commit e104a2edf3c459b2e79e86d4de388fb7a1efe198
author Elliott Hughes <enh@google.com> Fri May 06 15:55:36 2016 -0700
committer Elliott Hughes <enh@google.com> Fri May 06 16:37:00 2016 -0700
tree 4f50d2627e6f47f6908561114d95b32a168691bc
parent 5ac45a2add41fd7c1640815accf09a29bd14aac6

Generalize the clone function slightly.

* Allow clone where both the child function and stack are null. It's
obviously wrong to ask to call a function without a stack, but it's not
necessarily wrong to supply no stack if you're also not supplying a
function.

* Reimplement fork in terms of the clone function, rather than using the
clone system call directly.

This is intended as a step towards enabling use of pid namespaces.

Change-Id: I03c89bd1dc540d8b4ed1c8fdf6644290744b9e91

libc/bionic/clone.cpp

diff --git a/libc/bionic/clone.cpp b/libc/bionic/clone.cpp
index af63977..8281ac8 100644
--- a/libc/bionic/clone.cpp
+++ b/libc/bionic/clone.cpp
@@ -47,7 +47,7 @@
   void* new_tls = NULL;
   int* child_tid = NULL;
 
-  if (!child_stack) {
+  if (fn != nullptr && child_stack == nullptr) {
     errno = EINVAL;
     return -1;
   }
@@ -76,7 +76,16 @@
   pid_t parent_pid = self->invalidate_cached_pid();
 
   // Actually do the clone.
-  int clone_result = __bionic_clone(flags, child_stack, parent_tid, new_tls, child_tid, fn, arg);
+  int clone_result;
+  if (fn != nullptr) {
+    clone_result = __bionic_clone(flags, child_stack, parent_tid, new_tls, child_tid, fn, arg);
+  } else {
+#if defined(__x86_64__) // sys_clone's last two arguments are flipped on x86-64.
+    clone_result = syscall(__NR_clone, flags, child_stack, parent_tid, child_tid, new_tls);
+#else
+    clone_result = syscall(__NR_clone, flags, child_stack, parent_tid, new_tls, child_tid);
+#endif
+  }
 
   // We're the parent, so put our known pid back in place.
   // We leave the child without a cached pid, but:

libc/bionic/fork.cpp

diff --git a/libc/bionic/fork.cpp b/libc/bionic/fork.cpp
index 6cfc736..ffe94f4 100644
--- a/libc/bionic/fork.cpp
+++ b/libc/bionic/fork.cpp
@@ -31,8 +31,6 @@
 
 #include "pthread_internal.h"
 
-#define FORK_FLAGS (CLONE_CHILD_SETTID | CLONE_CHILD_CLEARTID | SIGCHLD)
-
 int fork() {
   __bionic_atfork_run_prepare();
 
@@ -41,11 +39,13 @@
   // Remember the parent pid and invalidate the cached value while we fork.
   pid_t parent_pid = self->invalidate_cached_pid();
 
-#if defined(__x86_64__) // sys_clone's last two arguments are flipped on x86-64.
-  int result = syscall(__NR_clone, FORK_FLAGS, NULL, NULL, &(self->tid), NULL);
-#else
-  int result = syscall(__NR_clone, FORK_FLAGS, NULL, NULL, NULL, &(self->tid));
-#endif
+  int result = clone(nullptr,
+                     nullptr,
+                     (CLONE_CHILD_SETTID | CLONE_CHILD_CLEARTID | SIGCHLD),
+                     nullptr,
+                     nullptr,
+                     nullptr,
+                     &(self->tid));
   if (result == 0) {
     self->set_cached_pid(gettid());
     __bionic_atfork_run_child();