[adding x86 folks]
Am 20.06.2017 um 10:49 schrieb Thomas Meyer:
> 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@xxxxxxxxx>
> 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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