Running Miri on Playground does not error when printing non-UTF-8 output

[elichai]

So it seems that somehow miri bypasses the the check for UTF8 validity when printing a str

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=393c2e1fe450d4a03e7a1d293d54d04e

It's a bit too late here for me to debug where it's happening in libstd right now.

[RalfJung]

Hm, that is curious.

First of all, I don't think this code has UB, at least not UB that Miri currently is designed to find. The only possible UB here is "creating a non-UTF8 &str", but that is invalid data behind a referece, and as the README states:

In particular, Miri does currently not check that integers/floats are initialized or that references point to valid data.

Moreover, the UTF-8 condition is not something the compiler actually exploits currently. Also see rust-lang/unsafe-code-guidelines#78; the general consensus was that we should just remove the UTF-8 clause from the language spec and make it purely a case of library UB. (In other words, UTF-8 would be a safety invariant, but not a validity invariant.)

---

Now, to the error you are seeing when pressing "Run" above. I think that error is actually coming from the playground:

Execution operation failed: Output was not valid UTF-8: invalid utf-8 sequence of 1 bytes from index 167

@shepmaster, could you confirm?
If so, then the question is rather why the Miri playground integration does not get the same error. (I am not sure if I would call this a bug, but it is curious.)

@elichai did you try running this example locally with rustc, to exclude playground effects?

[elichai]

@elichai did you try running this example locally with rustc, to exclude playground effects?

I did not know that "playground effects" were a thing, how/what are these?
Just checked locally and you're right, I don't get any error locally.
[src/main.rs:3] s = "2"

[RalfJung]

I did not know that "playground effects" were a thing, how/what are these?

I did not know either. But the output looked different from when the program itself prints something, where we also get

   Compiling playground v0.0.1 (/playground)
    Finished dev [unoptimized + debuginfo] target(s) in 0.40s
     Running `target/debug/playground`

so I concluded it wasn't the program printing this, but playground. Which also made more sense because the error message did not at all sound like UB, and if this was a regular assertion inside the program then Miri would also evaluate it.

[shepmaster]

rust-lang/rust-playground#386 ?

[RalfJung]

Ah yes, looks like it's that (though curiously that bug seems to not affect Miri output).

Closing as duplicate of rust-lang/rust-playground#386 then.

[shepmaster]

There's still some difference in how Miri treats this code that seems suspicious:

% xxd miri
00000000: 5b73 7263 2f6d 6169 6e2e 7273 3a33 5d20  [src/main.rs:3]
00000010: 7320 3d20 22ef bfbd 3222 0a              s = "...2".

% xxd run
00000000: 5b73 7263 2f6d 6169 6e2e 7273 3a33 5d20  [src/main.rs:3]
00000010: 7320 3d20 228b 3222 0a                   s = ".2".

Shouldn't Miri output the same as cargo run?

[RalfJung]

Hm, strange. Looks like "8b" turned into "ef bf bd", whatever that means.

The Miri code that does the printing is here:

miri/src/shims/foreign_items/posix.rs

Lines 68 to 81 in d935f62

	let buf_cont = this.memory.read_bytes(buf, Size::from_bytes(n))?;
	// We need to flush to make sure this actually appears on the screen
	let res = if fd == 1 {
	// Stdout is buffered, flush to make sure it appears on the screen.
	// This is the write() syscall of the interpreted program, we want it
	// to correspond to a write() syscall on the host -- there is no good
	// in adding extra buffering here.
	let res = io::stdout().write(buf_cont);
	io::stdout().flush().unwrap();
	res
	} else {
	// No need to flush, stderr is not buffered.
	io::stderr().write(buf_cont)
	};

This should not do anything "weird" wrt. UTF-8.

[RalfJung]

@shepmaster I cannot reproduce this locally, both are the same here.

$ MIRI_SYSROOT=~/.cache/miri/HOST CARGO_EXTRA_FLAGS="-q" ./miri run utf8.rs 2> miri-output
$ xxd miri-output 
00000000: 5b75 7466 382e 7273 3a33 5d20 7320 3d20  [utf8.rs:3] s = 
00000010: 228b 3222 0a                             ".2".
$ rustc utf8.rs && ./utf8 2> rustc-output
$ xxd rustc-output 
00000000: 5b75 7466 382e 7273 3a33 5d20 7320 3d20  [utf8.rs:3] s = 
00000010: 228b 3222 0

What are the exact commands you used to create your two files?

[shepmaster]

% cargo +nightly-2020-04-07 miri 2> miri

% cargo +nightly-2020-04-07 run 2> run

% cat src/main.rs
fn main() {
    let s = unsafe { std::str::from_utf8_unchecked(&[139, 50]) };
    dbg!(s);
}

I did do some small editing in vim to remove Cargo noise from the generated files, but AFAICT nothing changed.

I am on macOS 10.15.3 (19D76).

[shepmaster]

Changing to stdout has the same behavior but easier to see:

fn main() {
    let s = unsafe { std::str::from_utf8_unchecked(&[139, 50]) };
    println!("{}", s);
}

% xxd run
00000000: 8b32 0a                                  .2.

% xxd miri
00000000: efbf bd32 0a                             ...2.