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SubjectRe: [PATCH] MIPS: Validate PR_SET_FP_MODE prctl(2) requests against the ABI of the task
Hi Maciej,

On Mon, Nov 27, 2017 at 09:39:10PM +0000, Maciej W. Rozycki wrote:
> > > Always succeed however without taking any further action if the mode
> > > requested is the same as one already in effect, regardless of whether
> > > any mode change, should it be requested, would actually be allowed for
> > > the task concerned.
> >
> > This seems like a distinct change that I think would be worth splitting
> > out to a separate patch.
>
> I've been thinking about it before posting and decided it's inherent.
>
> Indeed in developing this fix this part was the last one I realised that
> had to be done for the change to be overall self-consistent, following a
> principle typically applied to hardware registers where the programmer is
> architecturally allowed to write individual bits with the values
> previously read from them even if these bits are undefined in the
> specification of hardware concerned.
>
> So here you'll be able to issue a PR_SET_FP_MODE request with a value
> previously obtained with PR_GET_FP_MODE and it will succeed, even if all
> the bits are actually read-only for the ABI in effect. This is important
> as GDB will soon be using these calls and expect PR_SET_FP_MODE not to
> fail if an attempt is made to write back a value previously obtained with
> PR_GET_FP_MODE.
>
> I could have buried this check in the two conditions that follow, making
> execution fall through if the mode remains unchanged, however I have
> realised that making the check upfront makes the resulting code cleaner.
>
> That written, I could make it 1/2 with the ABI checks becoming 2/2, but
> then 1/2 wouldn't make sense on its own (except perhaps as a
> microoptimisation, but that would be an entirely different purpose) and
> would have to be considered in conjunction with 2/2 anyway.

Ah - OK, I see. Prior to this patch the value returned by PR_GET_FP_MODE
would always be one accepted by PR_SET_FP_MODE anyway, but with the
patch that will cease to be true for non-o32 ABIs without the special
case. Gotcha.

> > Both changes look good to me though, so feel free to add:
> >
> > Reviewed-by: Paul Burton <paul.burton@mips.com>
>
> Thanks for your review. Do you feel convinced with the justification I
> gave?

Yes - I follow, please consider the Reviewed-by tag valid for the patch
as-is.

Thanks,
Paul

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