Use memfd_create when available

Author: am11

src/libraries/Common/src/Interop/Unix/System.Native/Interop.MemfdCreate.cs

@@ -0,0 +1,34 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Runtime.InteropServices;
+using System.Threading;
+using Microsoft.Win32.SafeHandles;
+
+internal static partial class Interop
+{
+    internal static partial class Sys
+    {
+        [LibraryImport(Libraries.SystemNative, EntryPoint = "SystemNative_MemfdCreate", StringMarshalling = StringMarshalling.Utf8, SetLastError = true)]
+        internal static partial SafeFileHandle MemfdCreate(string name);
+
+        [LibraryImport(Libraries.SystemNative, EntryPoint = "SystemNative_MemfdSupported", SetLastError = true)]
+        private static partial int MemfdSupportedImpl();
+
+        private static volatile int s_memfdSupported = -1;
+
+        internal static bool MemfdSupported
+        {
+            get
+            {
+                int result = Interlocked.CompareExchange(ref s_memfdSupported, -1, -1);
+                if (result == -1)
+                {
+                    result = MemfdSupportedImpl();
+                    Interlocked.Exchange(ref s_memfdSupported, result);
+                }
+                return result == 1;
+            }
+        }
+    }
+}

src/libraries/System.IO.MemoryMappedFiles/src/System.IO.MemoryMappedFiles.csproj

@@ -91,6 +91,8 @@
              Link="Common\Interop\Unix\Interop.Libraries.cs" />
     <Compile Include="$(CommonPath)Interop\Unix\Interop.Errors.cs"
              Link="Common\Interop\Unix\Interop.Errors.cs" />
+    <Compile Include="$(CommonPath)Interop\Unix\System.Native\Interop.Close.cs"
+             Link="Common\Interop\Unix\System.Native\Interop.Close.cs" />
     <Compile Include="$(CommonPath)Interop\Unix\System.Native\Interop.Fcntl.cs"
              Link="Common\Interop\Unix\Interop.Fcntl.cs" />
     <Compile Include="$(CommonPath)Interop\Unix\Interop.IOErrors.cs"
@@ -119,6 +121,8 @@
              Link="Common\Interop\Unix\Interop.MAdvise.cs" />
     <Compile Include="$(CommonPath)Interop\Unix\System.Native\Interop.ShmOpen.cs"
              Link="Common\Interop\Unix\Interop.ShmOpen.cs" />
+    <Compile Include="$(CommonPath)Interop\Unix\System.Native\Interop.MemfdCreate.cs"
+             Link="Common\Interop\Unix\Interop.MemfdCreate.cs" />
     <Compile Include="$(CommonPath)Interop\Unix\System.Native\Interop.Unlink.cs"
              Link="Common\Interop\Unix\Interop.Unlink.cs" />
   </ItemGroup>

src/libraries/System.IO.MemoryMappedFiles/src/System/IO/MemoryMappedFiles/MemoryMappedFile.Unix.cs

@@ -190,7 +190,14 @@ private static SafeFileHandle CreateSharedBackingObject(Interop.Sys.MemoryMapped
             do
             {
                 mapName = GenerateMapName();
-                fd = Interop.Sys.ShmOpen(mapName, flags, (int)perms); // Create the shared memory object.
+                if (Interop.Sys.MemfdSupported)
+                {
+                    fd = Interop.Sys.MemfdCreate(mapName);
+                }
+                else
+                {
+                    fd = Interop.Sys.ShmOpen(mapName, flags, (int)perms); // Create the shared memory object.
+                }
 
                 if (fd.IsInvalid)
                 {
@@ -204,7 +211,7 @@ private static SafeFileHandle CreateSharedBackingObject(Interop.Sys.MemoryMapped
                         // the result of native shm_open does not work well with our subsequent call to mmap.
                         return null;
                     }
-                    else if (errorInfo.Error == Interop.Error.ENAMETOOLONG)
+                    else if (!Interop.Sys.MemfdSupported && errorInfo.Error == Interop.Error.ENAMETOOLONG)
                     {
                         Debug.Fail($"shm_open failed with ENAMETOOLONG for {Encoding.UTF8.GetByteCount(mapName)} byte long name.");
                         // in theory it should not happen anymore, but just to be extra safe we use the fallback
@@ -219,10 +226,13 @@ private static SafeFileHandle CreateSharedBackingObject(Interop.Sys.MemoryMapped
 
