tools: ynl: submsg: reverse parse / error reporting

Reverse parsing lets YNL convert bad and missing attr pointers
from extack into a string like "missing attribute nest1.nest2.attr_name".
It's a feature that's unique to YNL C AFAIU (even the Python YNL
can't do nested reverse parsing). Add support for reverse-parsing
of sub-messages.

To simplify the logic and the code annotate the type policies
with extra metadata. Mark the selectors and the messages with
the information we need. We assume that key / selector always
precedes the sub-message while parsing (and also if there are
multiple sub-messages like in rt-link they are interleaved
selector 1 ... submsg 1 ... selector 2 .. submsg 2, not
selector 1 ... selector 2 ... submsg 1 ... submsg 2).

The rt-link sample in a subsequent changes shows reverse parsing
of sub-messages in action.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250515231650.1325372-8-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/net/ynl/lib/ynl-priv.h

@@ -43,7 +43,10 @@ typedef int (*ynl_parse_cb_t)(const struct nlmsghdr *nlh,
 			      struct ynl_parse_arg *yarg);
 
 struct ynl_policy_attr {
-	enum ynl_policy_type type;
+	enum ynl_policy_type type:8;
+	__u8 is_submsg:1;
+	__u8 is_selector:1;
+	__u16 selector_type;
 	unsigned int len;
 	const char *name;
 	const struct ynl_policy_nest *nest;

tools/net/ynl/lib/ynl.c

@@ -45,8 +45,39 @@
 #define perr(_ys, _msg)			__yerr(&(_ys)->err, errno, _msg)
 
 /* -- Netlink boiler plate */
+static bool
+ynl_err_walk_is_sel(const struct ynl_policy_nest *policy,
+		    const struct nlattr *attr)
+{
+	unsigned int type = ynl_attr_type(attr);
+
+	return policy && type <= policy->max_attr &&
+		policy->table[type].is_selector;
+}
+
+static const struct ynl_policy_nest *
+ynl_err_walk_sel_policy(const struct ynl_policy_attr *policy_attr,
+			const struct nlattr *selector)
+{
+	const struct ynl_policy_nest *policy = policy_attr->nest;
+	const char *sel;
+	unsigned int i;
+
+	if (!policy_attr->is_submsg)
+		return policy;
+
+	sel = ynl_attr_get_str(selector);
+	for (i = 0; i <= policy->max_attr; i++) {
+		if (!strcmp(sel, policy->table[i].name))
+			return policy->table[i].nest;
+	}
+
+	return NULL;
+}
+
 static int
-ynl_err_walk_report_one(const struct ynl_policy_nest *policy, unsigned int type,
+ynl_err_walk_report_one(const struct ynl_policy_nest *policy,
+			const struct nlattr *selector, unsigned int type,
 			char *str, int str_sz, int *n)
 {
 	if (!policy) {
@@ -67,9 +98,34 @@ ynl_err_walk_report_one(const struct ynl_policy_nest *policy, unsigned int type,
 		return 1;
 	}
 
-	if (*n < str_sz)
-		*n += snprintf(str, str_sz - *n,
-			       ".%s", policy->table[type].name);
+	if (*n < str_sz) {
+		int sz;
+
+		sz = snprintf(str, str_sz - *n,
+			      ".%s", policy->table[type].name);
+		*n += sz;
+		str += sz;
+	}
+
+	if (policy->table[type].is_submsg) {
+		if (!selector) {
+			if (*n < str_sz)
+				*n += snprintf(str, str_sz, "(!selector)");
+			return 1;
+		}
+
+		if (ynl_attr_type(selector) !=
+		    policy->table[type].selector_type) {
+			if (*n < str_sz)
+				*n += snprintf(str, str_sz, "(!=selector)");
+			return 1;
+		}
+
+		if (*n < str_sz)
+			*n += snprintf(str, str_sz - *n, "(%s)",
+				       ynl_attr_get_str(selector));
+	}
+
 	return 0;
 }
 
