bsd-user: Create special-errno.h

Pull the internal errno used by qemu internally its own
header file, for use by safe-syscall.S.

Reviewed-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

bsd-user/errno_defs.h

@@ -151,6 +151,10 @@
 /* Internal errors: */
 #define TARGET_EJUSTRETURN      254             /* Just return without modifing regs */
 #define TARGET_ERESTART         255             /* Restart syscall */
-#define QEMU_ERESTARTSYS        TARGET_ERESTART
+
+#include "special-errno.h"
+
+_Static_assert(TARGET_ERESTART == QEMU_ERESTARTSYS,
+               "TARGET_ERESTART and QEMU_ERESTARTSYS expected to match");
 
 #endif /* !  _ERRNO_DEFS_H_ */

bsd-user/special-errno.h

@@ -0,0 +1,24 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
+/*
+ * QEMU internal errno values for implementing user-only POSIX.
+ *
+ *  Copyright (c) 2021 Linaro, Ltd.
+ */
+
+#ifndef SPECIAL_ERRNO_H
+#define SPECIAL_ERRNO_H
+
+/*
+ * All of these are QEMU internal, not visible to the guest.
+ * They should be chosen so as to not overlap with any host
+ * or guest errno.
+ */
+
+/*
+ * This is returned when a system call should be restarted, to tell the
+ * main loop that it should wind the guest PC backwards so it will
+ * re-execute the syscall after handling any pending signals.
+ */
+#define QEMU_ERESTARTSYS  255
+
+#endif /* SPECIAL_ERRNO_H */