Correct documentation errors in regards to UTF8 params

This fixes numerous bugs in documentation in regards to UTF8
params and their sizes. The returned size should always be without the
terminating NUL byte. On the other hand on the requestor side
the size of the buffer should include the NUL byte if it expects it
being included in the returned string.

Also make this clear in the EVP_PKEY_get_group_name() documentation
which uses utf8 string params under the hood.

Fixes #16287

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16296)

doc/man3/EVP_PKEY_fromdata.pod

@@ -161,7 +161,7 @@ TODO Write a set of cookbook documents and link to them.
      0x5c, 0xcd, 0x86, 0x71, 0xa8, 0xbf, 0x1a, 0x47
  };
  const OSSL_PARAM params[] = {
-     OSSL_PARAM_utf8_string("group", "prime256v1"),
+     OSSL_PARAM_utf8_string("group", "prime256v1", 10),
      OSSL_PARAM_BN("priv", priv, sizeof(priv)),
      OSSL_PARAM_BN("pub", pub, sizeof(pub)),
      OSSL_PARAM_END

doc/man3/EVP_PKEY_get_group_name.pod

@@ -15,8 +15,8 @@ EVP_PKEY_get_group_name - get group name of a key
 
 EVP_PKEY_get_group_name() fills in the group name of the I<pkey> into
 I<gname>, up to at most I<gname_sz> bytes including the ending NUL byte
-and assigns I<*gname_len> the actual size of the name, if I<pkey>'s key type
-supports it.
+and assigns I<*gname_len> the actual length of the name not including
+the NUL byte, if I<pkey>'s key type supports it.
 I<gname> as well as I<gname_len> may individually be NULL, and won't be
 filled in or assigned in that case.
 

doc/man3/EVP_PKEY_gettable_params.pod

@@ -49,7 +49,8 @@ is allocated by the method.
 
 EVP_PKEY_get_utf8_string_param() get a key I<pkey> UTF8 string value int a buffer
 I<str> of maximum size I<max_buf_sz> associated with a name of I<key_name>.
-I<*out_sz> is the returned size of the string if it is not NULL.
+If I<out_sz> is not NULL the I<*out_sz> is set to the length of the string
+not including the terminating NUL byte.
 
 EVP_PKEY_get_octet_string_param() copy a I<pkey>'s octet string value into a buffer
 I<buf> of maximum size I<max_buf_sz> associated with a name of I<key_name>.

doc/man3/OSSL_PARAM.pod

@@ -306,11 +306,11 @@ This example is for setting parameters on some object:
     #include <openssl/core.h>
 
     const char *foo = "some string";
-    size_t foo_l = strlen(foo) + 1;
+    size_t foo_l = strlen(foo);
     const char bar[] = "some other string";
     OSSL_PARAM set[] = {
         { "foo", OSSL_PARAM_UTF8_STRING_PTR, &foo, foo_l, 0 },
-        { "bar", OSSL_PARAM_UTF8_STRING, &bar, sizeof(bar), 0 },
+        { "bar", OSSL_PARAM_UTF8_STRING, &bar, sizeof(bar) - 1, 0 },
         { NULL, 0, NULL, 0, 0 }
     };
 
@@ -338,10 +338,10 @@ could fill in the parameters like this:
     for (i = 0; params[i].key != NULL; i++) {
         if (strcmp(params[i].key, "foo") == 0) {
             *(char **)params[i].data = "foo value";
-            params[i].return_size = 10; /* size of "foo value" */
+            params[i].return_size = 9; /* length of "foo value" string */
         } else if (strcmp(params[i].key, "bar") == 0) {
             memcpy(params[i].data, "bar value", 10);
-            params[i].return_size = 10; /* size of "bar value" */
+            params[i].return_size = 9; /* length of "bar value" string */
         }
         /* Ignore stuff we don't know */
     }

doc/man3/OSSL_PARAM_BLD.pod

@@ -91,7 +91,8 @@ must exist until after OSSL_PARAM_BLD_to_param() has been called.
 
 OSSL_PARAM_BLD_push_utf8_string() is a function that will create an OSSL_PARAM
 object that references the UTF8 string specified by I<buf>.
-If the length of the string, I<bsize>, is zero then it will be calculated.
+The length of the string I<bsize> should not include the terminating NUL byte.
+If it is zero then it will be calculated.
 The string that I<buf> points to is stored by reference and must remain in
 scope until after OSSL_PARAM_BLD_to_param() has been called.
 
@@ -102,7 +103,8 @@ scope until after OSSL_PARAM_BLD_to_param() has been called.
 
 OSSL_PARAM_BLD_push_utf8_ptr() is a function that will create an OSSL_PARAM
 object that references the UTF8 string specified by I<buf>.
-If the length of the string, I<bsize>, is zero then it will be calculated.
+The length of the string I<bsize> should not include the terminating NUL byte.
+If it is zero then it will be calculated.
 The string I<buf> points to is stored by reference and must remain in
 scope until the OSSL_PARAM array is freed.
 

doc/man3/OSSL_PARAM_int.pod

@@ -200,7 +200,7 @@ OSSL_PARAM_construct_octet_string() is a function that constructs an OCTET
 string B<OSSL_PARAM> structure.
 A parameter with name I<key>, storage I<buf> and size I<bsize> is created.
 
-OSSL_PARAM_construct_utf8_ptr() is a function that constructs a UTF string
+OSSL_PARAM_construct_utf8_ptr() is a function that constructs a UTF8 string
 pointer B<OSSL_PARAM> structure.
 A parameter with name I<key>, storage pointer I<*buf> and size I<bsize>
 is created.
@@ -241,17 +241,18 @@ will be assigned the size the parameter's I<data> buffer should have.
 OSSL_PARAM_get_utf8_string() retrieves a UTF8 string from the parameter
 pointed to by I<p>.
 The string is stored into I<*val> with a size limit of I<max_len>,
-which must be large enough to accomodate a terminating NUL byte,
-otherwise this function will fail.
-If I<*val> is NULL, memory is allocated for the string and I<max_len>
-is ignored.
+which must be large enough to accomodate the string. A terminating NUL byte
+is added only if the buffer is longer than the string length otherwise the
+string will not be NUL terminated.
+If I<*val> is NULL, memory is allocated for the string (including the
+terminating  NUL byte) and I<max_len> is ignored.
 If memory is allocated by this function, it must be freed by the caller.
 
 OSSL_PARAM_set_utf8_string() sets a UTF8 string from the parameter pointed to
 by I<p> to the value referenced by I<val>.
 If the parameter's I<data> field is NULL, then only its I<return_size> field
 will be assigned the minimum size the parameter's I<data> buffer should have
-to accomodate the string, including a terminating NUL byte.
+to accomodate the string, not including a terminating NUL byte.
 
 OSSL_PARAM_get_octet_string() retrieves an OCTET string from the parameter
 pointed to by I<p>.