Comment by kragen

Well, for example, the extremely exotic scenario of passing command-line arguments to a program on little-known operating systems like Linux and FreeBSD; https://doc.rust-lang.org/book/ch12-01-accepting-command-lin... recommends:

  use std::env;

  fn main() {
      let args: Vec<String> = env::args().collect();
      ...
  }

When I run this code, a literal example from the official manual, with this filename I have here, it panics:

    $ ./main $'\200'
    thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: "\x80"', library/std/src/env.rs:805:51
    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

($'\200' is bash's notation for a single byte with the value 128. We'll see it below in the strace output.)

So, literally any program anyone writes in Rust will crash if you attempt to pass it that filename, if it uses the manual's recommended way to accept command-line arguments. It might work fine for a long time, in all kinds of tests, and then blow up in production when a wild file appears with a filename that fails to be valid Unicode.

This C program I just wrote handles it fine:

  #include <unistd.h>
  #include <fcntl.h>
  #include <stdio.h>
  #include <stdlib.h>

  char buf[4096];

  void
  err(char *s)
  {
    perror(s);
    exit(-1);
  }

  int
  main(int argc, char **argv)
  {
    int input, output;
    if ((input = open(argv[1], O_RDONLY)) < 0) err(argv[1]);
    if ((output = open(argv[2], O_WRONLY | O_CREAT, 0666)) < 0) err(argv[2]);
    for (;;) {
      ssize_t size = read(input, buf, sizeof buf);
      if (size < 0) err("read");
      if (size == 0) return 0;
      ssize_t size2 = write(output, buf, (size_t)size);
      if (size2 != size) err("write");
    }
  }

(I probably should have used O_TRUNC.)

Here you can see that it does successfully copy that file:

    $ cat baz
    cat: baz: No such file or directory
    $ strace -s4096 ./cp $'\200' baz
    execve("./cp", ["./cp", "\200", "baz"], 0x7ffd7ab60058 /* 50 vars */) = 0
    brk(NULL)                               = 0xd3ec000
    brk(0xd3ecd00)                          = 0xd3ecd00
    arch_prctl(ARCH_SET_FS, 0xd3ec380)      = 0
    set_tid_address(0xd3ec650)              = 4153012
    set_robust_list(0xd3ec660, 24)          = 0
    rseq(0xd3ecca0, 0x20, 0, 0x53053053)    = 0
    prlimit64(0, RLIMIT_STACK, NULL, {rlim_cur=9788*1024, rlim_max=RLIM64_INFINITY}) = 0
    readlink("/proc/self/exe", ".../cp", 4096) = 22
    getrandom("\xcf\x1f\xb7\xd3\xdb\x4c\xc7\x2c", 8, GRND_NONBLOCK) = 8
    brk(NULL)                               = 0xd3ecd00
    brk(0xd40dd00)                          = 0xd40dd00
    brk(0xd40e000)                          = 0xd40e000
    mprotect(0x4a2000, 16384, PROT_READ)    = 0
    openat(AT_FDCWD, "\200", O_RDONLY)      = 3
    openat(AT_FDCWD, "baz", O_WRONLY|O_CREAT, 0666) = 4
    read(3, "foo\n", 4096)                  = 4
    write(4, "foo\n", 4)                    = 4
    read(3, "", 4096)                       = 0
    exit_group(0)                           = ?
    +++ exited with 0 +++
    $ cat baz
    foo

The Rust manual page linked above explains why they think introducing this bug by default into all your programs is a good idea, and how to avoid it:

> Note that std::env::args will panic if any argument contains invalid Unicode. If your program needs to accept arguments containing invalid Unicode, use std::env::args_os instead. That function returns an iterator that produces OsString values instead of String values. We’ve chosen to use std::env::args here for simplicity because OsString values differ per platform and are more complex to work with than String values.

I don't know what's "complex" about OsString, but for the time being I'll take the manual's word for it.

So, Rust's approach evidently makes it extremely hard not to introduce problems like that, even in the simplest programs.

Go's approach doesn't have that problem; this program works just as well as the C program, without the Rust footgun:

  package main

  import (
          "io"
          "log"
          "os"
  )

  func main() {
          src, err := os.Open(os.Args[1])
          if err != nil {
                  log.Fatalf("open source: %v", err)
          }

          dst, err := os.OpenFile(os.Args[2], os.O_CREATE|os.O_WRONLY, 0666)
          if err != nil {
                  log.Fatalf("create dest: %v", err)
          }

          if _, err := io.Copy(dst, src); err != nil {
                  log.Fatalf("copy: %v", err)
          }
  }

(O_CREATE makes me laugh. I guess Ken did get to spell "creat" with an "e" after all!)

This program generates a much less clean strace, so I am not going to include it.

You might wonder how such a filename could arise other than as a deliberate attack. The most common scenario is when the filenames are encoded in a non-Unicode encoding like Shift-JIS or Latin-1, followed by disk corruption, but the deliberate attack scenario is nothing to sneeze at either. You don't want attackers to be able to create filenames your tools can't see, or turn to stone if they examine, like Medusa.

Note that the log message on error also includes the ill-formed Unicode filename:

  $ ./cp $'\201' baz
  2025/08/22 21:53:49 open source: open ζ: no such file or directory

But it didn't say ζ. It actually emitted a byte with value 129, making the error message ill-formed UTF-8. This is obviously potentially dangerous, depending on where that logfile goes because it can include arbitrary terminal escape sequences. But note that Rust's UTF-8 validation won't protect you from that, or from things like this:

  $ ./cp $'\n2025/08/22 21:59:59 oh no' baz
  2025/08/22 21:59:09 open source: open 
  2025/08/22 21:59:59 oh no: no such file or directory

I'm not bagging on Rust. There are a lot of good things about Rust. But its string handling is not one of them.