[lxc-devel] [PATCH 1/2] Modify lxc_attach and lxc_init to use clone instead of fork.

[Vladimir Smirnov]

Clone allows more flexible control. Currently, if there is any inherited fd,
lxc-start exits with error. With clone it's possible not to pass open fd's to childs.

Signed-off-by: Vladimir Smirnov <civil at yandex-team.ru>
---
 src/lxc/lxc_attach.c |   79 +++++++++++++++++++++++++++++---------------------
 src/lxc/lxc_init.c   |   46 ++++++++++++++++++-----------
 2 files changed, 75 insertions(+), 50 deletions(-)

diff --git a/src/lxc/lxc_attach.c b/src/lxc/lxc_attach.c
index ed3d5a4..930865e 100644
--- a/src/lxc/lxc_attach.c
+++ b/src/lxc/lxc_attach.c
@@ -20,6 +20,7 @@
  * License along with this library; if not, write to the Free Software
  * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
  */
+#define STACKSIZE 16384
 
 #define _GNU_SOURCE
 #include <unistd.h>
@@ -29,6 +30,7 @@
 #include <sys/param.h>
 #include <sys/types.h>
 #include <sys/wait.h>
+#include <sched.h>
 
 #include "commands.h"
 #include "arguments.h"
@@ -56,14 +58,48 @@ Options :\n\
 	.checker  = NULL,
 };
 
+static char *envp_global;
+
+int child_main()
+{
+	struct passwd *passwd;
+	uid_t uid;
+
+	if (my_args.argc) {
+		execve(my_args.argv[0], my_args.argv, envp_global);
+		SYSERROR("failed to exec '%s'", my_args.argv[0]);
+		return -1;
+	}
+
+	uid = getuid();
+
+	passwd = getpwuid(uid);
+	if (!passwd) {
+		SYSERROR("failed to get passwd "		\
+			 "entry for uid '%d'", uid);
+		return -1;
+	}
+
+	{
+		char *const args[] = {
+			passwd->pw_shell,
+			NULL,
+		};
+
+		execve(args[0], args, envp_global);
+		SYSERROR("failed to exec '%s'", args[0]);
+		return -1;
+	}
+}
+
 int main(int argc, char *argv[], char *envp[])
 {
 	int ret;
 	pid_t pid;
-	struct passwd *passwd;
-	uid_t uid;
 	char *curdir;
 
+	envp_global = envp;
+
 	ret = lxc_caps_init();
 	if (ret)
 		return ret;
@@ -96,7 +132,14 @@ int main(int argc, char *argv[], char *envp[])
 
 	free(curdir);
 
-	pid = fork();
+	void **newstack;
+	newstack = (void **) malloc(STACKSIZE);
+	if (!newstack)
+		exit(newstack);
+
+	newstack = (void **)(STACKSIZE + (char *)newstack);
+	*--newstack = 0;
+	pid = clone(child_main, newstack, CLONE_VM | CLONE_FS | CLONE_SIGHAND, 0);
 
 	if (pid < 0) {
 		SYSERROR("failed to fork");
@@ -120,35 +163,5 @@ int main(int argc, char *argv[], char *envp[])
 		return -1;
 	}
 
-	if (!pid) {
-
-		if (my_args.argc) {
-			execve(my_args.argv[0], my_args.argv, envp);
-			SYSERROR("failed to exec '%s'", my_args.argv[0]);
-			return -1;
-		}
-
-		uid = getuid();
-
-		passwd = getpwuid(uid);
-		if (!passwd) {
-			SYSERROR("failed to get passwd "		\
-				 "entry for uid '%d'", uid);
-			return -1;
-		}
-
-		{
-			char *const args[] = {
-				passwd->pw_shell,
-				NULL,
-			};
-
-			execve(args[0], args, envp);
-			SYSERROR("failed to exec '%s'", args[0]);
-			return -1;
-		}
-
-	}
-
 	return 0;
 }
diff --git a/src/lxc/lxc_init.c b/src/lxc/lxc_init.c
index a534b51..7debd6f 100644
--- a/src/lxc/lxc_init.c
+++ b/src/lxc/lxc_init.c
@@ -20,6 +20,7 @@
  * License along with this library; if not, write to the Free Software
  * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
  */
+#define STACKSIZE 16384
 
 #include <stdio.h>
 #include <unistd.h>
@@ -32,6 +33,7 @@
 #include <sys/wait.h>
 #define _GNU_SOURCE
 #include <getopt.h>
+#include <sched.h>
 
 #include "log.h"
 #include "caps.h"
@@ -40,6 +42,9 @@
 
 lxc_log_define(lxc_init, lxc);
 
+static sigset_t mask, omask;
+static char **aargv;
+
 static int quiet;
 
 static struct option options[] = {
@@ -49,6 +54,23 @@ static struct option options[] = {
 
 static	int was_interrupted = 0;
 
+int child_main()
+{
+	int i;
+	int err = -1;
+	/* restore default signal handlers */
+	for (i = 1; i < NSIG; i++)
+		signal(i, SIG_DFL);
+
+	sigprocmask(SIG_SETMASK, &omask, NULL);
+
+	NOTICE("about to exec '%s'", aargv[0]);
+
+	execvp(aargv[0], aargv);
+	ERROR("failed to exec: '%s' : %m", aargv[0]);
+	exit(err);
+}
+
 int main(int argc, char *argv[])
 {
 
@@ -61,8 +83,6 @@ int main(int argc, char *argv[])
 	pid_t pid;
 	int nbargs = 0;
 	int err = -1;
-	char **aargv;
-	sigset_t mask, omask;
 	int i, shutdown = 0;
 
 	while (1) {
@@ -115,25 +135,17 @@ int main(int argc, char *argv[])
 	if (lxc_caps_reset())
 		exit(err);
 
-	pid = fork();
-
-	if (pid < 0)
+	void **newstack;
+	newstack = (void **) malloc(STACKSIZE);
+	if (!newstack)
 		exit(err);
 
-	if (!pid) {
-
-		/* restore default signal handlers */
-		for (i = 1; i < NSIG; i++)
-			signal(i, SIG_DFL);
+	newstack = (void **)(STACKSIZE + (char *)newstack);
+	*--newstack = 0;
+	pid = clone(child_main, newstack, CLONE_VM | CLONE_FS | CLONE_SIGHAND, 0);
 
-		sigprocmask(SIG_SETMASK, &omask, NULL);
-
-		NOTICE("about to exec '%s'", aargv[0]);
-
-		execvp(aargv[0], aargv);
-		ERROR("failed to exec: '%s' : %m", aargv[0]);
+	if (pid < 0)
 		exit(err);
-	}
 
 	/* let's process the signals now */
 	sigdelset(&omask, SIGALRM);
-- 
1.7.6