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Wed, 14 Feb 2024 14:59:08 -0500 (EST) Message-ID: <2d94d420-fca2-47c1-aee7-bbce7a1505cf@ryhl.io> Date: Wed, 14 Feb 2024 20:59:06 +0100 Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] rust: str: add {make,to}_{upper,lower}case() to CString To: Boqun Feng , Danilo Krummrich 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> Content-Language: en-US-large From: Alice Ryhl In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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 >> --- >> 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::()` 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