Add support for clone3() to replace fork()

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/Makefile.am

@@ -56,6 +56,7 @@ logit_LDADD          = $(lite_LIBS)
 
 finit_SOURCES      = api.c	cgroup.c	cgroup.h	\
 		     client.c	client.h			\
+		     clone3.c	clone3.h			\
 		     cond.c	cond-w.c	cond.h		\
 		     conf.c	conf.h				\
 		     devmon.c   devmon.h			\

src/clone3.c

@@ -0,0 +1,95 @@
+/* Portable clone3() wrapper with fallback to fork()
+ *
+ * Copyright (c) 2025  Joachim Wiberg <troglobit@gmail.com>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include "config.h"
+
+#include <errno.h>
+#include <sched.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <string.h>
+#include <sys/syscall.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include "clone3.h"
+#include "finit.h"
+#include "log.h"
+
+/* Clone3 flags from linux/sched.h */
+#ifndef CLONE_INTO_CGROUP
+#define CLONE_INTO_CGROUP 0x200000000ULL
+#endif
+
+/* Linux kernel struct clone_args for clone3() syscall */
+struct clone3_args {
+	uint64_t flags;            /* Flags bit mask */
+	uint64_t pidfd;            /* File descriptor for PID */
+	uint64_t child_tid;        /* Child TID */
+	uint64_t parent_tid;       /* Parent TID */
+	uint64_t exit_signal;      /* Signal to deliver on exit */
+	uint64_t stack;            /* Stack address */
+	uint64_t stack_size;       /* Stack size */
+	uint64_t tls;              /* TLS descriptor */
+	uint64_t set_tid;          /* Set PID */
+	uint64_t set_tid_size;     /* Number of set_tid entries */
+	uint64_t cgroup;           /* Cgroup file descriptor */
+} __attribute__((aligned(8)));
+
+#ifdef __NR_clone3
+static int use_clone3 = 1;
+#else
+static int use_clone3 = 0;
+#endif
+
+int has_clone3(void)
+{
+	return use_clone3 == 1;
+}
+
+pid_t call_clone3(uint64_t flags,  int cgroup_fd)
+{
+	struct clone3_args cl = {
+		.flags = flags,
+		.exit_signal = SIGCHLD,
+	};
+	pid_t pid;
+
+	if (!use_clone3)
+		goto fallback;
+
+	if (cgroup_fd >= 0) {
+		cl.flags |= CLONE_INTO_CGROUP;
+		cl.cgroup = cgroup_fd;
+	}
+
+	pid = syscall(__NR_clone3, &cl, sizeof(cl));
+	if (pid != -1)
+		return pid;
+
+	logit(LOG_WARNING, "clone3() failed, falling back to fork(): %s", strerror(errno));
+	use_clone3 = 0;
+fallback:
+	return fork();
+}

src/clone3.h

@@ -0,0 +1,33 @@
+/* Portable clone3() wrapper with fallback to fork()
+ *
+ * Copyright (c) 2025  Joachim Wiberg <troglobit@gmail.com>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#ifndef FINIT_CLONE3_H_
+#define FINIT_CLONE3_H_
+
+#include <stdint.h>
+#include <sys/types.h>
+
+pid_t call_clone3 (uint64_t flags,  int cgroup_fd);
+int   has_clone3  (void);
+
+#endif /* FINIT_CLONE3_H_ */