Re: [PATCH v2] rust: page: Simplify overflow check using checked_add()

[Dirk Behme <dirk.behme@gmail.com>]

On 23.12.25 12:57, Gary Guo wrote:
> On Tue, 23 Dec 2025 12:06:17 +0200
> Kari Argillander <kari.argillander@gmail.com> wrote:
> 
>> Replace the explicit bounds comparisons with a single checked_add()-based
>> range check. This avoids redundant comparisons, makes the overflow case
>> explicit, and results in simpler generated code (checked with godbolt
>> for x86).
>>
>> No functional change intended.
>>
>> Reviewed-by: Dirk Behme <dirk.behme@de.bosch.com>
>> Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
>> Signed-off-by: Kari Argillander <kari.argillander@gmail.com>
>> ---
>> Changes in v2:
>> - Added MSVR todo (Dirk Behme)
>> - Link to v1: https://lore.kernel.org/r/20251219-rust-page-check-v1-1-df2e52fa3bd5@gmail.com
>> ---
>>  rust/kernel/page.rs | 19 +++++++++----------
>>  1 file changed, 9 insertions(+), 10 deletions(-)
>>
>> diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs
>> index 432fc0297d4a..cd2af7e4c357 100644
>> --- a/rust/kernel/page.rs
>> +++ b/rust/kernel/page.rs
>> @@ -239,17 +239,16 @@ fn with_pointer_into_page<T>(
>>          len: usize,
>>          f: impl FnOnce(*mut u8) -> Result<T>,
>>      ) -> Result<T> {
>> -        let bounds_ok = off <= PAGE_SIZE && len <= PAGE_SIZE && (off + len) <= PAGE_SIZE;
>> -
>> -        if bounds_ok {
>> -            self.with_page_mapped(move |page_addr| {
>> -                // SAFETY: The `off` integer is at most `PAGE_SIZE`, so this pointer offset will
>> -                // result in a pointer that is in bounds or one off the end of the page.
>> -                f(unsafe { page_addr.add(off) })
>> -            })
>> -        } else {
>> -            Err(EINVAL)
>> +        // TODO: Replace `map_or` with `is_none_or` once the MSRV is >= 1.82.
> 
> I was about to suggest just enable the feature gate, but turns out it was
> only added in 1.81. That's a fast one to stabilize!


With Alexandre's `matches!` proposal I was thinking that we could drop
the TODO comment and we won't need to touch this file again once MSRV
is >= 1.82. Opinions?

Cheers

Dirk


> Reviewed-by: Gary Guo <gary@garyguo.net>
> 
> Best,
> Gary
> 
>> +        if off.checked_add(len).map_or(true, |end| end > PAGE_SIZE) {
>> +            return Err(EINVAL);
>>          }
>> +
>> +        self.with_page_mapped(move |page_addr| {
>> +            // SAFETY: The `off` integer is at most `PAGE_SIZE`, so this pointer offset will
>> +            // result in a pointer that is in bounds or one off the end of the page.
>> +            f(unsafe { page_addr.add(off) })
>> +        })
>>      }
>>  
>>      /// Maps the page and reads from it into the given buffer.
>>
>> ---
>> base-commit: cc3aa43b44bdb43dfbac0fcb51c56594a11338a8
>> change-id: 20251219-rust-page-check-819ccc39c53a
>>
>> Best regards,
> 
>