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Date:   Thu, 30 Nov 2023 16:26:39 +0000
To:     Alice Ryhl <aliceryhl@google.com>
From:   Benno Lossin <benno.lossin@proton.me>
Cc:     Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
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        Matthew Wilcox <willy@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Daniel Xu <dxu@dxuuu.xyz>, linux-kernel@vger.kernel.org,
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Subject: Re: [PATCH 3/7] rust: security: add abstraction for secctx
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On 11/29/23 14:11, Alice Ryhl wrote:
> +/// A security context string.
> +///
> +/// The struct has the invariant that it always contains a valid securit=
y context.

Refactor to use the `# Invariants` section:

    # Invariants
    `secdata` points to a valid security context.

I also do not know what a "valid security context" is, so a link to the
definition wouldn't hurt.

> +pub struct SecurityCtx {
> +    secdata: *mut core::ffi::c_char,
> +    seclen: usize,
> +}
> +
> +impl SecurityCtx {
> +    /// Get the security context given its id.
> +    pub fn from_secid(secid: u32) -> Result<Self> {
> +        let mut secdata =3D core::ptr::null_mut();
> +        let mut seclen =3D 0;
> +        // 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,
> +            ))?;
> +        }
> +
> +        // If the above call did not fail, then we have a valid security
> +        // context, so the invariants are not violated.

Should be tagged `INVARIANT`.

> +        Ok(Self {
> +            secdata,
> +            seclen: usize::try_from(seclen).unwrap(),
> +        })
> +    }
> +
> +    /// Returns whether the security context is empty.
> +    pub fn is_empty(&self) -> bool {
> +        self.seclen =3D=3D 0
> +    }
> +
> +    /// Returns the length of this security context.
> +    pub fn len(&self) -> usize {
> +        self.seclen
> +    }
> +
> +    /// Returns the bytes for this security context.
> +    pub fn as_bytes(&self) -> &[u8] {
> +        let mut ptr =3D self.secdata;
> +        if ptr.is_null() {
> +            // Many C APIs will use null pointers for strings of length =
zero, but

I would just write that the secctx API uses null pointers to denote a
string of length zero.

> +            // `slice::from_raw_parts` doesn't allow the pointer to be n=
ull even if the length is
> +            // zero. Replace the pointer with a dangling but non-null po=
inter in this case.
> +            debug_assert_eq!(self.seclen, 0);

I am feeling a bit uncomfortable with this, why can't we just return
an empty slice in this case?

> +            ptr =3D core::ptr::NonNull::dangling().as_ptr();
> +        }
> +
> +        // SAFETY: The call to `security_secid_to_secctx` guarantees tha=
t the pointer is valid for
> +        // `seclen` bytes. Furthermore, if the length is zero, then we h=
ave 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`.

This should be part of the type invariant.

--=20
Cheers,
Benno

> +        unsafe {
> +            bindings::security_release_secctx(self.secdata, self.seclen =
as u32);
> +        }
> +    }
> +}
> --
> 2.43.0.rc1.413.gea7ed67945-goog
>