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References: <20240214172505.5044-1-dakr@redhat.com> <Zc0UNUGbmmBlzBAv@boqun-archlinux>
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From: Alice Ryhl <aliceryhl@google.com>
Date: Thu, 15 Feb 2024 10:38:07 +0100
Message-ID: <CAH5fLghO6Jy_hJXhRU_+eBSDHHveAvEOJA6fNkmMS9mqHvS6iQ@mail.gmail.com>
Subject: Re: [PATCH v3] rust: str: add {make,to}_{upper,lower}case() to CString
To: Boqun Feng <boqun.feng@gmail.com>
Cc: Alice Ryhl <alice@ryhl.io>, Danilo Krummrich <dakr@redhat.com>, ojeda@kernel.org, 
	alex.gaynor@gmail.com, wedsonaf@gmail.com, gary@garyguo.net, 
	bjorn3_gh@protonmail.com, benno.lossin@proton.me, a.hindborg@samsung.com, 
	rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org
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On Thu, Feb 15, 2024 at 2:18=E2=80=AFAM Boqun Feng <boqun.feng@gmail.com> w=
rote:
>
> 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 performin=
g 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`. Beside=
s,
> > > 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: &mu=
t [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.

Alice