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

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Date:   Fri,  1 Dec 2023 10:48:31 +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 3/7] rust: security: add abstraction for secctx

Benno Lossin <benno.lossin@...ton.me> writes:
> On 11/29/23 14:11, Alice Ryhl wrote:
>> +    /// 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.

I don't actually know whether it can ever be null, I just wanted to stay
on the safe side.

>> +            // `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?

I can do that, but to be clear, what I'm doing here is also definitely
okay.

Alice