Re: [PATCH 3/7] rust: security: add abstraction for secctx

[Benno Lossin]

On 11/29/23 14:11, Alice Ryhl wrote:
> +/// A security context string.
> +///
> +/// The struct has the invariant that it always contains a valid security 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 = core::ptr::null_mut();
> +        let mut seclen = 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 == 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 = 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 null even if the length is
> +            // zero. Replace the pointer with a dangling but non-null pointer 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 = core::ptr::NonNull::dangling().as_ptr();
> +        }
> +
> +        // 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`.

This should be part of the type invariant.

-- 
Cheers,
Benno

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