Only call FlushFileBuffers() when writing executables on Windows

This is a follow-up for "r325274: Call FlushFileBuffers on output files."

Previously, FlushFileBuffers() was called in all cases when writing a file. The objective was to go around a bug in the Windows kernel (as described here: https://randomascii.wordpress.com/2018/02/25/compiler-bug-linker-bug-windows-kernel-bug/). However that is required only when writing EXEs, any other file type doesn't need flushing.

This patch calls FlushFileBuffers() only for EXEs. In addition, we completly disable FlushFileBuffers() for known Windows 10 versions that do not exhibit the original kernel bug.

Differential Revision: https://reviews.llvm.org/D53727

llvm-svn: 346152

Author: aganea

llvm/lib/Support/Windows/Path.inc

@@ -854,16 +854,37 @@ mapped_file_region::mapped_file_region(int fd, mapmode mode, size_t length,
     Mapping = 0;
 }
 
+static bool hasFlushBufferKernelBug() {
+  static bool Ret{GetWindowsOSVersion() < llvm::VersionTuple(10, 0, 0, 17763)};
+  return Ret;
+}
+
+static bool isEXE(StringRef Magic) {
+  static const char PEMagic[] = {'P', 'E', '\0', '\0'};
+  if (Magic.startswith(StringRef("MZ")) && Magic.size() >= 0x3c + 4) {
+    uint32_t off = read32le(Magic.data() + 0x3c);
+    // PE/COFF file, either EXE or DLL.
+    if (Magic.substr(off).startswith(StringRef(PEMagic, sizeof(PEMagic))))
+      return true;
+  }
+  return false;
+}
+
 mapped_file_region::~mapped_file_region() {
   if (Mapping) {
+
+    bool Exe = isEXE(StringRef((char *)Mapping, Size));
+
     ::UnmapViewOfFile(Mapping);
 
-    if (Mode == mapmode::readwrite) {
+    if (Mode == mapmode::readwrite && Exe && hasFlushBufferKernelBug()) {
       // There is a Windows kernel bug, the exact trigger conditions of which
       // are not well understood.  When triggered, dirty pages are not properly
       // flushed and subsequent process's attempts to read a file can return
       // invalid data.  Calling FlushFileBuffers on the write handle is
       // sufficient to ensure that this bug is not triggered.
+      // The bug only occurs when writing an executable and executing it right
+      // after, under high I/O pressure.
       ::FlushFileBuffers(FileHandle);
     }
 

llvm/lib/Support/Windows/WindowsSupport.h

@@ -41,6 +41,7 @@
 #include "llvm/Config/config.h" // Get build system configuration settings
 #include "llvm/Support/Chrono.h"
 #include "llvm/Support/Compiler.h"
+#include "llvm/Support/VersionTuple.h"
 #include <cassert>
 #include <string>
 #include <system_error>
@@ -71,6 +72,40 @@ inline bool RunningWindows8OrGreater() {
                             Mask) != FALSE;
 }
 
+typedef NTSTATUS(WINAPI* RtlGetVersionPtr)(PRTL_OSVERSIONINFOW);
+#define STATUS_SUCCESS ((NTSTATUS)0x00000000L)
+
+inline llvm::VersionTuple GetWindowsOSVersion() {
+  HMODULE hMod = ::GetModuleHandleW(L"ntdll.dll");
+  if (hMod) {
+    auto getVer = (RtlGetVersionPtr)::GetProcAddress(hMod, "RtlGetVersion");
+    if (getVer) {
+      RTL_OSVERSIONINFOEXW info{};
+      info.dwOSVersionInfoSize = sizeof(info);
+      if (getVer((PRTL_OSVERSIONINFOW)&info) == STATUS_SUCCESS) {
+        return llvm::VersionTuple(info.dwMajorVersion, info.dwMinorVersion, 0,
+                                  info.dwBuildNumber);
+      }
+    }
+  }
+
+  OSVERSIONINFOEX info;
+  ZeroMemory(&info, sizeof(OSVERSIONINFOEX));
+  info.dwOSVersionInfoSize = sizeof(OSVERSIONINFOEX);
+#pragma warning(push)
+#pragma warning(disable : 4996)
+  // Starting with Microsoft SDK for Windows 8.1, this function is deprecated
+  // in favor of the new Windows Version Helper APIs.  Since we don't specify a
+  // minimum SDK version, it's easier to simply disable the warning rather than
+  // try to support both APIs.
+  if (GetVersionEx((LPOSVERSIONINFO)&info) == 0)
+    return llvm::VersionTuple();
+#pragma warning(pop)
+
+  return llvm::VersionTuple(info.dwMajorVersion, info.dwMinorVersion, 0,
+                            info.dwBuildNumber);
+}
+
 inline bool MakeErrMsg(std::string *ErrMsg, const std::string &prefix) {
   if (!ErrMsg)
     return true;