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From: Dmitry Safonov <0x7f454c46@gmail.com>
Date: Fri, 8 Apr 2016 12:39:22 +0300
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Subject: Re: [PATCH v3 2/7] x86/arch_prctl: Fix ARCH_GET_FS and ARCH_GET_GS
To: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
        security@kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Rudolf Marek <r.marek@assembler.cz>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>, "H. Peter Anvin" <hpa@zytor.com>,
        gorcunov@openvz.org
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2016-04-08 10:13 GMT+03:00 Ingo Molnar <mingo@kernel.org>:
>
> * Andy Lutomirski <luto@kernel.org> wrote:
>
>> ARCH_GET_FS and ARCH_GET_GS attempted to figure out the fsbase and
>> gsbase respectively from saved thread state.  This was wrong: fsbase
>> and gsbase live in registers while a thread is running, not in
>> memory.
>
> So I'm wondering, the current code looks totally broken,what user-space code can
> possibly use this? I checked glibc and Wine, and neither of them does. Wine uses
> ARCH_SET_GS and glibc uses ARCH_SET_FS, but that's all - neither actually tries to
> use the ARCH_GET_* reading APIs.

Just FYI, CRIU also does not use ARCH_GET_{FS,GS}, only ARCH_SET_{FS,GS}.
It gets them with the help of PTRACE_GETREGSET.

> So for backporting purposes I'd be much happier about simply returning -EINVAL or
> -ENOSYS, and we could re-introduce this code in v4.7.