linux-kernel - Re: um: PTRACE_SETREGSET failure with XSTATE on Kabylake CPU

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Date:   Tue, 20 Jun 2017 20:59:24 +0200
From:   Richard Weinberger <richard@....at>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     Thomas Meyer <thomas@...3r.de>, elicooper@....com,
        linux-kernel@...r.kernel.org,
        "open list:USER-MODE LINUX (UML)" 
        <user-mode-linux-devel@...ts.sourceforge.net>,
        linux-x86_64@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: um: PTRACE_SETREGSET failure with XSTATE on Kabylake CPU

Yu-cheng,

Am 20.06.2017 um 20:17 schrieb Richard Weinberger:
> Yu-cheng,
> 
> Am 20.06.2017 um 20:04 schrieb Yu-cheng Yu:
>>>> So to summarize:
>>>>
>>>> - PTRACE_GETREGSET with NT_X86_XSTATE gets 832 and return 832, with no
>>>> error.
>>>>
>>>> - PTRACE_SETREGSET get 832 (sizeof struct _xstate) but wants at least
>>>> 1088, otherwise it will fail with -EFAULT (why not -EINVAL?)
>>>>
>>>> Ideas?
>>
>> We considered allowing a partial XSAVE buffer for PTRACE_SETREGSET, but
>> it was that the XSAVE instruction requires a full-size buffer led to
>> this choice.  Using a smaller buffer for XSAVE causes a fault.
> 
> So, this code is not supposed to work?
> 
> iov.iov_base = fp_regs;
> iov.iov_len = sizeof(struct _xstate);
> ptrace(PTRACE_GETREGSET, pid, NT_X86_XSTATE, &iov);
> ptrace(PTRACE_SETREGSET, pid, NT_X86_XSTATE, &iov);
> 
> This is what UML does and on Thomas's new Laptop PTRACE_SETREGSET is failing.

Hmm, I think we need to do what gdb does, it uses a buffer of size X86_XSTATE_MAX_SIZE.

Thanks,
//richard