Re: [PATCH 2/2] rust: block/mq: replace mem::zeroed() with Zeroable trait

[Paolo Bonzini <pbonzini@redhat.com>]

On 11/29/24 10:41, Alice Ryhl wrote:
> On Thu, Nov 28, 2024 at 3:13 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
>>
>> Isolate the unsafety in the declaration of the Zeroable trait, instead of having
>> to use "unsafe" just to declare a struct.  This is more similar to how you would
>> use "..Default::default()" (which is also a possibility here, but arguably
>> less efficient).
>>
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> ---
>>   rust/kernel/block/mq/gen_disk.rs |  8 +++++---
>>   rust/kernel/block/mq/tag_set.rs  | 10 ++++++----
>>   2 files changed, 11 insertions(+), 7 deletions(-)
>>
>> diff --git a/rust/kernel/block/mq/gen_disk.rs b/rust/kernel/block/mq/gen_disk.rs
>> index 708125dce96a..65342d065296 100644
>> --- a/rust/kernel/block/mq/gen_disk.rs
>> +++ b/rust/kernel/block/mq/gen_disk.rs
>> @@ -6,7 +6,7 @@
>>   //! C header: [`include/linux/blk_mq.h`](srctree/include/linux/blk_mq.h)
>>
>>   use crate::block::mq::{raw_writer::RawWriter, Operations, TagSet};
>> -use crate::{bindings, error::from_err_ptr, error::Result, sync::Arc};
>> +use crate::{bindings, error::from_err_ptr, error::Result, init::Zeroable, sync::Arc};
>>   use crate::{error, static_lock_class};
>>   use core::fmt::{self, Write};
>>
>> @@ -31,6 +31,9 @@ fn default() -> Self {
>>       }
>>   }
>>
>> +// SAFETY: `bindings::queue_limits` contains only fields that are valid when zeroed.
>> +unsafe impl Zeroable for bindings::queue_limits {}
>> +
>>   impl GenDiskBuilder {
>>       /// Create a new instance.
>>       pub fn new() -> Self {
>> @@ -93,8 +96,7 @@ pub fn build<T: Operations>(
>>           name: fmt::Arguments<'_>,
>>           tagset: Arc<TagSet<T>>,
>>       ) -> Result<GenDisk<T>> {
>> -        // SAFETY: `bindings::queue_limits` contain only fields that are valid when zeroed.
>> -        let mut lim: bindings::queue_limits = unsafe { core::mem::zeroed() };
>> +        let mut lim: bindings::queue_limits = Zeroable::ZERO;
>>
>>           lim.logical_block_size = self.logical_block_size;
>>           lim.physical_block_size = self.physical_block_size;
>> diff --git a/rust/kernel/block/mq/tag_set.rs b/rust/kernel/block/mq/tag_set.rs
>> index f9a1ca655a35..1ff7366ca549 100644
>> --- a/rust/kernel/block/mq/tag_set.rs
>> +++ b/rust/kernel/block/mq/tag_set.rs
>> @@ -10,6 +10,7 @@
>>       bindings,
>>       block::mq::{operations::OperationsVTable, request::RequestDataWrapper, Operations},
>>       error,
>> +    init::Zeroable,
>>       prelude::PinInit,
>>       try_pin_init,
>>       types::Opaque,
>> @@ -32,6 +33,10 @@ pub struct TagSet<T: Operations> {
>>       _p: PhantomData<T>,
>>   }
>>
>> +// SAFETY: `blk_mq_tag_set` only contains integers and pointers, which
>> +// all are allowed to be 0.
>> +unsafe impl Zeroable for bindings::blk_mq_tag_set {}
> 
> This will have to be reverted if we want to split up the kernel crate
> due to the orphan rule.

Ok, I will look into using the bindgen doc comments then.  It looks like 
the idea is not dead on arrival at least!  Thanks for the review.

Paolo