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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> Reviewed-by: Alice Ryhl <aliceryhl@google.com> Reviewed-by: Alexandre Courbot <acourbot@nvidia.com> Acked-by: Miguel Ojeda <ojeda@kernel.org> [ Replace map_or() with let-else; use saturating_add(). - Danilo ] Signed-off-by: Danilo Krummrich <dakr@kernel.org>