On Thu Jun 12, 2025 at 3:40 PM CEST, Andreas Hindborg wrote:
> +pub trait ParseInt: private::FromStrRadix + TryFrom<u64> {
> + /// Parse a string according to the description in [`Self`].
> + fn from_str(src: &BStr) -> Result<Self> {
> + match src.deref() {
> + [b'-', rest @ ..] => {
> + let (radix, digits) = strip_radix(rest.as_ref());
> + // 2's complement values range from -2^(b-1) to 2^(b-1)-1.
> + // So if we want to parse negative numbers as positive and
> + // later multiply by -1, we have to parse into a larger
> + // integer. We choose `u64` as sufficiently large.
> + //
> + // NOTE: 128 bit integers are not available on all
> + // platforms, hence the choice of 64 bits.
> + let val =
> +
> u64::from_str_radix(core::str::from_utf8(digits).map_err(|_| EINVAL)?, radix)
> + .map_err(|_| EINVAL)?;
> +
> + if val > Self::abs_min() {
> + return Err(EINVAL);
> + }
> +
> + if val == Self::abs_min() {
> + return Ok(Self::MIN);
> + }
> +
> + // SAFETY: We checked that `val` will fit in `Self` above.
Sorry that it took me this long to realize, but this seems pretty weird.
I guess this is why the `FromStrRadix` is `unsafe`.
Can we just move this part of the code to `FromStrRadix` and make that
trait safe?
So essentially have:
fn from_u64(value: u64) -> Result<Self>;
in `FromStrRadix` and remove `MIN`, `abs_min` and `complement`. Then
implement it like this in the macro below:
const ABS_MIN = /* existing abs_min impl */;
if value > ABS_MIN {
return Err(EINVAL);
}
if val == ABS_MIN {
return Ok(<$ty>::MIN);
}
// SAFETY: We checked that `val` will fit in `Self` above.
let val: $ty = unsafe { val.try_into().unwrap_unchecked() };
(!val).wrapping_add(1)
The reason that this is fine and the above is "weird" is the following:
The current version only has `Self: FromStrRadix` which gives it access
to the following guarantee from the `unsafe` trait:
/// The member functions of this trait must be implemented according to
/// their documentation.
///
/// [`&BStr`]: kernel::str::BStr
This doesn't mention `TryFrom<u64>` and thus the comment "We checked
that `val` will fit in `Self` above" doesn't really apply: how does
checking with the bounds given in `FromStrRadix` make `TryFrom` return
`Ok`?
If we move this code into the implementation of `FromStrRadix`, then we
are locally in a context where we *know* the concrete type of `Self` and
can thus rely on "checking" being the correct thing for `TryFrom`.
With this adjustment, I can give my RB, but please let me take a look
before you send it again :)
---
Cheers,
Benno
> + let val: Self = unsafe { val.try_into().unwrap_unchecked() };
> +
> + Ok(val.complement())
> + }
> + _ => {
> + let (radix, digits) = strip_radix(src);
> + Self::from_str_radix(digits, radix).map_err(|_| EINVAL)
> + }
> + }
> + }
> +}
