tools: ynl-gen: validate 0 len strings from kernel

Strings from the kernel are guaranteed to be null terminated and
ynl_attr_validate() checks for this. But it doesn't check if the string
has a len of 0, which would cause problems when trying to access
data[len - 1]. Fix this by checking that len is positive.

Signed-off-by: David Wei <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: spikeh

tools/net/ynl/lib/ynl.c

@@ -364,7 +364,7 @@ int ynl_attr_validate(struct ynl_parse_arg *yarg, const struct nlattr *attr)
 		     "Invalid attribute (binary %s)", policy->name);
 		return -1;
 	case YNL_PT_NUL_STR:
-		if ((!policy->len || len <= policy->len) && !data[len - 1])
+		if (len && (!policy->len || len <= policy->len) && !data[len - 1])
 			break;
 		yerr(yarg->ys, YNL_ERROR_ATTR_INVALID,
 		     "Invalid attribute (string %s)", policy->name);