raw_syscalls: add lxc_raw_clone{_cb}()

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

src/lxc/attach.c

@@ -59,6 +59,7 @@
 #include "macro.h"
 #include "mainloop.h"
 #include "namespace.h"
+#include "raw_syscalls.h"
 #include "terminal.h"
 #include "utils.h"
 

src/lxc/namespace.c

@@ -75,80 +75,6 @@ pid_t lxc_clone(int (*fn)(void *), void *arg, int flags)
 	return ret;
 }
 
-/**
- * This is based on raw_clone in systemd but adapted to our needs. This uses
- * copy on write semantics and doesn't pass a stack. CLONE_VM is tricky and
- * doesn't really matter to us so disallow it.
- *
- * The nice thing about this is that we get fork() behavior. That is
- * lxc_raw_clone() returns 0 in the child and the child pid in the parent.
- */
-pid_t lxc_raw_clone(unsigned long flags)
-{
-
-	/* These flags don't interest at all so we don't jump through any hoopes
-	 * of retrieving them and passing them to the kernel.
-	 */
-	errno = EINVAL;
-	if ((flags & (CLONE_VM | CLONE_PARENT_SETTID | CLONE_CHILD_SETTID |
-		      CLONE_CHILD_CLEARTID | CLONE_SETTLS)))
-		return -EINVAL;
-
-#if defined(__s390x__) || defined(__s390__) || defined(__CRIS__)
-	/* On s390/s390x and cris the order of the first and second arguments
-	 * of the system call is reversed.
-	 */
-	return (int)syscall(__NR_clone, NULL, flags | SIGCHLD);
-#elif defined(__sparc__) && defined(__arch64__)
-	{
-		/**
-		 * sparc64 always returns the other process id in %o0, and
-		 * a boolean flag whether this is the child or the parent in
-		 * %o1. Inline assembly is needed to get the flag returned
-		 * in %o1.
-		 */
-		int in_child;
-		int child_pid;
-		asm volatile("mov %2, %%g1\n\t"
-			     "mov %3, %%o0\n\t"
-			     "mov 0 , %%o1\n\t"
-			     "t 0x6d\n\t"
-			     "mov %%o1, %0\n\t"
-			     "mov %%o0, %1"
-			     : "=r"(in_child), "=r"(child_pid)
-			     : "i"(__NR_clone), "r"(flags | SIGCHLD)
-			     : "%o1", "%o0", "%g1");
-
-		if (in_child)
-			return 0;
-		else
-			return child_pid;
-	}
-#elif defined(__ia64__)
-	/* On ia64 the stack and stack size are passed as separate arguments. */
-	return (int)syscall(__NR_clone, flags | SIGCHLD, NULL, 0);
-#else
-	return (int)syscall(__NR_clone, flags | SIGCHLD, NULL);
-#endif
-}
-
-pid_t lxc_raw_clone_cb(int (*fn)(void *), void *args, unsigned long flags)
-{
-	pid_t pid;
-
-	pid = lxc_raw_clone(flags);
-	if (pid < 0)
-		return -1;
-
-	/* exit() is not thread-safe and might mess with the parent's signal
-	 * handlers and other stuff when exec() fails.
-	 */
-	if (pid == 0)
-		_exit(fn(args));
-
-	return pid;
-}
-
 /* Leave the user namespace at the first position in the array of structs so
  * that we always attach to it first when iterating over the struct and using
  * setns() to switch namespaces. This especially affects lxc_attach(): Suppose

src/lxc/namespace.h

@@ -128,55 +128,13 @@ int clone(int (*fn)(void *), void *child_stack,
  *   corresponding libc wrapper. glibc currently does not run pthread_atfork()
  *   handlers but does not guarantee that they are not. Other libcs might or
  *   might not run pthread_atfork() handlers. If you require guarantees please
- *   refer to the lxc_raw_clone*() functions below.
+ *   refer to the lxc_raw_clone*() functions in raw_syscalls.{c,h}.
  *
  * - should call lxc_raw_getpid():
  *   The child should use lxc_raw_getpid() to retrieve its pid.
  */
 extern pid_t lxc_clone(int (*fn)(void *), void *arg, int flags);
 
