Re: um: PTRACE_SETREGSET failure with XSTATE on Kabylake CPU

[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? 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.

Thanks,
//richard