libcasper: tolerate kernels without PD_NOWAITPID

Commit 1a296762b3 made libcasper pass PD_NOWAITPID to pdfork(2).
Kernels predating the flag (15.1 and earlier, since the flag first
ships in 15.2) reject it with EINVAL, which makes cap_init() and every
service fork fail when a newer world runs on an older kernel, for
example in a poudriere jail.

Add casper_pdfork(), which retries without the flag on EINVAL, and use
it at both pdfork(2) call sites.  The retry is safe because the kernel
validates pdfork flags before creating a child.  On such kernels the
zombie must still be reaped with waitpid(2), as before the flag was
introduced.

The fallback is compiled out once __FreeBSD_version reaches 1700000,
so it disappears from main when stable/16 branches while remaining in
the stable/15 and stable/16 branches that need it.

Reviewed by:	kib
MFC after:	3 days

Differential Revision: https://reviews.freebsd.org/D60401

lib/libcasper/libcasper/libcasper.c

@@ -95,7 +95,7 @@ cap_init(void)
 		return (NULL);
 	}
 
-	pid = pdfork(&pfd, PD_NOWAITPID);
+	pid = casper_pdfork(&pfd);
 	if (pid == 0) {
 		/* Child. */
 		close(sock[0]);

lib/libcasper/libcasper/libcasper_impl.h

@@ -33,6 +33,10 @@
 #ifndef	_LIBCASPER_IMPL_H_
 #define	_LIBCASPER_IMPL_H_
 
+#include <sys/param.h>
+#include <sys/procdesc.h>
+
+#include <errno.h>
 #include <stdbool.h>
 
 #include "libcasper.h"
@@ -44,6 +48,25 @@ struct service_connection;
 bool fd_is_valid(int fd);
 void fd_fix_environment(int *fdp);
 
+static inline pid_t
+casper_pdfork(int *fdp)
+{
+	pid_t pid;
+
+	pid = pdfork(fdp, PD_NOWAITPID);
+#if __FreeBSD_version < 1700000
+	/*
+	 * Kernels predating PD_NOWAITPID (introduced in 15.2) reject the
+	 * flag with EINVAL before creating a child, so retrying without
+	 * it is safe.  The child must then be reaped with waitpid(2), as
+	 * before.  This fallback is compiled out starting with 17.0.
+	 */
+	if (pid == -1 && errno == EINVAL)
+		pid = pdfork(fdp, 0);
+#endif
+	return (pid);
+}
+
 /* Private service functions. */
 struct service	*service_alloc(const char *name,
 		    service_limit_func_t *limitfunc,

lib/libcasper/libcasper/zygote.c

@@ -149,7 +149,7 @@ zygote_main(int *sockp)
 			error = errno;
 			goto send;
 		}
-		pid = pdfork(&procfd, PD_NOWAITPID);
+		pid = casper_pdfork(&procfd);
 		switch (pid) {
 		case -1:
 			/* Failure. */