-/**
- * lxc_raw_clone() - create a new process
- *
- * - fork() behavior:
- *   This function returns 0 in the child and > 0 in the parent.
- *
- * - copy-on-write:
- *   This function does not allocate a new stack and relies on copy-on-write
- *   semantics.
- *
- * - supports subset of ClONE_* flags:
- *   lxc_raw_clone() intentionally only supports a subset of the flags available
- *   to the actual system call. Please refer to the implementation what flags
- *   cannot be used. Also, please don't assume that just because a flag isn't
- *   explicitly checked for as being unsupported that it is supported. If in
- *   doubt or not sufficiently familiar with process creation in the kernel and
- *   interactions with libcs this function should be used.
- *
- * - no pthread_atfork() handlers:
- *   This function circumvents - as much as this this is possible - any libc
- *   wrappers and thus does not run any pthread_atfork() handlers. Make sure
- *   that this is safe to do in the context you are trying to call this
- *   function.
- *
- * - must call lxc_raw_getpid():
- *   The child must use lxc_raw_getpid() to retrieve its pid.
- */
-extern pid_t lxc_raw_clone(unsigned long flags);
-/**
- * lxc_raw_clone_cb() - create a new process
- *
- * - non-fork() behavior:
- *   Function does return pid of the child or -1 on error. Pass in a callback
- *   function via the "fn" argument that gets executed in the child process. The
- *   "args" argument is passed to "fn".
- *
- * All other comments that apply to lxc_raw_clone() apply to lxc_raw_clone_cb()
- * as well.
- */
-extern pid_t lxc_raw_clone_cb(int (*fn)(void *), void *args,
-			      unsigned long flags);
-
 extern int lxc_namespace_2_cloneflag(const char *namespace);
 extern int lxc_namespace_2_ns_idx(const char *namespace);
 extern int lxc_namespace_2_std_identifiers(char *namespaces);

src/lxc/raw_syscalls.c

@@ -2,12 +2,16 @@
 #define _GNU_SOURCE 1
 #endif
 #include <errno.h>
+#include <sched.h>
+#include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <sys/syscall.h>
 #include <unistd.h>
 
 #include "config.h"
+#include "macro.h"
+#include "raw_syscalls.h"
 
 int lxc_raw_execveat(int dirfd, const char *pathname, char *const argv[],
 		     char *const envp[], int flags)
@@ -19,3 +23,78 @@ int lxc_raw_execveat(int dirfd, const char *pathname, char *const argv[],
 	return -1;
 #endif
 }
+
+/*
+ * This is based on raw_clone in systemd but adapted to our needs. This uses
+ * copy on write semantics and doesn't pass a stack. CLONE_VM is tricky and
+ * doesn't really matter to us so disallow it.
+ *
+ * The nice thing about this is that we get fork() behavior. That is
+ * lxc_raw_clone() returns 0 in the child and the child pid in the parent.
+ */
+pid_t lxc_raw_clone(unsigned long flags)
+{
+
+	/*
+	 * These flags don't interest at all so we don't jump through any hoopes
+	 * of retrieving them and passing them to the kernel.
+	 */
+	errno = EINVAL;
+	if ((flags & (CLONE_VM | CLONE_PARENT_SETTID | CLONE_CHILD_SETTID |
+		      CLONE_CHILD_CLEARTID | CLONE_SETTLS)))
+		return -EINVAL;
+
+#if defined(__s390x__) || defined(__s390__) || defined(__CRIS__)
+	/* On s390/s390x and cris the order of the first and second arguments
+	 * of the system call is reversed.
+	 */
+	return (int)syscall(__NR_clone, NULL, flags | SIGCHLD);
+#elif defined(__sparc__) && defined(__arch64__)
+	{
+		/*
+		 * sparc64 always returns the other process id in %o0, and a
+		 * boolean flag whether this is the child or the parent in %o1.
+		 * Inline assembly is needed to get the flag returned in %o1.
+		 */
+		int in_child;
+		int child_pid;
+		asm volatile("mov %2, %%g1\n\t"
+			     "mov %3, %%o0\n\t"
+			     "mov 0 , %%o1\n\t"
+			     "t 0x6d\n\t"
+			     "mov %%o1, %0\n\t"
+			     "mov %%o0, %1"
+			     : "=r"(in_child), "=r"(child_pid)
+			     : "i"(__NR_clone), "r"(flags | SIGCHLD)
+			     : "%o1", "%o0", "%g1");
+
+		if (in_child)
+			return 0;
+		else
+			return child_pid;
+	}
+#elif defined(__ia64__)
+	/* On ia64 the stack and stack size are passed as separate arguments. */
+	return (int)syscall(__NR_clone, flags | SIGCHLD, NULL, prctl_arg(0));
+#else
+	return (int)syscall(__NR_clone, flags | SIGCHLD, NULL);
+#endif
+}
+
+pid_t lxc_raw_clone_cb(int (*fn)(void *), void *args, unsigned long flags)
+{
+	pid_t pid;
+
+	pid = lxc_raw_clone(flags);
+	if (pid < 0)
+		return -1;
+
+	/*
+	 * exit() is not thread-safe and might mess with the parent's signal
+	 * handlers and other stuff when exec() fails.
+	 */
+	if (pid == 0)
+		_exit(fn(args));
+
+	return pid;
+}

