mctpd: fix return value check in control socket reads

Currently, we're checking the return value of read_message against 0 to
detect EOF, but this isn't a length value. This means that any
successful read will hit the errx.

Instead, check the actual length, and improve the read_message case
where we see an EOF from the recvmsg().

Fixes: ea5c9a3 ("mctpd: Exit on control socket EOF")
Signed-off-by: Jeremy Kerr <[email protected]>

CHANGELOG.md

@@ -33,4 +33,6 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
    pre-configured EID that would conflict with other (already enumerated)
    endpoints. The new endpoint will get a non-conflicting address assigned.
 
+2. mctpd: fix incorrect error detection on control socket reads
+
 ## [1.1] - 2023-04-13

src/mctpd.c

@@ -426,6 +426,13 @@ static int read_message(ctx *ctx, int sd, uint8_t **ret_buf, size_t *ret_buf_siz
 		goto out;
 	}
 
+	if (len == 0) {
+		*ret_buf = NULL;
+		*ret_buf_size = 0;
+		rc = 0;
+		goto out;
+	}
+
 	buf_size = len;
 	buf = malloc(buf_size);
 	if (!buf) {
@@ -721,7 +728,7 @@ static int cb_listen_control_msg(sd_event_source *s, int sd, uint32_t revents,
 	if (rc < 0)
 		goto out;
 
-	if (rc == 0)
+	if (buf_size == 0)
 		errx(EXIT_FAILURE, "Control socket returned EOF");
 
 	if (addr.smctp_base.smctp_type != MCTP_CTRL_HDR_MSG_TYPE) {