Re: [PATCH] xen: Don't cast away const-ness in vcpu_show_registers()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 26/02/2025 7:33 am, Jan Beulich wrote:
> On 26.02.2025 00:02, Andrew Cooper wrote:
>> The final hunk is `(struct vcpu *)v` in disguise, expressed using a runtime
>> pointer chase through memory and a technicality of the C type system to work
>> around the fact that get_hvm_registers() strictly requires a mutable pointer.
>>
>> For anyone interested, this is one reason why C cannot optimise any reads
>> across sequence points, even for a function purporting to take a const 
>> object.
>>
>> Anyway, have the function correctly state that it needs a mutable vcpu.  All
>> callers have a mutable vCPU to hand, and it removes the runtime pointer chase
>> in x86.
>>
>> Make one style adjustment in ARM while adjusting the parameter type.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> ---
>> CC: Anthony PERARD <anthony.perard@xxxxxxxxxx>
>> CC: Michal Orzel <michal.orzel@xxxxxxx>
>> CC: Jan Beulich <jbeulich@xxxxxxxx>
>> CC: Julien Grall <julien@xxxxxxx>
>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>> CC: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
>> CC: Bertrand Marquis <bertrand.marquis@xxxxxxx>
>>
>> RISC-V and PPC don't have this helper yet, not even in stub form.
>>
>> I expect there will be one objection to this patch.  Since the last time we
>> fought over this, speculative vulnerabilities have demonstrated how dangerous
>> pointer chases are, and this is a violation of Misra Rule 11.8, even if it's
>> not reasonable for Eclair to be able to spot and reject it.
> On these grounds
> Acked-by: Jan Beulich <jbeulich@xxxxxxxx>

Thanks.

> irrespective of the fact that a function of this name and purpose really, 
> really
> should be taking a pointer-to-const.

No - this is a perfect example of why most functions should *not* take
pointer-to-const for complex objects.

There is no such thing as an actually-const vcpu or domain; they are all
mutable.  The reason why x86 needs a strictly-mutable pointer is because
it needs to take a spinlock to negotiate for access to a hardware
resource to read some of the registers it needs.

This is where there is a semantic gap between "logically doesn't modify"
and what the C keyword means.

Anything except the-most-trivial trivial predates may reasonably need to
take a spinlock or some other safety primitive, even just to read
information.


Because this was gratuitously const in the first place, bad code was put
in place of making the prototype match reality.

This demonstrates a bigger failing in how code is reviewed and
maintained.  It is far too frequent that requests to const things don't
even compile.  It is also far too frequent that requests to const things
haven't read the full patch series to realise why not.  Both of these
are a source of friction during review.

But more than that, even if something could technically be const right
now, the request to do so forces churn into a future patch to de-const
it in order to make a clean change.  And for contributors who aren't
comfortable saying a firm no to a maintainer, this turns into a bad hack
instead.

i.e. requests to const accessors for complexity objects are making Xen
worse, not better, and we should stop doing it.

~Andrew