linux-kernel - Re: [PATCH v2 3/7] rust: security: add abstraction for secctx

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Message-ID: <bynTQw4ZTfXBA0m3PYPL50jFnGQIzZnONT_L0TUNuWGtLwJhk6m0jeYQktfEIRmcVZIvKX9MOHwu4RgLWuH3nm5E_AiWNDKuKt_D2HSqsQw=@proton.me>
Date:   Fri, 08 Dec 2023 16:22:48 +0000
From:   Benno Lossin <benno.lossin@...ton.me>
To:     Alice Ryhl <aliceryhl@...gle.com>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Andreas Hindborg <a.hindborg@...sung.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Carlos Llamas <cmllamas@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Matthew Wilcox <willy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Xu <dxu@...uu.xyz>, linux-kernel@...r.kernel.org,
        rust-for-linux@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2 3/7] rust: security: add abstraction for secctx

On 12/6/23 12:59, Alice Ryhl wrote:
> +impl SecurityCtx {
> +    /// Get the security context given its id.
> +    pub fn from_secid(secid: u32) -> Result<Self> {
> +        let mut secdata = core::ptr::null_mut();
> +        let mut seclen = 0u32;
> +        // SAFETY: Just a C FFI call. The pointers are valid for writes.
> +        unsafe {
> +            to_result(bindings::security_secid_to_secctx(
> +                secid,
> +                &mut secdata,
> +                &mut seclen,
> +            ))?;
> +        }

Can you move the `unsafe` block inside of the `to_result` call? That way
we only have the unsafe operation in the unsafe block. Additionally, on
my side it fits perfectly into 100 characters.

> +        // INVARIANT: If the above call did not fail, then we have a valid security context.
> +        Ok(Self {
> +            secdata,
> +            seclen: seclen as usize,
> +        })
> +    }

[...]

> +    /// Returns the bytes for this security context.
> +    pub fn as_bytes(&self) -> &[u8] {
> +        let ptr = self.secdata;
> +        if ptr.is_null() {
> +            // We can't pass a null pointer to `slice::from_raw_parts` even if the length is zero.
> +            debug_assert_eq!(self.seclen, 0);

Would this be interesting enough to emit some kind of log message when
this fails?

> +            return &[];
> +        }
> +
> +        // SAFETY: The call to `security_secid_to_secctx` guarantees that the pointer is valid for
> +        // `seclen` bytes. Furthermore, if the length is zero, then we have ensured that the
> +        // pointer is not null.
> +        unsafe { core::slice::from_raw_parts(ptr.cast(), self.seclen) }
> +    }
> +}
> +
> +impl Drop for SecurityCtx {
> +    fn drop(&mut self) {
> +        // SAFETY: This frees a pointer that came from a successful call to
> +        // `security_secid_to_secctx` and has not yet been destroyed by `security_release_secctx`.
> +        unsafe {
> +            bindings::security_release_secctx(self.secdata, self.seclen as u32);
> +        }

If you move the `;` to the outside of the `unsafe` block this also fits
on a single line.

-- 
Cheers,
Benno

> +    }
> +}