powerpc/ptrace: Preserve previous fprs/vsrs on short regset write

commit 99dfe80a upstream.

Ensure that if userspace supplies insufficient data to PTRACE_SETREGSET
to fill all the registers, the thread's old registers are preserved.

Fixes: c6e6771b ("powerpc: Introduce VSX thread_struct and CONFIG_VSX")
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

arch/powerpc/kernel/ptrace.c

@@ -394,6 +394,10 @@ static int fpr_set(struct task_struct *target, const struct user_regset *regset,
 	flush_fp_to_thread(target);
 
 #ifdef CONFIG_VSX
+	for (i = 0; i < 32 ; i++)
+		buf[i] = target->thread.TS_FPR(i);
+	buf[32] = target->thread.fp_state.fpscr;
+
 	/* copy to local buffer then write that out */
 	i = user_regset_copyin(&pos, &count, &kbuf, &ubuf, buf, 0, -1);
 	if (i)
@@ -537,6 +541,9 @@ static int vsr_set(struct task_struct *target, const struct user_regset *regset,
 
 	flush_vsx_to_thread(target);
 
+	for (i = 0; i < 32 ; i++)
+		buf[i] = target->thread.fp_state.fpr[i][TS_VSRLOWOFFSET];
+
 	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf,
 				 buf, 0, 32 * sizeof(double));
 	for (i = 0; i < 32 ; i++)