linux-kernel - Re: [PATCH v3] rust: str: add {make,to}

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Date: Thu, 15 Feb 2024 08:51:50 -0800
From: Boqun Feng <boqun.feng@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Alice Ryhl <alice@...l.io>, Danilo Krummrich <dakr@...hat.com>,
	ojeda@...nel.org, alex.gaynor@...il.com, wedsonaf@...il.com,
	gary@...yguo.net, bjorn3_gh@...tonmail.com, benno.lossin@...ton.me,
	a.hindborg@...sung.com, rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] rust: str: add {make,to}_{upper,lower}case() to
 CString

On Thu, Feb 15, 2024 at 10:38:07AM +0100, Alice Ryhl wrote:
> On Thu, Feb 15, 2024 at 2:18 AM Boqun Feng <boqun.feng@...il.com> wrote:
> >
> > On Wed, Feb 14, 2024 at 08:59:06PM +0100, Alice Ryhl wrote:
> > > On 2/14/24 20:27, Boqun Feng wrote:
> > > > On Wed, Feb 14, 2024 at 06:24:10PM +0100, Danilo Krummrich wrote:
> > > > > --- a/rust/kernel/str.rs
> > > > > +++ b/rust/kernel/str.rs
> > > > > @@ -5,7 +5,7 @@
> > > > >   use alloc::alloc::AllocError;
> > > > >   use alloc::vec::Vec;
> > > > >   use core::fmt::{self, Write};
> > > > > -use core::ops::{self, Deref, Index};
> > > > > +use core::ops::{self, Deref, DerefMut, Index};
> > > > >   use crate::{
> > > > >       bindings,
> > > > > @@ -143,6 +143,19 @@ pub const fn from_bytes_with_nul(bytes: &[u8]) -> Result<&Self, CStrConvertError
> > > > >           unsafe { core::mem::transmute(bytes) }
> > > > >       }
> > > > > +    /// Creates a mutable [`CStr`] from a `[u8]` without performing any
> > > > > +    /// additional checks.
> > > > > +    ///
> > > > > +    /// # Safety
> > > > > +    ///
> > > > > +    /// `bytes` *must* end with a `NUL` byte, and should only have a single
> > > > > +    /// `NUL` byte (or the string will be truncated).
> > > > > +    #[inline]
> > > > > +    pub const unsafe fn from_bytes_with_nul_unchecked_mut(bytes: &mut [u8]) -> &mut CStr {
> > > > > +        // SAFETY: Properties of `bytes` guaranteed by the safety precondition.
> > > > > +        unsafe { &mut *(bytes as *mut [u8] as *mut CStr) }
> > > >
> > > > First `.cast::<[u8]>().cast::<CStr>()` is preferred than `as`. Besides,
> > > > I think the dereference (or reborrow) is only safe if `CStr` is
> > > > `#[repr(transparent)]. I.e.
> > > >
> > > >     #[repr(transparent)]
> > > >     pub struct CStr([u8]);
> > > >
> > > > with that you can implement the function as (you can still use `cast()`
> > > > implementation, but I sometimes find `transmute` is more simple).
> > > >
> > > >      pub const unsafe fn from_bytes_with_nul_unchecked_mut(bytes: &mut [u8]) -> &mut CStr {
> > > >     // SAFETY: `CStr` is transparent to `[u8]`, so the transmute is
> > > >     // safe to do, and per the function safety requirement, `bytes`
> > > >     // is a valid `CStr`.
> > > >     unsafe { core::mem::transmute(bytes) }
> > > >      }
> > > >
> > > > but this is just my thought, better wait for others' feedback as well.
> > >
> > > Transmuting references is generally frowned upon. It's better to use a
> > > pointer cast.
> > >
> >
> > Ok, but honestly, I don't think the pointer casting is better ;-) What
> > wants to be done here is simply converting a `&mut [u8]` to `&mut CStr`,
> > adding two levels of pointer casting is kinda noise. (Also
> > `from_bytes_with_nul` uses `transmute` as well).
> 
> Here's my logic for preferring pointer casts: Transmute raises
> questions about the layout of fat pointers, whereas pointer casts are
> obviously okay.
> 

But in this case, eventually you need to worry about fat pointer layout
when you dereference the `*mut CStr`, right? In other words, the
dereference is only safe if `*mut [u8]` has the same fat pointer layout
as `*mut CStr`. I prefer to transmute here because it's a newtype
paradigm, and transmute kinda makes that clear.

Regards,
Boqun

> Alice