@@ -78,6 +134,8 @@ ynl_err_walk(struct ynl_sock *ys, void *start, void *end, unsigned int off,
 	     const struct ynl_policy_nest *policy, char *str, int str_sz,
 	     const struct ynl_policy_nest **nest_pol)
 {
+	const struct ynl_policy_nest *next_pol;
+	const struct nlattr *selector = NULL;
 	unsigned int astart_off, aend_off;
 	const struct nlattr *attr;
 	unsigned int data_len;
@@ -96,6 +154,10 @@ ynl_err_walk(struct ynl_sock *ys, void *start, void *end, unsigned int off,
 	ynl_attr_for_each_payload(start, data_len, attr) {
 		astart_off = (char *)attr - (char *)start;
 		aend_off = (char *)ynl_attr_data_end(attr) - (char *)start;
+
+		if (ynl_err_walk_is_sel(policy, attr))
+			selector = attr;
+
 		if (aend_off <= off)
 			continue;
 
@@ -109,16 +171,20 @@ ynl_err_walk(struct ynl_sock *ys, void *start, void *end, unsigned int off,
 
 	type = ynl_attr_type(attr);
 
-	if (ynl_err_walk_report_one(policy, type, str, str_sz, &n))
+	if (ynl_err_walk_report_one(policy, selector, type, str, str_sz, &n))
+		return n;
+
+	next_pol = ynl_err_walk_sel_policy(&policy->table[type], selector);
+	if (!next_pol)
 		return n;
 
 	if (!off) {
 		if (nest_pol)
-			*nest_pol = policy->table[type].nest;
+			*nest_pol = next_pol;
 		return n;
 	}
 
-	if (!policy->table[type].nest) {
+	if (!next_pol) {
 		if (n < str_sz)
 			n += snprintf(str, str_sz, "!nest");
 		return n;
@@ -128,7 +194,7 @@ ynl_err_walk(struct ynl_sock *ys, void *start, void *end, unsigned int off,
 	start =  ynl_attr_data(attr);
 	end = start + ynl_attr_data_len(attr);
 
-	return n + ynl_err_walk(ys, start, end, off, policy->table[type].nest,
+	return n + ynl_err_walk(ys, start, end, off, next_pol,
 				&str[n], str_sz - n, nest_pol);
 }
 
@@ -231,7 +297,7 @@ ynl_ext_ack_check(struct ynl_sock *ys, const struct nlmsghdr *nlh,
 		}
 
 		n2 = 0;
-		ynl_err_walk_report_one(nest_pol, type, &miss_attr[n],
+		ynl_err_walk_report_one(nest_pol, NULL, type, &miss_attr[n],
 					sizeof(miss_attr) - n, &n2);
 		n += n2;
 

tools/net/ynl/pyynl/ynl_gen_c.py

@@ -57,6 +57,8 @@ class Type(SpecAttr):
         self.request = False
         self.reply = False
 
+        self.is_selector = False
+
         if 'len' in attr:
             self.len = attr['len']
 
@@ -484,7 +486,10 @@ class TypeString(Type):
         ri.cw.p(f"char *{self.c_name};")
 
     def _attr_typol(self):
-        return f'.type = YNL_PT_NUL_STR, '
+        typol = f'.type = YNL_PT_NUL_STR, '
+        if self.is_selector:
+            typol += '.is_selector = 1, '
+        return typol
 
     def _attr_policy(self, policy):
         if 'exact-len' in self.checks:
@@ -878,6 +883,16 @@ class TypeNestTypeValue(Type):
 
 
 class TypeSubMessage(TypeNest):
+    def __init__(self, family, attr_set, attr, value):
+        super().__init__(family, attr_set, attr, value)
+
+        self.selector = Selector(attr, attr_set)
+
+    def _attr_typol(self):
+        typol = f'.type = YNL_PT_NEST, .nest = &{self.nested_render_name}_nest, '
+        typol += f'.is_submsg = 1, .selector_type = {self.attr_set[self["selector"]].value} '
+        return typol
+
     def _attr_get(self, ri, var):
         sel = c_lower(self['selector'])
         get_lines = [f'if (!{var}->{sel})',
@@ -890,6 +905,18 @@ class TypeSubMessage(TypeNest):
         return get_lines, init_lines, None
 
 
+class Selector:
+    def __init__(self, msg_attr, attr_set):
+        self.name = msg_attr["selector"]
+
+        if self.name in attr_set:
+            self.attr = attr_set[self.name]
+            self.attr.is_selector = True
+            self._external = False
+        else:
+            raise Exception("Passing selectors from external nests not supported")
+
+
 class Struct:
     def __init__(self, family, space_name, type_list=None,
                  inherited=None, submsg=None):