linux-kernel - Re: [PATCH v5 2/4] rust: debugfs: Bind file creation for long-lived Display

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Message-ID: <549e2bc42037ca6b366947160784794091069a07.camel@nvidia.com>
Date: Mon, 12 May 2025 20:51:53 +0000
From: Timur Tabi <ttabi@...dia.com>
To: "tmgross@...ch.edu" <tmgross@...ch.edu>, "benno.lossin@...ton.me"
	<benno.lossin@...ton.me>, "gregkh@...uxfoundation.org"
	<gregkh@...uxfoundation.org>, "gary@...yguo.net" <gary@...yguo.net>,
	"mmaurer@...gle.com" <mmaurer@...gle.com>, "a.hindborg@...nel.org"
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CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"rust-for-linux@...r.kernel.org" <rust-for-linux@...r.kernel.org>
Subject: Re: [PATCH v5 2/4] rust: debugfs: Bind file creation for long-lived
 Display

On Mon, 2025-05-05 at 23:51 +0000, Matthew Maurer wrote:
> +    pub(crate) unsafe extern "C" fn display_act<T: Display>(
> +        seq: *mut bindings::seq_file,
> +        _: *mut core::ffi::c_void,
> +    ) -> i32 {
> +        // SAFETY: By caller precondition, this pointer is live, points to a value of type `T`,
> and
> +        // is not being mutated.
> +        let data = unsafe { &*((*seq).private as *mut T) };
> +        // SAFETY: By caller precondition, `seq_file` points to a live `seq_file`, so we can lift
> +        // it.
> +        let seq_file = unsafe { SeqFile::from_raw(seq) };
> +        seq_print!(seq_file, "{}", data);

Doesn't this restrict T to data types that are supported by "{}"?  So, for example, T cannot be a
Vec, correct?

For nova-core, we need to be able to "print" an array of bytes as-is.  Specifically, a DMA buffer
that just contains binary data.  But by using seq_print!, aren't we forcing T to contain only
printable characters?