[PATCH v18 2/7] rust: str: add radix prefixed integer parsing functions

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

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.

Suggested-by: Benno Lossin <benno.lossin@proton.me>
Tested-by: Daniel Gomez <da.gomez@samsung.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/str.rs           |   2 +
 rust/kernel/str/parse_int.rs | 148 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 150 insertions(+)

diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
index 6c892550c0ba9..23fe924070e7c 100644
--- a/rust/kernel/str.rs
+++ b/rust/kernel/str.rs
@@ -8,6 +8,8 @@
 
 use crate::prelude::*;
 
+pub mod parse_int;
+
 /// Byte string without UTF-8 validity guarantee.
 #[repr(transparent)]
 pub struct BStr([u8]);
diff --git a/rust/kernel/str/parse_int.rs b/rust/kernel/str/parse_int.rs
new file mode 100644
index 0000000000000..48eb4c202984c
--- /dev/null
+++ b/rust/kernel/str/parse_int.rs
@@ -0,0 +1,148 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Integer parsing functions.
+//!
+//! Integer parsing functions for parsing signed and unsigned integers
+//! potentially prefixed with `0x`, `0o`, or `0b`.
+
+use crate::prelude::*;
+use crate::str::BStr;
+use core::ops::Deref;
+
+// Make `FromStrRadix` a public type with a private name. This seals
+// `ParseInt`, that is, prevents downstream users from implementing the
+// trait.
+mod private {
+    use crate::prelude::*;
+    use crate::str::BStr;
+
+    /// Trait that allows parsing a [`&BStr`] to an integer with a radix.
+    pub trait FromStrRadix: Sized {
+        /// Parse `src` to [`Self`] using radix `radix`.
+        fn from_str_radix(src: &BStr, radix: u32) -> Result<Self>;
+
+        /// Tries to convert `value` into [`Self`] and negates the resulting value.
+        fn from_u64_negated(value: u64) -> Result<Self>;
+    }
+}
+
+/// 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', rest @ ..] => (16, rest.as_ref()),
+        [b'0', b'o' | b'O', rest @ ..] => (8, rest.as_ref()),
+        [b'0', b'b' | b'B', rest @ ..] => (2, rest.as_ref()),
+        // NOTE: We are including the leading zero to be able to parse
+        // literal `0` here. If we removed it as a radix prefix, we would
+        // not be able to parse `0`.
+        [b'0', ..] => (8, src),
+        _ => (10, src),
+    }
+}
+
+/// Trait for parsing string representations of integers.
+///
+/// Strings beginning with `0x`, `0o`, or `0b` are parsed as hex, octal, or
+/// binary respectively. Strings beginning with `0` otherwise are parsed as
+/// octal. Anything else is parsed as decimal. A leading `+` or `-` is also
+/// permitted. Any string parsed by [`kstrtol()`] or [`kstrtoul()`] will be
+/// successfully parsed.
+///
+/// [`kstrtol()`]: https://docs.kernel.org/core-api/kernel-api.html#c.kstrtol
+/// [`kstrtoul()`]: https://docs.kernel.org/core-api/kernel-api.html#c.kstrtoul
+///
+/// # Examples
+///
+/// ```
+/// # use kernel::str::parse_int::ParseInt;
+/// # use kernel::b_str;
+///
+/// assert_eq!(Ok(0u8), u8::from_str(b_str!("0")));
+///
+/// assert_eq!(Ok(0xa2u8), u8::from_str(b_str!("0xa2")));
+/// assert_eq!(Ok(-0xa2i32), i32::from_str(b_str!("-0xa2")));
+///
+/// assert_eq!(Ok(-0o57i8), i8::from_str(b_str!("-0o57")));
+/// assert_eq!(Ok(0o57i8), i8::from_str(b_str!("057")));
+///
+/// assert_eq!(Ok(0b1001i16), i16::from_str(b_str!("0b1001")));
+/// assert_eq!(Ok(-0b1001i16), i16::from_str(b_str!("-0b1001")));
+///
+/// assert_eq!(Ok(127i8), i8::from_str(b_str!("127")));
+/// assert!(i8::from_str(b_str!("128")).is_err());
+/// assert_eq!(Ok(-128i8), i8::from_str(b_str!("-128")));
+/// assert!(i8::from_str(b_str!("-129")).is_err());
+/// assert_eq!(Ok(255u8), u8::from_str(b_str!("255")));
+/// assert!(u8::from_str(b_str!("256")).is_err());
+/// ```
+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)?;
+                Self::from_u64_negated(val)
+            }
+            _ => {
+                let (radix, digits) = strip_radix(src);
+                Self::from_str_radix(digits, radix).map_err(|_| EINVAL)
+            }
+        }
+    }
+}
+
+macro_rules! impl_parse_int {
+    ($($ty:ty),*) => {
+        $(
+            impl private::FromStrRadix for $ty {
+                fn from_str_radix(src: &BStr, radix: u32) -> Result<Self> {
+                    <$ty>::from_str_radix(core::str::from_utf8(src).map_err(|_| EINVAL)?, radix)
+                        .map_err(|_| EINVAL)
+                }
+
+                fn from_u64_negated(value: u64) -> Result<Self> {
+                    const ABS_MIN: u64 = {
+                        #[allow(unused_comparisons)]
+                        if <$ty>::MIN < 0 {
+                            1u64 << (<$ty>::BITS - 1)
+                        } else {
+                            0
+                        }
+                    };
+
+                    if value > ABS_MIN {
+                        return Err(EINVAL);
+                    }
+
+                    if value == ABS_MIN {
+                        return Ok(<$ty>::MIN);
+                    }
+
+                    // SAFETY: The above checks guarantee that `value` fits into `Self`:
+                    // - if `Self` is unsigned, then `ABS_MIN == 0` and thus we have returned above
+                    //   (either `EINVAL` or `MIN`).
+                    // - if `Self` is signed, then we have that `0 <= value < ABS_MIN`. And since
+                    //   `ABS_MIN - 1` fits into `Self` by construction, `value` also does.
+                    let value: Self = unsafe { value.try_into().unwrap_unchecked() };
+
+                    Ok((!value).wrapping_add(1))
+                }
+            }
+
+            impl ParseInt for $ty {}
+        )*
+    };
+}
+
+impl_parse_int![i8, u8, i16, u16, i32, u32, i64, u64, isize, usize];

-- 
2.47.2