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.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Matthew Maurer <mmaurer@google.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 rust/kernel/uaccess.rs | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
index a8fb4764185a..c1cd3a76cff8 100644
--- a/rust/kernel/uaccess.rs
+++ b/rust/kernel/uaccess.rs
@@ -287,6 +287,22 @@ 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: usize) -> Result<usize> {
+        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

On Wed Oct 22, 2025 at 11:30 PM JST, Danilo Krummrich wrote:
> 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.
>
> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Reviewed-by: Matthew Maurer <mmaurer@google.com>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
>  rust/kernel/uaccess.rs | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
>
> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> index a8fb4764185a..c1cd3a76cff8 100644
> --- a/rust/kernel/uaccess.rs
> +++ b/rust/kernel/uaccess.rs
> @@ -287,6 +287,22 @@ 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: usize) -> Result<usize> {
> +        let end = offset
> +            .checked_add(self.len())
> +            .unwrap_or(out.len())
> +            .min(out.len());

IIUC you should be able to replace the `checked_add().unwrap_or()` with
`saturating_add()` and end up with the same result.

Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>

On Wed, Oct 22, 2025 at 04:30:36PM +0200, Danilo Krummrich wrote:
> 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.
> 
> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Reviewed-by: Matthew Maurer <mmaurer@google.com>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
>  rust/kernel/uaccess.rs | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> index a8fb4764185a..c1cd3a76cff8 100644
> --- a/rust/kernel/uaccess.rs
> +++ b/rust/kernel/uaccess.rs
> @@ -287,6 +287,22 @@ 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: usize) -> Result<usize> {
> +        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()))

Same comment applies as write_slice_partial().

Reviewed-by: Alice Ryhl <aliceryhl@google.com>