diff --git a/libc/intrin/__is_wsl.c b/libc/intrin/__is_wsl.c
new file mode 100644
index 000000000..bb0ee25e6
--- /dev/null
+++ b/libc/intrin/__is_wsl.c
@@ -0,0 +1,49 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2022 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/calls/calls.h"
+#include "libc/calls/syscall-sysv.internal.h"
+#include "libc/dce.h"
+#include "libc/errno.h"
+#include "libc/runtime/internal.h"
+#include "libc/sysv/consts/map.h"
+#include "libc/sysv/consts/prot.h"
+
+#define MAP_GROWSDOWN_linux 0x00000100
+#define MAP_ANONYMOUS_linux 0x00000020
+
+/**
+ * Returns true if host platform is WSL.
+ */
+bool __is_wsl(void) {
+  int e;
+  void *p;
+  bool res;
+  if (!IsLinux()) return false;
+  e = errno;
+  p = __sys_mmap(0, 4096, PROT_READ | PROT_WRITE,
+                 MAP_PRIVATE | MAP_ANONYMOUS_linux | MAP_GROWSDOWN_linux, -1, 0,
+                 0);
+  if (p != MAP_FAILED) {
+    __sys_munmap(p, 4096);
+    return false;
+  }
+  res = errno == ENOTSUP;
+  errno = e;
+  return res;
+}
diff --git a/libc/runtime/internal.h b/libc/runtime/internal.h
index 6345e5a26..1fc9be768 100644
--- a/libc/runtime/internal.h
+++ b/libc/runtime/internal.h
@@ -29,6 +29,7 @@ extern unsigned char _tls_size[];
 extern unsigned char _tls_content[];
 
 void _init(void) hidden;
+bool __is_wsl(void);
 void __morph_tls(void);
 void __enable_tls(void);
 void __enable_threads(void) hidden;
diff --git a/test/libc/calls/fcntl_test.c b/test/libc/calls/fcntl_test.c
index 5af72435b..4229f241b 100644
--- a/test/libc/calls/fcntl_test.c
+++ b/test/libc/calls/fcntl_test.c
@@ -87,6 +87,10 @@ TEST(fcntl, getfd) {
   ASSERT_SYS(0, 0, fcntl(3, F_GETFD));
   ASSERT_SYS(0, 4, open("/dev/null", O_RDWR | O_CLOEXEC));
   ASSERT_SYS(0, FD_CLOEXEC, fcntl(4, F_GETFD));
+  ASSERT_SYS(0, 0, fcntl(4, F_SETFD, FD_CLOEXEC));
+  ASSERT_SYS(0, FD_CLOEXEC, fcntl(4, F_GETFD));
+  ASSERT_SYS(0, 0, fcntl(4, F_SETFD, 0));
+  ASSERT_SYS(0, 0, fcntl(4, F_GETFD));
   ASSERT_SYS(0, 0, close(4));
   ASSERT_SYS(0, 0, close(3));
 }
diff --git a/test/libc/sock/sendfile_test.c b/test/libc/sock/sendfile_test.c
index ad1570ca9..8bacbc972 100644
--- a/test/libc/sock/sendfile_test.c
+++ b/test/libc/sock/sendfile_test.c
@@ -24,6 +24,7 @@
 #include "libc/limits.h"
 #include "libc/mem/gc.internal.h"
 #include "libc/mem/mem.h"
+#include "libc/runtime/internal.h"
 #include "libc/runtime/runtime.h"
 #include "libc/sock/sock.h"
 #include "libc/sock/struct/sockaddr.h"
@@ -125,7 +126,10 @@ TEST(sendfile, testPositioning) {
     ASSERT_EQ(-1, sendfile(4, 5, 0, 6));
     ASSERT_TRUE(errno == EINVAL || errno == EPIPE);
     errno = 0;
-    ASSERT_EQ(12, GetFileOffset(5));
+    // XXX: WSL clobbers file offset on failure!
+    if (!__is_wsl()) {
+      ASSERT_EQ(12, GetFileOffset(5));
+    }
     _Exit(0);
   }
   ASSERT_SYS(0, 0, close(3));