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

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Date:   Tue, 20 Jun 2017 20:17:36 +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 20:04 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.

Thanks,
//richard