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

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Date:   Mon, 11 Dec 2023 15:34:32 +0000
From:   Alice Ryhl <aliceryhl@...gle.com>
To:     benno.lossin@...ton.me
Cc:     a.hindborg@...sung.com, alex.gaynor@...il.com,
        aliceryhl@...gle.com, arve@...roid.com, bjorn3_gh@...tonmail.com,
        boqun.feng@...il.com, brauner@...nel.org, cmllamas@...gle.com,
        dan.j.williams@...el.com, dxu@...uu.xyz, gary@...yguo.net,
        gregkh@...uxfoundation.org, joel@...lfernandes.org,
        keescook@...omium.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, maco@...roid.com, ojeda@...nel.org,
        peterz@...radead.org, rust-for-linux@...r.kernel.org,
        surenb@...gle.com, tglx@...utronix.de, tkjos@...roid.com,
        viro@...iv.linux.org.uk, wedsonaf@...il.com, willy@...radead.org
Subject: Re: [PATCH v2 3/7] rust: security: add abstraction for secctx

Benno Lossin <benno.lossin@...ton.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