Use the libc clone wrapper in sys_clone.

sandbox/linux/services/syscall_wrappers.cc

Index: sandbox/linux/services/syscall_wrappers.cc
diff --git a/sandbox/linux/services/syscall_wrappers.cc b/sandbox/linux/services/syscall_wrappers.cc
index c3685197e88c7313e713c803a962b8e2f920582c..6372a753eff4dc9bf49569851446e803e75fee32 100644
--- a/sandbox/linux/services/syscall_wrappers.cc
+++ b/sandbox/linux/services/syscall_wrappers.cc
@@ -4,6 +4,9 @@
 
 #include "sandbox/linux/services/syscall_wrappers.h"
 
+#include <pthread.h>
+#include <sched.h>
+#include <setjmp.h>
 #include <sys/resource.h>
 #include <sys/syscall.h>
 #include <sys/time.h>
@@ -24,12 +27,23 @@ pid_t sys_gettid(void) {
   return syscall(__NR_gettid);
 }
 
-long sys_clone(unsigned long flags) {
-  return sys_clone(flags, nullptr, nullptr, nullptr, nullptr);
+namespace {
+
+int CloneHelper(void* arg) {
+  jmp_buf* env_ptr = reinterpret_cast<jmp_buf*>(arg);
+  // This function runs on the stack specified on the clone call. Use longjmp to
+  // switch back to the original stack so the child can return from sys_clone.
+  longjmp(*env_ptr, 1);
+
+  // Should not be reached.
+  RAW_CHECK(false);
+  return 1;
 }
 
+}  // namespace
+
 long sys_clone(unsigned long flags,
-               void* child_stack,
+               decltype(nullptr) child_stack,
                pid_t* ptid,
                pid_t* ctid,
                decltype(nullptr) tls) {
@@ -37,21 +51,37 @@ long sys_clone(unsigned long flags,
   const bool invalid_ctid =
       (flags & (CLONE_CHILD_SETTID | CLONE_CHILD_CLEARTID)) && !ctid;
   const bool invalid_ptid = (flags & CLONE_PARENT_SETTID) && !ptid;
-  const bool invalid_stack = (flags & CLONE_VM) && !child_stack;
 
-  if (clone_tls_used || invalid_ctid || invalid_ptid || invalid_stack) {
+  // Since child_stack must be nullptr, we do not support CLONE_VM.
+  const bool clone_vm_used = flags & CLONE_VM;
+
+  if (clone_tls_used || invalid_ctid || invalid_ptid || clone_vm_used) {
     RAW_LOG(FATAL, "Invalid usage of sys_clone");
   }
 
-// See kernel/fork.c in Linux. There is different ordering of sys_clone
-// parameters depending on CONFIG_CLONE_BACKWARDS* configuration options.
-#if defined(ARCH_CPU_X86_64)
-  return syscall(__NR_clone, flags, child_stack, ptid, ctid, tls);
-#elif defined(ARCH_CPU_X86) || defined(ARCH_CPU_ARM_FAMILY) || \
-    defined(ARCH_CPU_MIPS_FAMILY) || defined(ARCH_CPU_MIPS64_FAMILY)
-  // CONFIG_CLONE_BACKWARDS defined.
-  return syscall(__NR_clone, flags, child_stack, ptid, tls, ctid);
+  jmp_buf env;
+  if (setjmp(env) == 0) {
+    // We use the libc clone wrapper instead of making the syscall
+    // directly because making the syscall may fail to update the libc's
+    // internal pid cache. The libc interface unfortunately requires
+    // specifying a new stack, so we use setjmp/longjmp to emulate
+    // fork-like behavior.
+    char stack_buf[PTHREAD_STACK_MIN];
+#if defined(ARCH_CPU_X86_FAMILY) || defined(ARCH_CPU_ARM_FAMILY) || \
+    defined(ARCH_CPU_MIPS64_FAMILY) || defined(ARCH_CPU_MIPS_FAMILY)
+    // The stack grows downward.
+    void* stack = stack_buf + sizeof(stack_buf);
+#else
+#error "Unsupported architecture"
 #endif
+    return clone(&CloneHelper, stack, flags, &env, ptid, tls, ctid);
+  }
+
+  return 0;
+}
+
+long sys_clone(unsigned long flags) {
+  return sys_clone(flags, nullptr, nullptr, nullptr, nullptr);
 }
 
 void sys_exit_group(int status) {