MIPS: prevent FP context set via ptrace being discarded

If a ptracee has not used the FPU and the ptracer sets its FP context
using PTRACE_POKEUSR, PTRACE_SETFPREGS or PTRACE_SETREGSET then that
context will be discarded upon either the ptracee using the FPU or a
further write to the context via ptrace. Prevent this loss by recording
that the task has "used" math once its FP context has been written to.
The context initialisation code that was present for the PTRACE_POKEUSR
case is reused for the other 2 cases to provide consistent behaviour
for the different ptrace requests.
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9166/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/ptrace.c

@@ -46,6 +46,26 @@
 #define CREATE_TRACE_POINTS
 #include <trace/events/syscalls.h>
 
+static void init_fp_ctx(struct task_struct *target)
+{
+	/* If FP has been used then the target already has context */
+	if (tsk_used_math(target))
+		return;
+
+	/* Begin with data registers set to all 1s... */
+	memset(&target->thread.fpu.fpr, ~0, sizeof(target->thread.fpu.fpr));
+
+	/* ...and FCSR zeroed */
+	target->thread.fpu.fcr31 = 0;
+
+	/*
+	 * Record that the target has "used" math, such that the context
+	 * just initialised, and any modifications made by the caller,
+	 * aren't discarded.
+	 */
+	set_stopped_child_used_math(target);
+}
+
 /*
  * Called by kernel/ptrace.c when detaching..
  *
@@ -142,6 +162,7 @@ int ptrace_setfpregs(struct task_struct *child, __u32 __user *data)
 	if (!access_ok(VERIFY_READ, data, 33 * 8))
 		return -EIO;
 
+	init_fp_ctx(child);
 	fregs = get_fpu_regs(child);
 
 	for (i = 0; i < 32; i++) {
@@ -439,6 +460,8 @@ static int fpr_set(struct task_struct *target,
 
 	/* XXX fcr31  */
 
+	init_fp_ctx(target);
+
 	if (sizeof(target->thread.fpu.fpr[i]) == sizeof(elf_fpreg_t))
 		return user_regset_copyin(&pos, &count, &kbuf, &ubuf,
 					  &target->thread.fpu,
@@ -660,12 +683,7 @@ long arch_ptrace(struct task_struct *child, long request,
 		case FPR_BASE ... FPR_BASE + 31: {
 			union fpureg *fregs = get_fpu_regs(child);
 
-			if (!tsk_used_math(child)) {
-				/* FP not yet used  */
-				memset(&child->thread.fpu, ~0,
-				       sizeof(child->thread.fpu));
-				child->thread.fpu.fcr31 = 0;
-			}
+			init_fp_ctx(child);
 #ifdef CONFIG_32BIT
 			if (test_thread_flag(TIF_32BIT_FPREGS)) {
 				/*