MIPS: ptrace: Expose FIR register through FP regset

Correct commit 7aeb753b5353 ("MIPS: Implement task_user_regset_view.")
and expose the FIR register using the unused 4 bytes at the end of the
NT_PRFPREG regset.  Without that register included clients cannot use
the PTRACE_GETREGSET request to retrieve the complete FPU register set
and have to resort to one of the older interfaces, either PTRACE_PEEKUSR
or PTRACE_GETFPREGS, to retrieve the missing piece of data.  Also the
register is irreversibly missing from core dumps.

This register is architecturally hardwired and read-only so the write
path does not matter.  Ignore data supplied on writes then.

Fixes: 7aeb753b5353 ("MIPS: Implement task_user_regset_view.")
Signed-off-by: James Hogan <jhogan@kernel.org>
Signed-off-by: Maciej W. Rozycki <macro@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: <stable@vger.kernel.org> # 3.13+
Patchwork: https://patchwork.linux-mips.org/patch/19273/
Signed-off-by: James Hogan <jhogan@kernel.org>

arch/mips/kernel/ptrace.c

@@ -463,7 +463,7 @@ static int fpr_get_msa(struct task_struct *target,
 /*
  * Copy the floating-point context to the supplied NT_PRFPREG buffer.
  * Choose the appropriate helper for general registers, and then copy
- * the FCSR register separately.
+ * the FCSR and FIR registers separately.
  */
 static int fpr_get(struct task_struct *target,
 		   const struct user_regset *regset,
@@ -471,6 +471,7 @@ static int fpr_get(struct task_struct *target,
 		   void *kbuf, void __user *ubuf)
 {
 	const int fcr31_pos = NUM_FPU_REGS * sizeof(elf_fpreg_t);
+	const int fir_pos = fcr31_pos + sizeof(u32);
 	int err;
 
 	if (sizeof(target->thread.fpu.fpr[0]) == sizeof(elf_fpreg_t))
@@ -483,6 +484,12 @@ static int fpr_get(struct task_struct *target,
 	err = user_regset_copyout(&pos, &count, &kbuf, &ubuf,
 				  &target->thread.fpu.fcr31,
 				  fcr31_pos, fcr31_pos + sizeof(u32));
+	if (err)
+		return err;
+
+	err = user_regset_copyout(&pos, &count, &kbuf, &ubuf,
+				  &boot_cpu_data.fpu_id,
+				  fir_pos, fir_pos + sizeof(u32));
 
 	return err;
 }
@@ -531,7 +538,8 @@ static int fpr_set_msa(struct task_struct *target,
 /*
  * Copy the supplied NT_PRFPREG buffer to the floating-point context.
  * Choose the appropriate helper for general registers, and then copy
- * the FCSR register separately.
+ * the FCSR register separately.  Ignore the incoming FIR register
+ * contents though, as the register is read-only.
  *
  * We optimize for the case where `count % sizeof(elf_fpreg_t) == 0',
  * which is supposed to have been guaranteed by the kernel before
@@ -545,6 +553,7 @@ static int fpr_set(struct task_struct *target,
 		   const void *kbuf, const void __user *ubuf)
 {
 	const int fcr31_pos = NUM_FPU_REGS * sizeof(elf_fpreg_t);
+	const int fir_pos = fcr31_pos + sizeof(u32);
 	u32 fcr31;
 	int err;
 
@@ -572,6 +581,11 @@ static int fpr_set(struct task_struct *target,
 		ptrace_setfcr31(target, fcr31);
 	}
 
+	if (count > 0)
+		err = user_regset_copyin_ignore(&pos, &count, &kbuf, &ubuf,
+						fir_pos,
+						fir_pos + sizeof(u32));
+
 	return err;
 }