Re: [PATCH] rust: uaccess: use to_result for error handling

["Onur Özkan" <work@onurozkan.dev>]

On Thu, 21 Aug 2025 13:04:18 +0200
Alice Ryhl <aliceryhl@google.com> wrote:

> On Thu, Aug 21, 2025 at 11:19 AM Onur Özkan <work@onurozkan.dev>
> wrote:
> >
> > Simplifies error handling by replacing the manual check
> > of the return value with the `to_result` helper.
> >
> > Signed-off-by: Onur Özkan <work@onurozkan.dev>
> > ---
> >  rust/kernel/uaccess.rs | 6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> >
> > diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> > index a8fb4764185a..9992eece2694 100644
> > --- a/rust/kernel/uaccess.rs
> > +++ b/rust/kernel/uaccess.rs
> > @@ -7,7 +7,7 @@
> >  use crate::{
> >      alloc::{Allocator, Flags},
> >      bindings,
> > -    error::Result,
> > +    error::{to_result, Result},
> >      ffi::{c_char, c_void},
> >      prelude::*,
> >      transmute::{AsBytes, FromBytes},
> > @@ -495,9 +495,7 @@ fn raw_strncpy_from_user(dst: &mut
> > [MaybeUninit<u8>], src: UserPtr) -> Result<us )
> >      };
> >
> > -    if res < 0 {
> > -        return Err(Error::from_errno(res as i32));
> > -    }
> > +    to_result(res as i32)?;
> 
> This is wrong. The type of `res` is isize, and casting a positive
> isize to i32 can result in a negative number and incorrectly trigger
> this error. For example: 2147483650i64 as i32 is -2147483646.
> 
> Alice

Nice catch. I could use `try_into().unwrap_or(0)` but that feels a bit
iffy since it would silently avoid the error if `isize` happens to be
much smaller than what `i32` can handle (though I am not sure if it's
possible in practice in the kernel codebase). Let's just ignore this
patch.

Thanks,
Onur