block: sed-opal: split generation of bytestring header and content

Split the header generation from the (normal) memcpy part if a
bytestring is copied into the command buffer. This allows in-place
generation of the bytestring content. For example, copy_from_user may be
used without an intermediate buffer.
Signed-off-by: Jonas Rabenstein <jonas.rabenstein@studium.uni-erlangen.de>
Signed-off-by: David Kozub <zub@linux.fjfi.cvut.cz>
Reviewed-by: Scott Bauer <sbauer@plzdonthack.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jon Derrick <jonathan.derrick@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/sed-opal.c

@@ -580,15 +580,11 @@ static void add_token_u64(int *err, struct opal_dev *cmd, u64 number)
 		add_token_u8(err, cmd, number >> (len * 8));
 }
 
-static void add_token_bytestring(int *err, struct opal_dev *cmd,
-				 const u8 *bytestring, size_t len)
+static u8 *add_bytestring_header(int *err, struct opal_dev *cmd, size_t len)
 {
 	size_t header_len = 1;
 	bool is_short_atom = true;
 
-	if (*err)
-		return;
-
 	if (len & ~SHORT_ATOM_LEN_MASK) {
 		header_len = 2;
 		is_short_atom = false;
@@ -596,7 +592,7 @@ static void add_token_bytestring(int *err, struct opal_dev *cmd,
 
 	if (!can_add(err, cmd, header_len + len)) {
 		pr_debug("Error adding bytestring: end of buffer.\n");
-		return;
+		return NULL;
 	}
 
 	if (is_short_atom)
@@ -604,9 +600,19 @@ static void add_token_bytestring(int *err, struct opal_dev *cmd,
 	else
 		add_medium_atom_header(cmd, true, false, len);
 
-	memcpy(&cmd->cmd[cmd->pos], bytestring, len);
-	cmd->pos += len;
+	return &cmd->cmd[cmd->pos];
+}
 
+static void add_token_bytestring(int *err, struct opal_dev *cmd,
+				 const u8 *bytestring, size_t len)
+{
+	u8 *start;
+
+	start = add_bytestring_header(err, cmd, len);
+	if (!start)
+		return;
+	memcpy(start, bytestring, len);
+	cmd->pos += len;
 }
 
 static int build_locking_range(u8 *buffer, size_t length, u8 lr)