Messages in this thread | | | Subject | um: PTRACE_SETREGSET failure with XSTATE on Kabylake CPU | From | Thomas Meyer <> | Date | Tue, 20 Jun 2017 03:56:46 +0200 |
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Hi,
I finally did figure out where in the host kernel the ptrace syscall fails with -EFAULT.
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 :-(
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.
with kind regards thomas
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