Be more wary about OpenSSL not setting errno on error.

OpenSSL will sometimes return SSL_ERROR_SYSCALL without having set
errno; this is apparently a reflection of recv(2)'s habit of not
setting errno when reporting EOF.  Ensure that we treat such cases
the same as read EOF.  Previously, we'd frequently report them like
"could not accept SSL connection: Success" which is confusing, or
worse report them with an unrelated errno left over from some
previous syscall.

To fix, ensure that errno is zeroed immediately before the call,
and report its value only when it's not zero afterwards; otherwise
report EOF.

For consistency, I've applied the same coding pattern in libpq's
pqsecure_raw_read().  Bare recv(2) shouldn't really return -1 without
setting errno, but in case it does we might as well cope.

Per report from Andres Freund.  Back-patch to all supported versions.

Discussion: https://postgr.es/m/20231208181451.deqnflwxqoehhxpe@awork3.anarazel.de

4 files changed, 46 insertions, 14 deletions

diff --git a/src/backend/libpq/be-secure-openssl.c b/src/backend/libpq/be-secure-openssl.c
index aed8a75345a..ed13e8b06df 100644
--- a/src/backend/libpq/be-secure-openssl.c
+++ b/src/backend/libpq/be-secure-openssl.c
@@ -457,6 +457,7 @@ aloop:
 	 * per-thread error queue following another call to an OpenSSL I/O
 	 * routine.
 	 */
+	errno = 0;
 	ERR_clear_error();
 	r = SSL_accept(port->ssl);
 	if (r <= 0)
@@ -493,7 +494,7 @@ aloop:
 										 WAIT_EVENT_SSL_OPEN_SERVER);
 				goto aloop;
 			case SSL_ERROR_SYSCALL:
-				if (r < 0)
+				if (r < 0 && errno != 0)
 					ereport(COMMERROR,
 							(errcode_for_socket_access(),
 							 errmsg("could not accept SSL connection: %m")));
@@ -727,7 +728,7 @@ be_tls_read(Port *port, void *ptr, size_t len, int *waitfor)
 			break;
 		case SSL_ERROR_SYSCALL:
 			/* leave it to caller to ereport the value of errno */
-			if (n != -1)
+			if (n != -1 || errno == 0)
 			{
 				errno = ECONNRESET;
 				n = -1;
@@ -785,8 +786,14 @@ be_tls_write(Port *port, void *ptr, size_t len, int *waitfor)
 			n = -1;
 			break;
 		case SSL_ERROR_SYSCALL:
-			/* leave it to caller to ereport the value of errno */
-			if (n != -1)
+
+			/*
+			 * Leave it to caller to ereport the value of errno.  However, if
+			 * errno is still zero then assume it's a read EOF situation, and
+			 * report ECONNRESET.  (This seems possible because SSL_write can
+			 * also do reads.)
+			 */
+			if (n != -1 || errno == 0)
 			{
 				errno = ECONNRESET;
 				n = -1;
diff --git a/src/backend/libpq/pqcomm.c b/src/backend/libpq/pqcomm.c
index 75392a8bb7c..bb9fa77cd4f 100644
--- a/src/backend/libpq/pqcomm.c
+++ b/src/backend/libpq/pqcomm.c
@@ -954,6 +954,8 @@ pq_recvbuf(void)
 	{
 		int			r;
 
+		errno = 0;
+
 		r = secure_read(MyProcPort, PqRecvBuffer + PqRecvLength,
 						PQ_RECV_BUFFER_SIZE - PqRecvLength);
 
@@ -966,10 +968,13 @@ pq_recvbuf(void)
 			 * Careful: an ereport() that tries to write to the client would
 			 * cause recursion to here, leading to stack overflow and core
 			 * dump!  This message must go *only* to the postmaster log.
+			 *
+			 * If errno is zero, assume it's EOF and let the caller complain.
 			 */
-			ereport(COMMERROR,
-					(errcode_for_socket_access(),
-					 errmsg("could not receive data from client: %m")));
+			if (errno != 0)
+				ereport(COMMERROR,
+						(errcode_for_socket_access(),
+						 errmsg("could not receive data from client: %m")));
 			return EOF;
 		}
 		if (r == 0)
@@ -1046,6 +1051,8 @@ pq_getbyte_if_available(unsigned char *c)
 	/* Put the socket into non-blocking mode */
 	socket_set_nonblocking(true);
 
