Seccomp sandbox changes (performance and correctness fixes, primarily targetting x86-32)

sandbox/linux/seccomp/syscall.cc

Index: sandbox/linux/seccomp/syscall.cc
===================================================================
--- sandbox/linux/seccomp/syscall.cc	(revision 39965)
+++ sandbox/linux/seccomp/syscall.cc	(working copy)
@@ -1,3 +1,7 @@
+// Copyright (c) 2010 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
 #include "debug.h"
 #include "sandbox_impl.h"
 #include "syscall_table.h"
@@ -146,14 +150,76 @@
 
     // Check range of system call
     "cmp playground$maxSyscall, %eax\n"
-    "ja  1f\n"
+    "ja  5f\n"
 
+    // We often have long sequences of calls to gettimeofday(). This is
+    // needlessly expensive. Coalesce them into a single call.
+    //
+    // We keep track of state in TLS storage that we can access through
+    // the %fs segment register. See trusted_thread.cc for the exact
+    // memory layout.
+    //
+    // TODO(markus): maybe, we should proactively call gettimeofday() and
+    //               clock_gettime(), whenever we talk to the trusted thread?
+    //               or maybe, if we have recently seen requests to compute
+    //               the time. There might be a repeated pattern of those.
+    "cmp  $78, %eax\n"             // __NR_gettimeofday
+    "jnz  2f\n"
+    "cmp  %eax, %fs:0x102C-0x54\n" // last system call
+    "jnz  0f\n"
+
+    // This system call and the last system call prior to this one both are
+    // calls to gettimeofday(). Try to avoid making the new call and just
+    // return the same result as in the previous call.
+    // Just in case the caller is spinning on the result from gettimeofday(),
+    // every so often, call the actual system call.
+    "decl %fs:0x1030-0x54\n"       // countdown calls to gettimofday()
+    "jz   0f\n"
+
+    // Atomically read the 64bit word representing last-known timestamp and
+    // return it to the caller. On x86-32 this is a little more complicated and
+    // requires the use of the cmpxchg8b instruction.
+    "mov  %ebx, %eax\n"
+    "mov  %ecx, %edx\n"
+    "lock; cmpxchg8b 100f\n"
+    "mov  %eax, 0(%ebx)\n"
+    "mov  %edx, 4(%ebx)\n"
+    "xor  %eax, %eax\n"
+    "add  $28, %esp\n"
+    "jmp  4f\n"
+
+    // This is a call to gettimeofday(), but we don't have a valid cached
+    // result, yet.
+  "0:mov  %eax, %fs:0x102C-0x54\n" // remember syscall number
+    "movl $500, %fs:0x1030-0x54\n" // make system call, each 500 invocations
+    "call playground$defaultSystemCallHandler\n"
+
+    // Returned from gettimeofday(). Remember return value, in case the
+    // application calls us again right away.
+    // Again, this has to happen atomically and requires cmpxchg8b.
+    "mov 4(%ebx), %ecx\n"
+    "mov 0(%ebx), %ebx\n"
+    "mov 100f, %eax\n"
+    "mov 101f, %edx\n"
+  "1:lock; cmpxchg8b 100f\n"
+    "jnz 1b\n"
+    "xor %eax, %eax\n"
+    "jmp 6f\n"
+
+    // Remember the number of the last system call made. We deliberately do
+    // not remember calls to gettid(), as we have often seen long sequences
+    // of calls to just gettimeofday() and gettid(). In that situation, we
+    // would still like to coalesce the gettimeofday() calls.
+  "2:cmp $224, %eax\n"             // __NR_gettid
+    "jz  3f\n"
+    "mov  %eax, %fs:0x102C-0x54\n" // remember syscall number
+
     // Retrieve function call from system call table (c.f. syscall_table.c).
     // We have three different types of entries; zero for denied system calls,
     // that should be handled by the defaultSystemCallHandler(); minus one
     // for unrestricted system calls that need to be forwarded to the trusted
     // thread; and function pointers to specific handler functions.
-    "shl  $3, %eax\n"
+  "3:shl  $3, %eax\n"
     "lea  playground$syscallTable, %ebx\n"
     "add  %ebx, %eax\n"
     "mov  0(%eax), %eax\n"
@@ -161,14 +227,13 @@
     // Jump to function if non-null and not UNRESTRICTED_SYSCALL, otherwise
     // jump to fallback handler.
     "cmp  $1, %eax\n"
-    "jbe  1f\n"
+    "jbe  5f\n"
     "add  $4, %esp\n"
     "call *%eax\n"
     "add  $24, %esp\n"
-  "0:"
 
     // Restore CPU registers, except for %eax which was set by the system call.
-    "pop  %ebp\n"
+  "4:pop  %ebp\n"
     "pop  %edi\n"
     "pop  %esi\n"
     "pop  %edx\n"
@@ -178,14 +243,17 @@
     // Return to caller
     "ret\n"
 
-  "1:"
     // Call default handler.
-    "push $2f\n"
-    "push $playground$defaultSystemCallHandler\n"
-    "ret\n"
-  "2:add  $28, %esp\n"
-    "jmp 0b\n"
+  "5:call playground$defaultSystemCallHandler\n"
+  "6:add  $28, %esp\n"
+    "jmp 4b\n"
 
+    ".pushsection \".bss\"\n"
+    ".balign 8\n"
+"100:.byte 0, 0, 0, 0\n"
+"101:.byte 0, 0, 0, 0\n"
+    ".popsection\n"
+
     #else
     #error Unsupported target platform
     #endif