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

[Alice Ryhl <aliceryhl@google.com>]

Benno Lossin <benno.lossin@proton.me> writes:
> 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.

Will do.

> > +    /// 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?

I'm not convinced that makes sense. I'm pretty sure that if this API
returns a null pointer under any circumstances, then we're in some sort
of context where security contexts don't exist at all, and then they
would be hard-coded to use a length zero as well.

> > +            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.

Will do.

Alice