Re: [uml-devel] um: PTRACE_SETREGSET failure with XSTATE on Kabylake CPU

[Thomas Meyer <thomas@m3y3r.de>]

Am Dienstag, den 20.06.2017, 08:58 +0200 schrieb Richard Weinberger:
> Thomas,
> 
> Am 20.06.2017 um 03:56 schrieb Thomas Meyer:
> > Hi,
> > 
> > I finally did figure out where in the host kernel the ptrace
> > syscall
> > fails with -EFAULT.
> 
> Nice! Thanks a lot for digging into this. I still had no chance to
> setup
> Ipv6 to connect to your host and figure myself. ;-\
> 
> > In arch/x86/kernel/fpu/regset.c:130:
> > 
> > 114 int xstateregs_set(struct task_struct *target, const struct
> > user_regset *regset,
> > 115                   unsigned int pos, unsigned int count,
> > 116                   const void *kbuf, const void __user *ubuf)
> > 117 {
> > 118         struct fpu *fpu = &target->thread.fpu;
> > 119         struct xregs_state *xsave;
> > 120         int ret;
> > 121 
> > 122         if (!boot_cpu_has(X86_FEATURE_XSAVE))
> > 123                 return -ENODEV;
> > 124 
> > 125         pr_info("in xstateregs_set");
> > 126 
> > 127         /*
> > 128          * A whole standard-format XSAVE buffer is needed:
> > 129          */
> > 130         if ((pos != 0) || (count < fpu_user_xstate_size)) {
> > 131                 pr_info("EFAULT from xstateregs_set");
> > 132->               pr_info("pos = %i, count = %i,
> > fpu_user_xstate_size= %i\n", pos, count, fpu_user_xstate_size);
> > 133                 return -EFAULT;
> > 134         }
> > 
> > Sadly I had to fallback to debugging by printk because kgdb/qemu
> > gdbstub, all didn't work for some unknown reason :-(
> 
> As always. printk is best debugger ever. ;-)
> 
> > output is:
> > [   69.598349] EFAULT from xstateregs_set
> > [   69.598350] pos = 0, count = 832, fpu_user_xstate_size= 1088
> > 
> > calling code is in arch/x86/um/os-Linux/registers.c:
> > 
> >  49 int restore_fp_registers(int pid, unsigned long *fp_regs)
> >  50 {
> >  51         struct iovec iov;
> >  52 
> >  53         if (have_xstate_support) {
> >  54                 iov.iov_base = fp_regs;
> >  55                 iov.iov_len = sizeof(struct _xstate);
> >  56                 if (ptrace(PTRACE_SETREGSET, pid,
> > NT_X86_XSTATE, &iov) < 0)
> >  57  ->                     return -errno;
> >  58                 return 0;
> >  59         } else {
> >  60                 return restore_i387_registers(pid, fp_regs);
> >  61         }
> >  62 }
> > 
> > it looks like _xstate is too short for above operation, I wonder
> > why
> > PTRACE_GETREGSET works without a warning of too short size.
> 
> Does PTRACE_GETREGSET return a size?

Yes, it returns 832. the size of struct _xstate.

> Maybe we have to take this into account.
> It could be that your host CPU has a smaller set.
> Also check whether PTRACE_SETREGSET always fails.

In UML the first userspace ptrace always fails, so init get's killed.

The check "count < fpu_user_xstate_size" was introduced by commit:

commit 91c3dba7dbc199191272f4a9863f86ea3bfd679f
Author: Yu-cheng Yu <yu-cheng.yu@intel.com>
Date:   Fri Jun 17 13:07:17 2016 -0700

    x86/fpu/xstate: Fix PTRACE frames for XSAVES
    
    XSAVES uses compacted format and is a kernel instruction. The kernel
    should use standard-format, non-supervisor state data for PTRACE.

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?

> 
> Thanks,
> //richard

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