genksyms: restrict direct-abstract-declarator to take one parameter-type-list

While there is no more grammatical ambiguity in genksyms, the parser
logic is still inaccurate.

For example, genksyms accepts the following invalid C code:

    void my_func(int ()(int));

This should result in a syntax error because () cannot be reduced to
<direct-abstract-declarator>.

( <abstract-declarator> ) can be reduced, but <abstract-declarator>
must not be empty in the following grammar from K&R [1]:

  <direct-abstract-declarator> ::=  ( <abstract-declarator> )
                                 | {<direct-abstract-declarator>}? [ {<constant-expression>}? ]
                                 | {<direct-abstract-declarator>}? ( {<parameter-type-list>}? )

Furthermore, genksyms accepts the following weird code:

    void my_func(int (*callback)(int)(int)(int));

The parser allows <direct-abstract-declarator> to recursively absorb
multiple ( {<parameter-type-list>}? ), but this behavior is incorrect.

In the example above, (*callback) should be followed by at most one
(int).

[1]: https://cs.wmich.edu/~gupta/teaching/cs4850/sumII06/The%20syntax%20of%20C%20in%20Backus-Naur%20form.htm

Signed-off-by: Masahiro Yamada <[email protected]>
Acked-by: Nicolas Schier <[email protected]>

Author: masahir0y

scripts/genksyms/parse.y

@@ -381,20 +381,24 @@ abstract_declarator:
 	;
 
 direct_abstract_declarator:
+	direct_abstract_declarator1
+	| direct_abstract_declarator1 open_paren parameter_declaration_clause ')'
+		{ $$ = $4; }
+	| open_paren parameter_declaration_clause ')'
+		{ $$ = $3; }
+	;
+
+direct_abstract_declarator1:
 	  IDENT
 		{ /* For version 2 checksums, we don't want to remember
 		     private parameter names.  */
 		  remove_node($1);
 		  $$ = $1;
 		}
-	| direct_abstract_declarator open_paren parameter_declaration_clause ')'
-		{ $$ = $4; }
-	| direct_abstract_declarator open_paren error ')'
+	| direct_abstract_declarator1 open_paren error ')'
 		{ $$ = $4; }
-	| direct_abstract_declarator BRACKET_PHRASE
+	| direct_abstract_declarator1 BRACKET_PHRASE
 		{ $$ = $2; }
-	| open_paren parameter_declaration_clause ')'
-		{ $$ = $3; }
 	| open_paren abstract_declarator ')'
 		{ $$ = $3; }
 	| open_paren error ')'