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Date: Wed, 14 Feb 2024 20:59:06 +0100
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Subject: Re: [PATCH v3] rust: str: add {make,to}_{upper,lower}case() to
 CString
To: Boqun Feng <boqun.feng@gmail.com>, Danilo Krummrich <dakr@redhat.com>
Cc: 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, aliceryhl@google.com,
 rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20240214172505.5044-1-dakr@redhat.com>
 <Zc0UNUGbmmBlzBAv@boqun-archlinux>
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From: Alice Ryhl <alice@ryhl.io>
In-Reply-To: <Zc0UNUGbmmBlzBAv@boqun-archlinux>
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On 2/14/24 20:27, Boqun Feng wrote:
> On Wed, Feb 14, 2024 at 06:24:10PM +0100, Danilo Krummrich wrote:
>> Add functions to convert a CString to upper- / lowercase, either
>> in-place or by creating a copy of the original CString.
>>
>> Naming followes the one from the Rust stdlib, where functions starting
>> with 'to' create a copy and functions starting with 'make' perform an
>> in-place conversion.
>>
>> This is required by the Nova project (GSP only Rust successor of
>> Nouveau) to convert stringified enum values (representing different GPU
>> chipsets) to strings in order to generate the corresponding firmware
>> paths. See also [1].
>>
>> [1] https://rust-for-linux.zulipchat.com/#narrow/stream/288089-General/topic/String.20manipulation.20in.20kernel.20Rust
>>
>> Signed-off-by: Danilo Krummrich <dakr@redhat.com>
>> ---
>> Changes in V3:
>>    - add an `impl DerefMut for CString`, such that these functions can be defined
>>      for `CStr` as `&mut self` and still be called on a `CString`
>> Changes in V2:
>>    - expand commit message mentioning the use case
>>    - expand function doc comments to match the ones from Rust's stdlib
>>    - rename to_* to make_* and add the actual to_* implementations
>> ---
>>   rust/kernel/str.rs | 81 +++++++++++++++++++++++++++++++++++++++++++++-
>>   1 file changed, 80 insertions(+), 1 deletion(-)
>>
>> diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
>> index 7d848b83add4..02d6e510b852 100644
>> --- 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.

As for .cast() vs the `as` operator, I'm not sure you can use .cast() in 
this case since the pointers are unsized. So you might have to use `as` 
instead.

Alice