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Date: Fri, 20 Mar 2020 12:34:56 -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 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.

Rich

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