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Date: Fri, 20 Mar 2020 23:18:56 +0100
From: Andreas Dröscher <musl@...free.ch>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: mips32 little endian -ENOSYS is not -(-ENOSYS)

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.

Andreas

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