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 | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
index a8fb4764185a..1b0b57e855c9 100644
--- a/rust/kernel/uaccess.rs
+++ b/rust/kernel/uaccess.rs
@@ -287,6 +287,19 @@ 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()).ok_or(EINVAL)?.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 Sat, Oct 04, 2025 at 12:26:38AM +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.
> 
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
>  rust/kernel/uaccess.rs | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> index a8fb4764185a..1b0b57e855c9 100644
> --- a/rust/kernel/uaccess.rs
> +++ b/rust/kernel/uaccess.rs
> @@ -287,6 +287,19 @@ 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()).ok_or(EINVAL)?.min(out.len());

Should this be?
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()))

So if out.len() < offset, then we return Ok(0)?

Alice

On Fri Oct 17, 2025 at 1:11 PM CEST, Alice Ryhl wrote:
> On Sat, Oct 04, 2025 at 12:26:38AM +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.
>> 
>> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
>> ---
>>  rust/kernel/uaccess.rs | 13 +++++++++++++
>>  1 file changed, 13 insertions(+)
>> 
>> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
>> index a8fb4764185a..1b0b57e855c9 100644
>> --- a/rust/kernel/uaccess.rs
>> +++ b/rust/kernel/uaccess.rs
>> @@ -287,6 +287,19 @@ 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()).ok_or(EINVAL)?.min(out.len());
>
> Should this be?
> let end = offset.checked_add(self.len()).unwrap_or(out.len()).min(out.len());

Yes, that seems reasonable.

>> +        out.get_mut(offset..end)
>> +            .map_or(Ok(0), |dst| self.read_slice(dst).map(|()| dst.len()))
>
> So if out.len() < offset, then we return Ok(0)?

Yes, because it tells userspace that there are no more bytes left to read.