Re: [PATCH] block: set bi_vcnt when cloning bio

[Andreas Hindborg <a.hindborg@kernel.org>]

"John Garry" <john.g.garry@oracle.com> writes:

> On 15/02/2025 10:58, Andreas Hindborg wrote:
>> When cloning a bio, the `bio.bi_vcnt` field is not cloned. This is a
>> problem if users want to perform bounds checks on the `bio.bi_io_vec`
>> field.
>
> Is this fixing a potential problem? Or fixing a real issue?

It is fixing a problem I ran into in rnull, the rust null block
implementation. When running with debug assertions enabled, a bound
check on `bi_io_vec` fails for split bio, because `bio_vcnt` becomes
zero in the cloned bio.

I can work around this by not using a slice type to represent
`bi_io_vec` in rust, not a big deal.

But I am genuinely curious if there is a reason for not setting
`bi_vcnt` during a clone. As far as I can tell, it should be safe to
set. `bi_vcnt` being zero does not seem to have any effect other than to
puzzle developers debugging the code.

Maybe I missed something?


Best regards,
Andreas Hindborg