[PATCH v13 1/6] 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.

Tested-by: Daniel Gomez <da.gomez@samsung.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/str.rs           |   2 +
 rust/kernel/str/parse_int.rs | 171 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 173 insertions(+)

diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
index a927db8e079c..2b6c8b4a0ae4 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 000000000000..0754490aec4b
--- /dev/null
+++ b/rust/kernel/str/parse_int.rs
@@ -0,0 +1,171 @@
+// 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::str::BStr;
+
+    /// Trait that allows parsing a [`&BStr`] to an integer with a radix.
+    ///
+    /// # Safety
+    ///
+    /// The member functions of this trait must be implemented according to
+    /// their documentation.
+    ///
+    /// [`&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 unsafe trait FromStrRadix: Sized {
+        /// The minimum value this integer type can assume.
+        const MIN: Self;
+
+        /// Parse `src` to [`Self`] using radix `radix`.
+        fn from_str_radix(src: &BStr, radix: u32) -> Result<Self, crate::error::Error>;
+
+        /// Return the absolute value of [`Self::MIN`].
+        fn abs_min() -> u64;
+
+        /// Perform bitwise 2's complement on `self`.
+        ///
+        /// Note: This function does not make sense for unsigned integers.
+        fn complement(self) -> 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)?;
+
+                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.
+                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)
+            }
+        }
+    }
+}
+
+macro_rules! impl_parse_int {
+    ($ty:ty) => {
+        // SAFETY: We implement the trait according to the documentation.
+        unsafe impl private::FromStrRadix for $ty {
+            const MIN: Self = <$ty>::MIN;
+
+            fn from_str_radix(src: &BStr, radix: u32) -> Result<Self, crate::error::Error> {
+                <$ty>::from_str_radix(core::str::from_utf8(src).map_err(|_| EINVAL)?, radix)
+                    .map_err(|_| EINVAL)
+            }
+
+            fn abs_min() -> u64 {
+                #[allow(unused_comparisons)]
+                if Self::MIN < 0 {
+                    1u64 << (Self::BITS - 1)
+                } else {
+                    0
+                }
+            }
+
+            fn complement(self) -> Self {
+                (!self).wrapping_add((1 as $ty))
+            }
+        }
+
+        impl ParseInt for $ty {}
+    };
+}
+
+impl_parse_int!(i8);
+impl_parse_int!(u8);
+impl_parse_int!(i16);
+impl_parse_int!(u16);
+impl_parse_int!(i32);
+impl_parse_int!(u32);
+impl_parse_int!(i64);
+impl_parse_int!(u64);
+impl_parse_int!(isize);
+impl_parse_int!(usize);

-- 
2.47.2