um: PTRACE_SETREGSET failure with XSTATE on Kabylake CPU

From: Thomas Meyer
Date: Mon Jun 19 2017 - 21:56:56 EST


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