Messages in this thread | | | Date | Wed, 14 Feb 2024 17:18:16 -0800 | From | Boqun Feng <> | Subject | Re: [PATCH v3] rust: str: add {make,to}_{upper,lower}case() to CString |
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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: > > > 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. >
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).
> 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. >
You're right, that's a bit unfortunate..
Regards, Boqun
> Alice
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