linux-kernel - Re: [PATCH v2] LoongArch: Clean useless vcsr in loongarch

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Thu, 7 Jul 2022 12:22:52 +0800
From:   WANG Xuerui <kernel@...0n.name>
To:     Xi Ruoyao <xry111@...111.site>, WANG Xuerui <kernel@...0n.name>,
        Qi Hu <huqi@...ngson.cn>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        Huacai Chen <chenhuacai@...nel.org>
Cc:     loongarch@...ts.linux.dev, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] LoongArch: Clean useless vcsr in loongarch_fpu.

On 2022/7/7 12:04, Xi Ruoyao wrote:
> On Thu, 2022-07-07 at 11:05 +0800, WANG Xuerui wrote:
>
>> To be frank, at this point I think you're trying to hide something.
>> (This is not your fault, blame someone else of course because they told
>> you the fact.) In the old-world kernel the VCSR a.k.a. FCSR16 is
>> certainly being saved/restored, and there's apparently no harm in doing
>> so. And if the contents are indeed "undefined", why are the code there
>> in the first place? Certainly the bits *are* meaningful, only that for
>> some reason you aren't revealing the semantics and pretending that they
>> are "undefined" and probably "do nothing externally observable" if
>> accessed in the first place.
> On a 3A5000LL, I did an experiment via a kernel module, which enables
> LSX/LASX and tries to write and read fcsr16.  I tried each bit (1, 2, 4,
> 8, ..., 1 << 31) one by one.  The result: no matter which bit I wrote
> into fcsr16, I always read out 0.
>
> And I've objdump'ed a kernel shipped in an early Loongnix release.  It
> seems the only reference to fcsr16 is a "movgr2fcsr $r16, $r0"
> instruction.

Hmm this is weird. I can't understand why the vcsr code was there in the 
first place then... I'd like to check a few Loongnix/Kylin/UOS kernels 
but currently I don't have the time.

If this is the case, indeed all vcsr-related code should be removed. 
Although I'm still not sure how to best word the commit message.