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
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