Re: [PATCH] ioreq: don't wrongly claim "success" in ioreq_send_buffered()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 16.09.2024 23:27, Julien Grall wrote:
> On 11/09/2024 13:19, Jan Beulich wrote:
>> Returning a literal number is a bad idea anyway when all other returns
>> use IOREQ_STATUS_* values. While that's maybe intended on Arm (mapping
>> to IO_ABORT), 
> 
> Arm doesn't support buffered ioreq (see ioreq_server_create()) and 
> AFAICT the "0" was already there before the code was moved.

Indeed, the bad conversion is older than the move.

>  > mapping to X86EMUL_OKAY is surely wrong on x86.
> 
> The code has been for nearly 10 years. So I would like to understand why 
> the change now. Did you see any issue?

Well, result of looking at the code. As said - returning success here is
definitely wrong on x86. The open question is whether IO_ABORT was actually
meant to be (implicitly) used here for Arm (but see below).

> The unclear part for me is the behavior change. Below...
> 
>>
>> Fixes: f6bf39f84f82 ("x86/hvm: add support for broadcast of buffered 
>> ioreqs...")
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> Judging from history, it may want to be IOREQ_STATUS_UNHANDLED instead,
>> eliminating the need for IOREQ_STATUS_BAD. That'll be a behavioral
>> change on Arm then too, though.
> 
> ... you mention Arm. But not x86. This would imply there are no behavior 
> change but I don't understand why.

The way the patch is written it keeps Arm's (perceived; again see below)
behavior unchanged, but fixes x86. The remark above is suggesting an
alternative without need for the new IOREQ_STATUS_BAD, yet then also
leading to a behavioral change on Arm. Hence the question whether the
present behavior is intended. However, ...

> For the Arm behavior change, per above, I don't think we can reach the 
> code on Arm so it should not be a problem to change it.

... with you pointing out that buffered ioreqs aren't supported on Arm,
I could indeed change this whichever way suits x86, without affecting
Arm at all. It would then be only an abstract consideration, for the
hypothetical case that buffered ioreqs became needed on Arm as well.

Buffered ioreqs not being supported on Arm of course means the function
as a whole is unreachable, i.e. in violation of Misra rule 2.1. Which I
find concerning, as that rule is marked as clean - indicating that
Eclair isn't smart enough to spot the case here. (Reason for the remark:
If the function had been marked / excluded accordingly, I would have
noticed Arm's unaffectedness of whichever way the change is done.)

>> Shouldn't IOREQ_READ requests also be rejected here, for the result of
>> a read not possibly coming from anywhere, yet a (bogus) caller then
>> assuming some data was actually returned?
> 
> I am not sure. I understand from an hardened PoV. But this would add an 
> extra check to something the caller should be aware of. This is 
> different from the address check because this is more of an 
> implementation details.
> 
> So maybe it should be an ASSERT()?

That might be an option, yet with the general movement towards also
providing safety on release builds that would likely end up being

    if ( dir != IOREQ_WRITE )
    {
        ASSERT_UNREACHABLE():
        return 0;
    }

i.e. still an extra check.

Jan