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Date: Fri, 20 Mar 2020 18:33:00 -0400
From: Rich Felker <dalias@...c.org>
To: Andreas Dröscher <musl@...free.ch>
Cc: musl@...ts.openwall.com
Subject: Re: mips32 little endian -ENOSYS is not -(-ENOSYS)

On Fri, Mar 20, 2020 at 11:18:56PM +0100, Andreas Dröscher wrote:
> Am 20.03.20 um 17:34 schrieb Rich Felker:
> >On Wed, Mar 11, 2020 at 07:08:48PM -0400, Rich Felker wrote:
> >>On Wed, Mar 11, 2020 at 10:08:11PM +0100, Andreas Dröscher wrote:
> >>>Am 11.03.20 um 03:18 schrieb Rich Felker:
> >>>>On Wed, Mar 11, 2020 at 03:08:22AM +0100, Andreas Dröscher wrote:
> >>>>>The current implementation of __syscall5, __syscall6 and __syscall7
> >>>>>(those use caller saved registers) violate the calling conventions
> >>>>>of MIPS32 Linux Kernels prior 2.6.35. Those were assuming that the
> >>>>>instruction immediately preceding the SYSCALL instruction was an
> >>>>>instruction for loading the syscall number.
> >>>>>
> >>>>>I’ll will try to rearrange the stack pushes to accommodate this
> >>>>>requirement and report back if I manage to come up with something
> >>>>>presentable.
> >>>>
> >>>>Uhg, so commit 604f8d3d8b08ee4f548de193050ef93a7753c2e0 was probably
> >>>>wrong and there was a reason for the nonsensical code it removed:
> >>>>making old broken kernels happy. I'm not sure if you can just revert
> >>>>it or need to make new changes.
> >>>>
> >>>>Do you know if this "rule" applies to n32/n64 too or just o32?
> >>>
> >>>I've reverted 604f8d3d8b08ee4f548de193050ef93a7753c2e0 and additionally
> >>>replaced all:
> >>>return r7 ? -r2 : r2;
> >>>with
> >>>return (r7 && r2 > 0) ? -r2 : r2;
> >>>
> >>>My software stack (built with OE-Core Zeus) now works almost flawlessly.
> >>>Some Daemons have hiccups but those most likely come from source
> >>>that expects syscalls to always succeed and on my system they are
> >>>simply missing.
> >>>
> >>>Thank you for your helping to sort this out.
> >>>
> >>>You asked about n32/n64. I am not familiar with more modern MIPS Architectures.
> >>>Therefore I can't give any informed answer. I found some documentation:
> >>>https://www.linux-mips.org/wiki/Syscall but it does not give a
> >>>definitive answer. It just points towards "all 3 mips are effected
> >>>by the ordering requirement".
> >>
> >>I'm posting a patch series now.
> >
> >I've pushed a version of this upstream now, a long with a lot of other
> >commits that had backed up in my queue. Please let me know if this
> >does or doesn't fix the issues with mips on old kernels.
> >
> 
> Sorry for now sending an update in a timely manner. I've tested your
> patches and they definitely get me to the point I had with my manual
> changes. However, the incompatibilities I reported earlier stil need
> to be ironed out. Sadly I got sidetracked by another project. There
> was no progress in the past week.
> 
> I’m planning to pick up the task on Monday. I was wondering if the
> inclusion of our fixes for a 13-year-old kernel benefits anyone
> except my niche use case.

I'm not sure, but critical regression on supported kernel is always a
bug that needs to be fixed. This is actually probably serious enough
to merit a follow-up to the 1.1.x series, though I'll hold off for
anything else that might come up in the next month or two.

Rich

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