apps: allow empty attribute values with -subj

Historically (i.e., OpenSSL 1.0.x), the openssl applications would
allow for empty subject attributes to be passed via the -subj argument,
e.g., `opensl req -subj '/CN=joe/O=/OU=local' ...`.  Commit
db4c08f0194d58c6192f0d8311bf3f20e251cf4f applied a badly needed rewrite
to the parse_name() helper function that parses these strings, but
in the process dropped a check that would skip attributes with no
associated value.  As a result, such strings are now treated as
hard errors and the operation fails.

Restore the check to skip empty attribute values and restore
the historical behavior.

Document the behavior for empty subject attribute values in the
corresponding applications' manual pages.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7349)

apps/apps.c

@@ -1831,6 +1831,12 @@ X509_NAME *parse_name(const char *cp, long chtype, int canmulti)
                       opt_getprog(), typestr);
             continue;
         }
+        if (*valstr == '\0') {
+            BIO_printf(bio_err,
+                       "%s: No value provided for Subject Attribute %s, skipped\n",
+                       opt_getprog(), typestr);
+            continue;
+        }
         if (!X509_NAME_add_entry_by_NID(n, nid, chtype,
                                         valstr, strlen((char *)valstr),
                                         -1, ismulti ? -1 : 0))

doc/man1/ca.pod

@@ -250,8 +250,10 @@ for all available algorithms.
 =item B<-subj arg>
 
 Supersedes subject name given in the request.
-The arg must be formatted as I</type0=value0/type1=value1/type2=...>,
-characters may be escaped by \ (backslash), no spaces are skipped.
+The arg must be formatted as I</type0=value0/type1=value1/type2=...>.
+Keyword characters may be escaped by \ (backslash), and whitespace is retained.
+Empty values are permitted, but the corresponding type will not be included
+in the resulting certificate.
 
 =item B<-utf8>
 

doc/man1/req.pod

@@ -221,8 +221,10 @@ see L<openssl(1)/COMMAND SUMMARY>.
 
 Sets subject name for new request or supersedes the subject name
 when processing a request.
-The arg must be formatted as I</type0=value0/type1=value1/type2=...>,
-characters may be escaped by \ (backslash), no spaces are skipped.
+The arg must be formatted as I</type0=value0/type1=value1/type2=...>.
+Keyword characters may be escaped by \ (backslash), and whitespace is retained.
+Empty values are permitted, but the corresponding type will not be included
+in the request.
 
 =item B<-multivalue-rdn>
 

doc/man1/storeutl.pod

@@ -82,8 +82,11 @@ returned.
 =item B<-subject arg>
 
 Search for an object having the subject name B<arg>.
-The arg must be formatted as I</type0=value0/type1=value1/type2=...>,
-characters may be escaped by \ (backslash), no spaces are skipped.
+The arg must be formatted as I</type0=value0/type1=value1/type2=...>.
+Keyword characters may be escaped by \ (backslash), and whitespace is retained.
+Empty values are permitted but are ignored for the search.  That is,
+a search with an empty value will have the same effect as not specifying
+the type at all.
 
 =item B<-issuer arg>