FS permission handling seems odd

The following test code runs perfectly fine when compiled with clang and ran, but when ran with emscripten the ftruncate operation throws a Permission error instead of going through as expected.

#include <assert.h>
#include <stdio.h>

#include <stdlib.h>
#include <unistd.h>
#include <sys/stat.h>

int main() {
  int fd = 0;
  char filename[] = "XXXXXX";

  // Test
  assert((fd = mkstemp(filename)) >= 0);
  assert(chmod(filename, 0) >= 0);
  assert(ftruncate(fd, 69) >= 0);

  // Cleanup
  assert(close(fd) >= 0);
  assert(remove(filename) >= 0);

  printf("ok\n");
  return 0;
}

Ran with

• clang main.c -o testfs && ./testfs
• emcc main.c -s ENVIRONMENT=node -s MAIN_MODULE -o out.js && node out.js

Current behavior

Clang build prints ok, emcc build fails (assertion error at ftruncate)

Expected behavior

Both should print "ok"

Environment

OS: Arch Linux clang version 12.0.1 emscripten version 2.0.26 (with clang 13.0.0) node version 16.4.2