Make the seccomp sandbox compatible with tcmalloc:...

third_party/tcmalloc/chromium/src/base/spinlock_linux-inl.h

Index: third_party/tcmalloc/chromium/src/base/spinlock_linux-inl.h
===================================================================
--- third_party/tcmalloc/chromium/src/base/spinlock_linux-inl.h	(revision 42483)
+++ third_party/tcmalloc/chromium/src/base/spinlock_linux-inl.h	(working copy)
@@ -33,12 +33,22 @@
 
 #include <sched.h>
 #include <time.h>
-#include "base/linux_syscall_support.h"
 
 #define FUTEX_WAIT 0
 #define FUTEX_WAKE 1
 #define FUTEX_PRIVATE_FLAG 128
 
+// Note: Instead of making direct system calls that are inlined, we rely
+//       on the syscall() function in glibc to do the right thing. This
+//       is necessary to make the code compatible with the seccomp sandbox,
+//       which needs to be able to find and patch all places where system
+//       calls are made. Scanning through and patching glibc is fast, but
+//       doing so on the entire Chrome binary would be prohibitively
+//       expensive.
+//       This is a notable change from the upstream version of tcmalloc,
+//       which prefers direct system calls in order to improve compatibility
+//       with older toolchains and runtime libraries.
+
 static bool have_futex;
 static int futex_private_flag = FUTEX_PRIVATE_FLAG;
 
@@ -49,9 +59,9 @@
     // futexes are ints, so we can use them only when
     // that's the same size as the lockword_ in SpinLock.
     have_futex = (sizeof (Atomic32) == sizeof (int) && 
-                  sys_futex(&x, FUTEX_WAKE, 1, 0) >= 0);
+                  syscall(__NR_futex, &x, FUTEX_WAKE, 1, 0) >= 0);
     if (have_futex &&
-        sys_futex(&x, FUTEX_WAKE | futex_private_flag, 1, 0) < 0) {
+        syscall(__NR_futex, &x, FUTEX_WAKE | futex_private_flag, 1, 0) < 0) {
       futex_private_flag = 0;
     }
   }
@@ -67,7 +77,7 @@
     tm.tv_nsec = 1000000;   // 1ms; really we're trying to sleep for one kernel
                             // clock tick
     while ((value = base::subtle::Acquire_CompareAndSwap(w, 0, 1)) != 0) {
-      sys_futex(reinterpret_cast<int *>(const_cast<Atomic32 *>(w)),
+      syscall(__NR_futex, reinterpret_cast<int *>(const_cast<Atomic32 *>(w)),
           FUTEX_WAIT | futex_private_flag,
           value, reinterpret_cast<struct kernel_timespec *>(&tm));
     }
@@ -85,7 +95,7 @@
 
 static void SpinLockWake(volatile Atomic32 *w) {
   if (have_futex) {
-    sys_futex(reinterpret_cast<int *>(const_cast<Atomic32 *>(w)),
+    syscall(__NR_futex, reinterpret_cast<int *>(const_cast<Atomic32 *>(w)),
               FUTEX_WAKE | futex_private_flag, 1, 0);
   }
 }