raw_syscalls: add lxc_raw_execveat()

src/lxc/Makefile.am

@@ -22,6 +22,7 @@ noinst_HEADERS = attach.h \
 		 macro.h \
 		 monitor.h \
 		 namespace.h \
+		 raw_syscalls.h \
 		 start.h \
 		 state.h \
 		 storage/btrfs.h \
@@ -114,6 +115,7 @@ liblxc_la_SOURCES = af_unix.c af_unix.h \
 		    network.c network.h \
 		    monitor.c monitor.h \
 		    parse.c parse.h \
+		    raw_syscalls.c raw_syscalls.h \
 		    ringbuf.c ringbuf.h \
 		    rtnl.c rtnl.h \
 		    state.c state.h \

src/lxc/conf.c

@@ -70,6 +70,7 @@
 #include "namespace.h"
 #include "network.h"
 #include "parse.h"
+#include "raw_syscalls.h"
 #include "ringbuf.h"
 #include "start.h"
 #include "storage.h"
@@ -3482,21 +3483,11 @@ static bool verify_start_hooks(struct lxc_conf *conf)
 
 static bool execveat_supported(void)
 {
-#ifdef __NR_execveat
-	/*
-	 * We use the syscall here, because it was introduced in kernel 3.19,
-	 * while glibc got support for using the syscall much later, in 2.27.
-	 * We don't want to use glibc because it falls back to /proc, and the
-	 * container may not have /proc mounted depending on its configuration.
-	 */
-	syscall(__NR_execveat, -1, "", NULL, NULL, AT_EMPTY_PATH);
+	lxc_raw_execveat(-1, "", NULL, NULL, AT_EMPTY_PATH);
 	if (errno == ENOSYS)
 		return false;
 
 	return true;
-#else
-	return false;
-#endif
 }
 
 int lxc_setup(struct lxc_handler *handler)

src/lxc/execute.c

@@ -35,6 +35,7 @@
 #include "config.h"
 #include "log.h"
 #include "start.h"
+#include "raw_syscalls.h"
 #include "utils.h"
 
 lxc_log_define(execute, start);
@@ -122,11 +123,7 @@ static int execute_start(struct lxc_handler *handler, void* data)
 	NOTICE("Exec'ing \"%s\"", my_args->argv[0]);
 
 	if (my_args->init_fd >= 0)
-#ifdef __NR_execveat
-		syscall(__NR_execveat, my_args->init_fd, "", argv, environ, AT_EMPTY_PATH);
-#else
-		ERROR("System seems to be missing execveat syscall number");
-#endif
+		lxc_raw_execveat(my_args->init_fd, "", argv, environ, AT_EMPTY_PATH);
 	else
 		execvp(argv[0], argv);
 	SYSERROR("Failed to exec %s", argv[0]);

src/lxc/raw_syscalls.c

+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE 1
+#endif
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+#include "config.h"
+
+int lxc_raw_execveat(int dirfd, const char *pathname, char *const argv[],
+		     char *const envp[], int flags)
+{
+#ifdef __NR_execveat
+	syscall(__NR_execveat, dirfd, pathname, argv, envp, flags);
+#else
+	errno = ENOSYS;
+	return -1;
+#endif
+}

src/lxc/raw_syscalls.h

+/* liblxcapi
+ *
+ * Copyright © 2018 Christian Brauner <christian.brauner@ubuntu.com>.
+ * Copyright © 2018 Canonical Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2, as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef __LXC_RAW_SYSCALL_H
+#define __LXC_RAW_SYSCALL_H
+
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE 1
+#endif
+#include <stdio.h>
+#include <stdlib.h>
+
+extern int lxc_raw_execveat(int dirfd, const char *pathname, char *const argv[],
+			    char *const envp[], int flags);
+
+#endif /* __LXC_RAW_SYSCALL_H */