Does anyone know why the `format!()` macro would cause a crash | Rust Programming Language Community | Page 1

edgy locust Jul 16, 2022, 9:36 AM

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I'm writing a hypervisor in Rust and allocations work but whenever I call the format! macro my system crashes. It doesn't crash when I allocate a string

vestal tapir Jul 16, 2022, 9:38 AM

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does format!("Hello") crash?

edgy locust Jul 16, 2022, 9:41 AM

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Yes

edgy locust Jul 16, 2022, 9:41 AM

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vestal tapir does `format!("Hello")` crash?

.

vestal tapir Jul 16, 2022, 9:41 AM

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hmmmm

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and i assume this is a segfault

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or something

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"crash" is very not specific

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what's the error?

edgy locust Jul 16, 2022, 9:42 AM

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I assume some CPU exception happens

vestal tapir Jul 16, 2022, 9:43 AM

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well, can you run it in a debugger and see which one?

edgy locust Jul 16, 2022, 9:43 AM

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It's kind of hard to debug because my code is running in vmx root

vestal tapir Jul 16, 2022, 9:43 AM

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ah

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can you register CPU exception handlers

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could be a stack overflow

edgy locust Jul 16, 2022, 9:43 AM

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actually yeah maybe

vestal tapir Jul 16, 2022, 9:43 AM

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also you've not declared your own format macro right

edgy locust Jul 16, 2022, 9:44 AM

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    serial.write_str(&String::from("asdf\n"));

vestal tapir Jul 16, 2022, 9:44 AM

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you're using the stdlib one

edgy locust Jul 16, 2022, 9:44 AM

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yes

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i'm using stdlib one

vestal tapir Jul 16, 2022, 9:44 AM

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ok

edgy locust Jul 16, 2022, 9:44 AM

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edgy locust ```rust serial.write_str(&String::from("asdf\n")); ```

this works

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    serial.write_str(&format!("asdf\n"));

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it's possible the stack is corrupted

vestal tapir Jul 16, 2022, 9:45 AM

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?expand

format!("Hello")

austere ospreyBOT Jul 16, 2022, 9:45 AM

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{
    let res = ::alloc::fmt::format(::core::fmt::Arguments::new_v1(&["Hello"], &[]));
    res
}

vestal tapir Jul 16, 2022, 9:45 AM

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hmmm

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maybe split it up into 2 parts

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once doing format_args!("Hello")

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the other doing a to_string() on that

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see where it crashes?

edgy locust Jul 16, 2022, 9:47 AM

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format_args! works

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that's interesting

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it's crashing in to_string

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    let buf = [0u8; 0x500];
    for i in buf {
        if i != 0 {
            serial.write_str("bad\n");
        }
    }

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this works fine so i assume it's not an issue with the stack

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args.as_str() works

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?expand

format_args!("hello")

austere ospreyBOT Jul 16, 2022, 9:56 AM

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::core::fmt::Arguments::new_v1(&["hello"], &[])

edgy locust Jul 16, 2022, 10:11 AM

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@vestal tapir Found the issue

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    if args.pieces.len() != args.args.len() {
        serial.write_str("3\n");
    }

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edgy locust Jul 16, 2022, 10:34 AM

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Ok I don't know if anyone is still reading this but this is the weirdest issue ever

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I found the issue is in the Write::write_str method

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If I put this function inside core/fmt/mod.rs it crashes

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But if I put it in my lib.rs it doesn't crash

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This works

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This crashes

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hollow raft Jul 16, 2022, 11:43 AM

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An explanation for this last thing would be the usage of a fmt::Write impl vs a io::Write one, maybe

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Regarding the initial question, have you checked that your realloc logic is fine. And also make sure you're not recursing into string formatting inside the alloc logic

edgy locust Jul 16, 2022, 10:19 PM

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hollow raft Regarding the initial question, have you checked that your realloc logic is fine...

I’m using the bump allocator

edgy locust Jul 16, 2022, 10:20 PM

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hollow raft An explanation for this last thing would be the usage of a `fmt::Write` impl vs ...

Yeah I think that’s it

edgy locust Jul 16, 2022, 10:43 PM

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hollow raft An explanation for this last thing would be the usage of a `fmt::Write` impl vs ...

They're both using fmt::Write

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Looks like it's an issue with the trait object

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It works if I use static dispatch

#Does anyone know why the `format!()` macro would cause a crash