Re: um: PTRACE_SETREGSET failure with XSTATE on Kabylake CPU

[Yu-cheng Yu]

On Tue, 2017-06-20 at 20:59 +0200, Richard Weinberger wrote:
> 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.
> 

Linux kernel determines XSAVE buffer size from CPUID:
http://elixir.free-electrons.com/linux/latest/source/arch/x86/kernel/fpu/xstate.c#L626

GDB has a fixed X86_XSTATE_MAX_SIZE of 2688.  That can become an issue.

Yu-cheng