Re: [PATCH v5 0/6] rust: reduce pointer casts, enable related lints

Tamir Duberstein
2025-03-24T20:55:52Z

On Mon, Mar 24, 2025 at 4:16 PM Benno Lossin <benno.lossin@proton.me> wrote:
>
> On Mon Mar 17, 2025 at 3:23 PM CET, Tamir Duberstein wrote:
> > This started with a patch that enabled `clippy::ptr_as_ptr`. Benno
> > Lossin suggested I also look into `clippy::ptr_cast_constness` and I
> > discovered `clippy::as_ptr_cast_mut`. This series now enables all 3
> > lints. It also enables `clippy::as_underscore` which ensures other
> > pointer casts weren't missed. The first commit reduces the need for
> > pointer casts and is shared with another series[1].
> >
> > The final patch also enables pointer provenance lints and fixes
> > violations. See that commit message for details. The build system
> > portion of that commit is pretty messy but I couldn't find a better way
> > to convincingly ensure that these lints were applied globally.
> > Suggestions would be very welcome.
>
> I applied the patches to v6.14-rc7 and did a quick pass with
>
> rg -nC 3 -t rust ' as ' | bat -l rust
>
> to see if there are any cases left that we could fix and I found a
> couple:
>
> * there are several cases of `number as int_type` (like `num as c_int`
> or `my_u32 as usize` etc.) not sure what we can do about these, some
> are probably unavoidable, but since the kernel doesn't support 16 bit
> systems (that is true, right?), we *could* have a `From<u32> for
> usize` impl...

Yeah, these are the most difficult ones to get rid of.

> * some instances of `'|' as u32` (samples/rust/rust_misc_device.rs:112).
> There is a `From<char> for u32` impl, so this can just be replaced
> with `.into()` (or maybe by using a byte literal `b'|'`?).

We can enable https://rust-lang.github.io/rust-clippy/master/index.html?levels=allow#cast_lossless
for this one.

> * `shared_ref as *const _` (for example in rust/kernel/uaccess.rs:247,
> rust/kernel/str.rs:32 and rust/kernel/fs/file.rs:367), these we can
> replace with `let ptr: *const ... = shared_ref;`. Don't know if there
> is a clippy lint for this.

I think there's not a focused one. There's a nuclear option:
https://rust-lang.github.io/rust-clippy/master/index.html?levels=allow#as_conversions

> * some pointer casts in rust/kernel/list/impl_list_item_mod.rs:{253,254}
> not sure if they can be converted though (maybe they are unsizing the
> pointer?)

I have a local series that gets rid of these by doing similar things
to https://lore.kernel.org/all/20250307-no-offset-v1-0-0c728f63b69c@gmail.com/.
I can send it later this week but it probably can't land until Alice
is back from vacation; she was the author of this code.

> Another pointer cast in rust/kernel/driver.rs:81 (I'm pretty sure this
> one can be replaced by a `.cast()`)
>
> Some clippy lints that we could also enable that share the spirit of
> this series:
>
> * `char_lit_as_u8` (maybe that also covers the `'|' as u32` case from
> above?)

It's already enabled, it's warn-by-default.

> * `cast_lossless` (maybe this catches some of the `num as int_type`
> conversions I mentioned above)

Yeah, suggested the same above. I had hoped this would deal with the
char as u32 pattern but it did not.

> I'll leave it up to you what you want to do with this: add it to this
> series, make a new one, or let someone else handle it. If you don't want
> to handle it, let me know, then I'll create a good-first-issue :)

I'll add a patch for `cast_lossless` -- the rest should probably go
into an issue.

>
> > ---
> > Tamir Duberstein (6):
> > rust: retain pointer mut-ness in `container_of!`
> > rust: enable `clippy::ptr_as_ptr` lint
> > rust: enable `clippy::ptr_cast_constness` lint
> > rust: enable `clippy::as_ptr_cast_mut` lint
> > rust: enable `clippy::as_underscore` lint
> > rust: use strict provenance APIs
> >
> > Makefile | 4 ++
> > init/Kconfig | 3 +
> > rust/bindings/lib.rs | 1 +
> > rust/kernel/alloc.rs | 2 +-
> > rust/kernel/alloc/allocator_test.rs | 2 +-
> > rust/kernel/alloc/kvec.rs | 4 +-
> > rust/kernel/block/mq/operations.rs | 2 +-
> > rust/kernel/block/mq/request.rs | 7 +-
> > rust/kernel/device.rs | 5 +-
> > rust/kernel/device_id.rs | 2 +-
> > rust/kernel/devres.rs | 19 +++---
> > rust/kernel/error.rs | 2 +-
> > rust/kernel/firmware.rs | 3 +-
> > rust/kernel/fs/file.rs | 2 +-
> > rust/kernel/io.rs | 16 ++---
> > rust/kernel/kunit.rs | 15 ++---
> > rust/kernel/lib.rs | 113 ++++++++++++++++++++++++++++++++-
> > rust/kernel/list/impl_list_item_mod.rs | 2 +-
> > rust/kernel/miscdevice.rs | 2 +-
> > rust/kernel/of.rs | 6 +-
> > rust/kernel/pci.rs | 15 +++--
> > rust/kernel/platform.rs | 6 +-
> > rust/kernel/print.rs | 11 ++--
> > rust/kernel/rbtree.rs | 23 +++----
> > rust/kernel/seq_file.rs | 3 +-
> > rust/kernel/str.rs | 18 ++----
> > rust/kernel/sync/poll.rs | 2 +-
> > rust/kernel/uaccess.rs | 12 ++--
> > rust/kernel/workqueue.rs | 12 ++--
> > rust/uapi/lib.rs | 1 +
> > 30 files changed, 218 insertions(+), 97 deletions(-)
> > ---
> > base-commit: 498f7ee4773f22924f00630136da8575f38954e8
>
> Btw I didn't find this commit anywhere I usually check, where is it
> from?

It was probably nowhere, a local frankenstein that included some fixes
from rust-fixes.