             try
             {
-                // Unlink the shared memory object immediately so that it'll go away once all handles
-                // to it are closed (as with opened then unlinked files, it'll remain usable via
-                // the open handles even though it's unlinked and can't be opened anew via its name).
-                Interop.CheckIo(Interop.Sys.ShmUnlink(mapName));
+                if (!Interop.Sys.MemfdSupported)
+                {
+                    // Unlink the shared memory object immediately so that it'll go away once all handles
+                    // to it are closed (as with opened then unlinked files, it'll remain usable via
+                    // the open handles even though it's unlinked and can't be opened anew via its name).
+                    Interop.CheckIo(Interop.Sys.ShmUnlink(mapName));
+                }
 
                 // Give it the right capacity.  We do this directly with ftruncate rather
                 // than via FileStream.SetLength after the FileStream is created because, on some systems,

src/native/libs/System.Native/entrypoints.c

@@ -62,6 +62,8 @@ static const Entry s_sysNative[] =
     DllImportEntry(SystemNative_Close)
     DllImportEntry(SystemNative_Dup)
     DllImportEntry(SystemNative_Unlink)
+    DllImportEntry(SystemNative_MemfdSupported)
+    DllImportEntry(SystemNative_MemfdCreate)
     DllImportEntry(SystemNative_ShmOpen)
     DllImportEntry(SystemNative_ShmUnlink)
     DllImportEntry(SystemNative_GetReadDirRBufferSize)

src/native/libs/System.Native/pal_io.c

@@ -369,6 +369,48 @@ int32_t SystemNative_Unlink(const char* path)
     return result;
 }
 
+int32_t SystemNative_MemfdSupported(void)
+{
+#ifdef MFD_ALLOW_SEALING
+#ifdef TARGET_LINUX
+    struct utsname uts;
+    int32_t major, minor;
+
+    // memfd_create is only known to work properly on kernel version > 3.17 and throws SIGSEGV instead of ENOTSUP
+    if (sscanf(uts.release, "%d.%d", &major, &minor) == 2 && (major < 3 || (major == 3 && minor < 17)))
+    {
+        return 0;
+    }
+#endif
+
+    int32_t fd = memfd_create("test", MFD_CLOEXEC | MFD_ALLOW_SEALING);
+    if (fd < 0) return 0;
+
+    close(fd);
+    return 1;
+#else
+    errno = ENOTSUP;
+    return 0;
+#endif
+}
+
+intptr_t SystemNative_MemfdCreate(const char* name)
+{
+#ifdef MFD_ALLOW_SEALING
+#if defined(SHM_NAME_MAX) // macOS
+    assert(strlen(name) <= SHM_NAME_MAX);
+#elif defined(PATH_MAX) // other Unixes
+    assert(strlen(name) <= PATH_MAX);
+#endif
+
+    return memfd_create(name, MFD_CLOEXEC | MFD_ALLOW_SEALING);
+#else
+    (void)name;
+    errno = ENOTSUP;
+    return -1;
+#endif
+}
+
 intptr_t SystemNative_ShmOpen(const char* name, int32_t flags, int32_t mode)
 {
 #if defined(SHM_NAME_MAX) // macOS

src/native/libs/System.Native/pal_io.h

@@ -369,6 +369,20 @@ PALEXPORT intptr_t SystemNative_Dup(intptr_t oldfd);
  */
 PALEXPORT int32_t SystemNative_Unlink(const char* path);
 
+/**
+ * Check if the system supports memfd_create(2). 
+ * 
+ * Returns 1 if memfd_create is supported, 0 if not supported, or -1 on failure. Sets errno on failure.
+ */
+PALEXPORT int32_t SystemNative_MemfdSupported(void);
+
+/**
+ * Create an anonymous file descriptor. Implemented as shim to memfd_create(2).
+ *
+ * Returns file descriptor or -1 on failure. Sets errno on failure.
+ */
+PALEXPORT intptr_t SystemNative_MemfdCreate(const char* name);
+
 /**
  * Open or create a shared memory object. Implemented as shim to shm_open(3).
  *