[WasmFS] Implement /dev/random and /dev/urandom (#16896)

Author: tlively

system/lib/standalone/standalone.c

@@ -116,6 +116,13 @@ __attribute__((__weak__)) int __syscall_fcntl64(int fd, int cmd, ...) {
   return -ENOSYS;
 }
 
+// There is no good source of entropy without an import. Make this weak so that
+// it can be replaced with a pRNG or a proper import.
+__attribute__((__weak__))
+int getentropy(void* buffer, size_t length) {
+  abort();
+}
+
 // Emscripten additions
 
 extern void emscripten_notify_memory_growth(size_t memory_index);

system/lib/wasmfs/special_files.cpp

@@ -7,6 +7,7 @@
 // See https://github.com/emscripten-core/emscripten/issues/15041.
 
 #include <emscripten/html5.h>
+#include <unistd.h>
 #include <vector>
 #include <wasi/api.h>
 
@@ -113,6 +114,31 @@ class StderrFile : public WritingStdFile {
   StderrFile() {}
 };
 
+class RandomFile : public DataFile {
+  void open(oflags_t) override {}
+  void close() override {}
+
+  ssize_t write(const uint8_t* buf, size_t len, off_t offset) override {
+    return -__WASI_ERRNO_INVAL;
+  }
+
+  ssize_t read(uint8_t* buf, size_t len, off_t offset) override {
+    uint8_t* end = buf + len;
+    for (; buf < end; buf += 256) {
+      int err = getentropy(buf, std::min(end - buf, 256l));
+      assert(err == 0);
+    }
+    return len;
+  };
+
+  void flush() override {}
+  size_t getSize() override { return 0; }
+  void setSize(size_t size) override {}
+
+public:
+  RandomFile() : DataFile(S_IRUGO, NullBackend, S_IFCHR) { seekable = false; }
+};
+
 } // anonymous namespace
 
 std::shared_ptr<DataFile> getStdin() {
@@ -130,4 +156,14 @@ std::shared_ptr<DataFile> getStderr() {
   return stderr;
 }
 
+std::shared_ptr<DataFile> getRandom() {
+  static auto random = std::make_shared<RandomFile>();
+  return random;
+}
+
+std::shared_ptr<DataFile> getURandom() {
+  static auto urandom = std::make_shared<RandomFile>();
+  return urandom;
+}
+
 } // namespace wasmfs::SpecialFiles

system/lib/wasmfs/special_files.h

@@ -18,4 +18,10 @@ std::shared_ptr<DataFile> getStdout();
 // /dev/stderr
 std::shared_ptr<DataFile> getStderr();
 
+// /dev/random
+std::shared_ptr<DataFile> getRandom();
+
+// /dev/urandom
+std::shared_ptr<DataFile> getURandom();
+
 } // namespace wasmfs::SpecialFiles

system/lib/wasmfs/wasmfs.cpp

@@ -74,6 +74,8 @@ std::shared_ptr<Directory> WasmFS::initRootDirectory() {
   lockedDev.mountChild("stdin", SpecialFiles::getStdin());
   lockedDev.mountChild("stdout", SpecialFiles::getStdout());
   lockedDev.mountChild("stderr", SpecialFiles::getStderr());
+  lockedDev.mountChild("random", SpecialFiles::getRandom());
+  lockedDev.mountChild("urandom", SpecialFiles::getURandom());
 
   return rootDirectory;
 }

tests/filesystem/dev_random.cpp

@@ -3,13 +3,24 @@
 // University of Illinois/NCSA Open Source License.  Both these licenses can be
 // found in the LICENSE file.
 
+#include <assert.h>
 #include <stdio.h>
+
 int main() {
-  int byte_count = 64;
-  char data[64];
+  int byte_count = 500;
+  char data[500];
   FILE *fp;
+  int nread;
+
   fp = fopen("/dev/random", "r");
-  fread(&data, 1, byte_count, fp);
+  nread = fread(&data, 1, byte_count, fp);
+  assert(nread == byte_count);
   fclose(fp);
+
+  fp = fopen("/dev/urandom", "r");
+  nread = fread(&data, 1, byte_count, fp);
+  assert(nread == byte_count);
+  fclose(fp);
+
   return 0;
 }

tests/test_browser.py

@@ -702,6 +702,7 @@ def test():
     # create_file('shell.html', read_file(path_from_root('src/shell.html')).replace('var Module = {', 'var Module = { locateFile: function (path) {return "http:/localhost:8888/cdn/" + path;}, '))
     # test()
 
+  @also_with_wasmfs
   def test_dev_random(self):
     self.btest_exit(Path('filesystem/dev_random.cpp'))
 

tests/wasmfs/wasmfs_getdents.out

@@ -24,6 +24,10 @@ d.d_name = ..
 d.d_reclen = 280
 d.d_type = directory
 
+d.d_name = random
+d.d_reclen = 280
+d.d_type = regular
+
 d.d_name = stderr
 d.d_reclen = 280
 d.d_type = regular
@@ -36,6 +40,10 @@ d.d_name = stdout
 d.d_reclen = 280
 d.d_type = regular
 
+d.d_name = urandom
+d.d_reclen = 280
+d.d_type = regular
+
 /root/working file position is: 1
 ------------- Reading one from /root/working Directory -------------
 d.d_name = ..