src/lxc/raw_syscalls.h

@@ -23,8 +23,52 @@
 #ifndef _GNU_SOURCE
 #define _GNU_SOURCE 1
 #endif
+#include <sched.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <sys/syscall.h>
+
+/*
+ * lxc_raw_clone() - create a new process
+ *
+ * - fork() behavior:
+ *   This function returns 0 in the child and > 0 in the parent.
+ *
+ * - copy-on-write:
+ *   This function does not allocate a new stack and relies on copy-on-write
+ *   semantics.
+ *
+ * - supports subset of ClONE_* flags:
+ *   lxc_raw_clone() intentionally only supports a subset of the flags available
+ *   to the actual system call. Please refer to the implementation what flags
+ *   cannot be used. Also, please don't assume that just because a flag isn't
+ *   explicitly checked for as being unsupported that it is supported. If in
+ *   doubt or not sufficiently familiar with process creation in the kernel and
+ *   interactions with libcs this function should be used.
+ *
+ * - no pthread_atfork() handlers:
+ *   This function circumvents - as much as this this is possible - any libc
+ *   wrappers and thus does not run any pthread_atfork() handlers. Make sure
+ *   that this is safe to do in the context you are trying to call this
+ *   function.
+ *
+ * - must call lxc_raw_getpid():
+ *   The child must use lxc_raw_getpid() to retrieve its pid.
+ */
+extern pid_t lxc_raw_clone(unsigned long flags);
+
+/*
+ * lxc_raw_clone_cb() - create a new process
+ *
+ * - non-fork() behavior:
+ *   Function does return pid of the child or -1 on error. Pass in a callback
+ *   function via the "fn" argument that gets executed in the child process.
+ *   The "args" argument is passed to "fn".
+ *
+ * All other comments that apply to lxc_raw_clone() apply to lxc_raw_clone_cb()
+ * as well.
+ */
+extern pid_t lxc_raw_clone_cb(int (*fn)(void *), void *args, unsigned long flags);
 
 extern int lxc_raw_execveat(int dirfd, const char *pathname, char *const argv[],
 			    char *const envp[], int flags);

src/lxc/start.c

@@ -70,6 +70,7 @@
 #include "monitor.h"
 #include "namespace.h"
 #include "network.h"
+#include "raw_syscalls.h"
 #include "start.h"
 #include "storage/storage.h"
 #include "storage/storage_utils.h"

src/lxc/utils.c

@@ -51,6 +51,7 @@
 #include "lxclock.h"
 #include "namespace.h"
 #include "parse.h"
+#include "raw_syscalls.h"
 #include "syscall_wrappers.h"
 #include "utils.h"
 

src/tests/lxc_raw_clone.c

@@ -38,7 +38,7 @@
 #include <sys/types.h>
 
 #include "lxctest.h"
-#include "namespace.h"
+#include "raw_syscalls.h"
 #include "utils.h"
 
 int main(int argc, char *argv[])