struct expr *expr)
 		binop_postprocess(ctx, expr, &expr->map);
 }
 
+static bool is_shift_by_zero(const struct expr *binop)
+{
+	struct expr *rhs;
+
+	if (binop->op != OP_RSHIFT && binop->op != OP_LSHIFT)
+		return false;
+
+	rhs = binop->right;
+	if (rhs->etype != EXPR_VALUE || rhs->len > 64)
+		return false;
+
+	return mpz_get_uint64(rhs->value) == 0;
+}
+
 static void relational_binop_postprocess(struct rule_pp_ctx *ctx,
 					 struct expr **exprp)
 {
@@ -2421,6 +2435,13 @@ static void relational_binop_postprocess(struct rule_pp_ctx *ctx,
 		 */
 
 		binop_postprocess(ctx, binop, &binop->left);
+		if (is_shift_by_zero(binop)) {
+			struct expr *lhs = binop->left;
+
+			expr_get(lhs);
+			expr_free(binop);
+			expr->left = lhs;
+		}
 	}
 }