[PATCH v2 2/8] rust: uaccess: add UserSliceReader::read_slice_partial()

[Danilo Krummrich <dakr@kernel.org>]

The existing read_slice() method is a wrapper around copy_from_user()
and expects the user buffer to be larger than the destination buffer.

However, userspace may split up writes in multiple partial operations
providing an offset into the destination buffer and a smaller user
buffer.

In order to support this common case, provide a helper for partial
reads.

Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 rust/kernel/uaccess.rs | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
index a8fb4764185a..2061a7e10c65 100644
--- a/rust/kernel/uaccess.rs
+++ b/rust/kernel/uaccess.rs
@@ -9,6 +9,7 @@
     bindings,
     error::Result,
     ffi::{c_char, c_void},
+    fs::file,
     prelude::*,
     transmute::{AsBytes, FromBytes},
 };
@@ -287,6 +288,30 @@ pub fn read_slice(&mut self, out: &mut [u8]) -> Result {
         self.read_raw(out)
     }
 
+    /// Reads raw data from the user slice into a kernel buffer partially.
+    ///
+    /// This is the same as [`Self::read_slice`] but considers the given `offset` into `out` and
+    /// truncates the read to the boundaries of `self` and `out`.
+    ///
+    /// On success, returns the number of bytes read.
+    pub fn read_slice_partial(&mut self, out: &mut [u8], offset: file::Offset) -> Result<usize> {
+        if offset < 0 {
+            return Err(EINVAL);
+        }
+
+        let Ok(offset) = usize::try_from(offset) else {
+            return Ok(0);
+        };
+
+        let end = offset
+            .checked_add(self.len())
+            .unwrap_or(out.len())
+            .min(out.len());
+
+        out.get_mut(offset..end)
+            .map_or(Ok(0), |dst| self.read_slice(dst).map(|()| dst.len()))
+    }
+
     /// Reads a value of the specified type.
     ///
     /// Fails with [`EFAULT`] if the read happens on a bad address, or if the read goes out of
-- 
2.51.0