[nft PATCH] evaluate: Reject quoted strings containing only wildcard

[Phil Sutter <phil@nwl.cc>]

Fix for an assertion fail when trying to match against an all-wildcard
interface name:

| % nft add rule t c iifname '"*"'
| nft: expression.c:402: constant_expr_alloc: Assertion `(((len) + (8) - 1) / (8)) > 0' failed.
| zsh: abort      nft add rule t c iifname '"*"'

Fix this by detecting the string in expr_evaluate_string() and returning
an error message:

| % nft add rule t c iifname '"*"'
| Error: All-wildcard strings are not supported
| add rule t c iifname "*"
|                      ^^^

While being at it, drop the 'datalen >= 1' clause from the following
conditional as together with the added check for 'datalen == 0', all
possible other values have been caught already.

Signed-off-by: Phil Sutter <phil@nwl.cc>
---
 src/evaluate.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/src/evaluate.c b/src/evaluate.c
index c8045e5ded729..5f17d7501ac0e 100644
--- a/src/evaluate.c
+++ b/src/evaluate.c
@@ -324,8 +324,11 @@ static int expr_evaluate_string(struct eval_ctx *ctx, struct expr **exprp)
 		return 0;
 	}
 
-	if (datalen >= 1 &&
-	    data[datalen - 1] == '\\') {
+	if (datalen == 0)
+		return expr_error(ctx->msgs, expr,
+				  "All-wildcard strings are not supported");
+
+	if (data[datalen - 1] == '\\') {
 		char unescaped_str[data_len];
 
 		memset(unescaped_str, 0, sizeof(unescaped_str));
-- 
2.28.0