Re: [PATCH v6 5/6] rust: str: add radix prefixed integer parsing functions

[Andreas Hindborg <a.hindborg@kernel.org>]

"Gary Guo" <gary@garyguo.net> writes:

> On Tue, 11 Feb 2025 21:13:10 +0100
> Andreas Hindborg <a.hindborg@kernel.org> wrote:
>
>> "Gary Guo" <gary@garyguo.net> writes:
>>
>> > On Tue, 11 Feb 2025 16:57:39 +0100
>> > Andreas Hindborg <a.hindborg@kernel.org> wrote:
>> >
>> >> Add the trait `ParseInt` for parsing string representations of integers
>> >> where the string representations are optionally prefixed by a radix
>> >> specifier. Implement the trait for the primitive integer types.
>> >>
>> >> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
>> >> ---
>> >>  rust/kernel/str.rs | 111 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>> >>  1 file changed, 111 insertions(+)
>> >>
>> >> diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
>> >> index c102adac32757..192cd0ff5974f 100644
>> >> --- a/rust/kernel/str.rs
>> >> +++ b/rust/kernel/str.rs
>> >> @@ -945,3 +945,114 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
>> >>  macro_rules! fmt {
>> >>      ($($f:tt)*) => ( core::format_args!($($f)*) )
>> >>  }
>> >> +
>> >> +pub mod parse_int {
>> >> +    //! Integer parsing functions for parsing signed and unsigned integers
>> >> +    //! potentially prefixed with `0x`, `0o`, or `0b`.
>> >> +
>> >> +    use crate::alloc::flags;
>> >> +    use crate::prelude::*;
>> >> +    use crate::str::BStr;
>> >> +    use core::ops::Deref;
>> >> +
>> >> +    /// Trait that allows parsing a [`&BStr`] to an integer with a radix.
>> >> +    ///
>> >> +    /// [`&BStr`]: kernel::str::BStr
>> >> +    // This is required because the `from_str_radix` function on the primitive
>> >> +    // integer types is not part of any trait.
>> >> +    pub trait FromStrRadix: Sized {
>> >> +        /// Parse `src` to `Self` using radix `radix`.
>> >> +        fn from_str_radix(src: &BStr, radix: u32) -> Result<Self, crate::error::Error>;
>> >> +    }
>> >> +
>> >> +    /// Extract the radix from an integer literal optionally prefixed with
>> >> +    /// one of `0x`, `0X`, `0o`, `0O`, `0b`, `0B`, `0`.
>> >> +    fn strip_radix(src: &BStr) -> (u32, &BStr) {
>> >> +        match src.deref() {
>> >> +            [b'0', b'x' | b'X', ..] => (16, &src[2..]),
>> >
>> > This can be written as
>> >
>> > 	[b'0', b'x' | b'X', rest @ ..] => (16, rest),
>> >
>> > to avoid manual indexing. Same for o and b below.
>>
>> error[E0308]: mismatched types
>>    --> /home/aeh/src/linux-rust/module-params/rust/kernel/str.rs:972:52
>>     |
>> 972 |             [b'0', b'x' | b'X', rest @ ..] => (16, rest),
>>     |                                                    ^^^^ expected `&BStr`, found `&[u8]`
>>     |
>>     = note: expected reference `&BStr`
>>                found reference `&[u8]`
>>
>> But I guess I could use the new AsRef impl. Or is it more idiomatic to
>> implement `From<&[u8]> for &BStr` and go with `rest.into()`?
>
> Ah, alright, I missed that this function is operating on BStr instead
> of slice. Keeping the current form is fine then.
>
>>
>> >
>> >> +            [b'0', b'o' | b'O', ..] => (8, &src[2..]),
>> >> +            [b'0', b'b' | b'B', ..] => (2, &src[2..]),
>> >> +            [b'0', ..] => (8, src),
>> >
>> > Perhaps add a comment saying that this isn't using `src[1..]` so `0`
>> > can be parsed.
>>
>> Good idea.
>>
>> >
>> >> +            _ => (10, src),
>> >> +        }
>> >> +    }
>
>> >> +    pub trait ParseInt: FromStrRadix {
>> >> +        /// Parse a string according to the description in [`Self`].
>> >> +        fn from_str(src: &BStr) -> Result<Self> {
>> >> +            match src.iter().next() {
>> >> +                None => Err(EINVAL),
>> >> +                Some(sign @ b'-') | Some(sign @ b'+') => {
>> >> +                    let (radix, digits) = strip_radix(BStr::from_bytes(&src[1..]));
>> >> +                    let mut n_digits: KVec<u8> =
>> >> +                        KVec::with_capacity(digits.len() + 1, flags::GFP_KERNEL)?;
>> >> +                    n_digits.push(*sign, flags::GFP_KERNEL)?;
>> >> +                    n_digits.extend_from_slice(digits, flags::GFP_KERNEL)?;
>> >
>> > I think my comment from a previous series saying that this shouldn't
>> > need allocation is not addressed.
>>
>> Thanks for noticing. This is the discussion from v4:
>>
>> >> I don't think we should allocate for parsing. This can trivially be a
>> >> non-allocating. Just check that the next byte is an ASCII digit (reject
>> >> if so, in case people give multiple signs), and then from_str_radix and
>> >> return as is or use `checked_neg`.
>> >
>> >The issue with that approach is that 2s complement signed integer types
>> >of width `b` can assume values from -2^(b-1) to (2^(b-1))-1. We would
>> >reject the value -2^(b-1) when trying to parse as 2^(b-1).
>> >
>> >We could parse into an unsigned type, but it gets kind of clunky.
>
> I would say either that or just call into kstrto* family.

Right. I'll rather parse into i128 than call into that.


Best regards,
Andreas Hindborg