Re: 3.18+: soft-float userland unusable due to .MIPS.abiflags patch

[Paul Burton <paul.burton@imgtec.com>]

On Sun, Jan 18, 2015 at 11:35:31AM +0100, Manuel Lauss wrote:
> On Sat, Jan 17, 2015 at 8:00 PM, Matthew Fortune
> <Matthew.Fortune@imgtec.com> wrote:
> > Aaro Koskinen <aaro.koskinen@iki.fi> writes:
> >> On Fri, Jan 16, 2015 at 08:36:12PM +0000, Matthew Fortune wrote:
> >> > You are right that it is the .MIPS.abiflags patch that is causing your
> >> > trouble. For a long time I had to put a restriction in the ABI plan
> >> > that soft-float binaries without an ABIFLAGS pheader could not be
> >> > linked against soft-float binaries with an ABIFLAGS pheader. We have
> >> > since found a way to relax that restriction without reducing the
> >> > effectiveness of the new compatibility checks. I would need to check
> >> > the code in the kernel but I suspect that is the issue. Markos has
> >> > done a significant update to this piece of code which he posted
> >> > earlier today. That updated version should allow the combination of
> >> > soft-float without ABIFLAGS and soft-float with ABIFLAGS.
> >>
> >> Are you referring to the series with 70 patches? I think a fix that
> >> passes stable kernel rules is needed.
> >
> > Yes it was just one patch though for this issue:
> > [PATCH RFC v2 68/70] MIPS: kernel: elf: Improve the overall ABI and FPU
> > mode checks
> >
> > I wasn't trying to suggest how to fix the existing code just explaining
> > how it came to be and what has been done about it for next release.
> > (I'm not a kernel developer I'm just interested as I did most of the
> > design work for the new ABI extensions.)
> >
> > I guess there are three options:
> > a) revert the patch - That would remove the new ABI safety measures from
> >    3.19 which is a shame given it has MSA support in it (I think anyway).
> >    equally given that the new prctl FPU mode options did not make 3.19
> >    then I suppose it doesn't lose too much either as the two features
> >    go hand in hand to some extent.
> 
> I favor this one.  I don't know how many systems with MSA are in the wild,
> and if there are any, I'm sure they're using some mti/imgtec-supplied kernel
> anyway.  Another thing I noticed last time is that companies shipping MIPS
> products rarely upgrade their toolchains, so I'm sure the ABI safety measures
> can wait for another release, but then function with all configurations
> in the wild.
> 
> Manuel

An alternative would be the patch I just submitted, which makes the mode
checks conditional upon CONFIG_MIPS_O32_FP64_SUPPORT:

  http://marc.info/?l=linux-mips&m=142164553017027&w=2

Assuming this fixes your problem, and I believe it should, it would
avoid the churn of reverting the patch & readding the modified logic
again later.

Thanks,
    Paul