+	errno = 0;
+
 	r = secure_read(MyProcPort, c, 1);
 	if (r < 0)
 	{
@@ -1062,10 +1069,13 @@ pq_getbyte_if_available(unsigned char *c)
 			 * Careful: an ereport() that tries to write to the client would
 			 * cause recursion to here, leading to stack overflow and core
 			 * dump!  This message must go *only* to the postmaster log.
+			 *
+			 * If errno is zero, assume it's EOF and let the caller complain.
 			 */
-			ereport(COMMERROR,
-					(errcode_for_socket_access(),
-					 errmsg("could not receive data from client: %m")));
+			if (errno != 0)
+				ereport(COMMERROR,
+						(errcode_for_socket_access(),
+						 errmsg("could not receive data from client: %m")));
 			r = EOF;
 		}
 	}
diff --git a/src/interfaces/libpq/fe-secure-openssl.c b/src/interfaces/libpq/fe-secure-openssl.c
index d863d279a07..61f37678a6b 100644
--- a/src/interfaces/libpq/fe-secure-openssl.c
+++ b/src/interfaces/libpq/fe-secure-openssl.c
@@ -209,7 +209,7 @@ rloop:
 			 */
 			goto rloop;
 		case SSL_ERROR_SYSCALL:
-			if (n < 0)
+			if (n < 0 && SOCK_ERRNO != 0)
 			{
 				result_errno = SOCK_ERRNO;
 				if (result_errno == EPIPE ||
@@ -317,7 +317,13 @@ pgtls_write(PGconn *conn, const void *ptr, size_t len)
 			n = 0;
 			break;
 		case SSL_ERROR_SYSCALL:
-			if (n < 0)
+
+			/*
+			 * If errno is still zero then assume it's a read EOF situation,
+			 * and report EOF.  (This seems possible because SSL_write can
+			 * also do reads.)
+			 */
+			if (n < 0 && SOCK_ERRNO != 0)
 			{
 				result_errno = SOCK_ERRNO;
 				if (result_errno == EPIPE || result_errno == ECONNRESET)
@@ -1479,10 +1485,12 @@ open_client_SSL(PGconn *conn)
 {
 	int			r;
 
+	SOCK_ERRNO_SET(0);
 	ERR_clear_error();
 	r = SSL_connect(conn->ssl);
 	if (r <= 0)
 	{
+		int			save_errno = SOCK_ERRNO;
 		int			err = SSL_get_error(conn->ssl, r);
 		unsigned long ecode;
 
@@ -1499,10 +1507,10 @@ open_client_SSL(PGconn *conn)
 				{
 					char		sebuf[PG_STRERROR_R_BUFLEN];
 
-					if (r == -1)
+					if (r == -1 && save_errno != 0)
 						appendPQExpBuffer(&conn->errorMessage,
 										  libpq_gettext("SSL SYSCALL error: %s\n"),
-										  SOCK_STRERROR(SOCK_ERRNO, sebuf, sizeof(sebuf)));
+										  SOCK_STRERROR(save_errno, sebuf, sizeof(sebuf)));
 					else
 						appendPQExpBufferStr(&conn->errorMessage,
 											 libpq_gettext("SSL SYSCALL error: EOF detected\n"));
diff --git a/src/interfaces/libpq/fe-secure.c b/src/interfaces/libpq/fe-secure.c
index a1dc7b796d1..4c85b7299cc 100644
--- a/src/interfaces/libpq/fe-secure.c
+++ b/src/interfaces/libpq/fe-secure.c
@@ -235,6 +235,8 @@ pqsecure_raw_read(PGconn *conn, void *ptr, size_t len)
 	int			result_errno = 0;
 	char		sebuf[PG_STRERROR_R_BUFLEN];
 
+	SOCK_ERRNO_SET(0);
+
 	n = recv(conn->sock, ptr, len, 0);
 
 	if (n < 0)
@@ -262,6 +264,11 @@ pqsecure_raw_read(PGconn *conn, void *ptr, size_t len)
 												   "\tbefore or while processing the request.\n"));
 				break;
 
+			case 0:
+				/* If errno didn't get set, treat it as regular EOF */
+				n = 0;
+				break;
+
 			default:
 				appendPQExpBuffer(&conn->errorMessage,
 								  libpq_gettext("could not receive data from server: %s\n"),