[PATCH 3.16 306/366] MIPS: ptrace: Expose FIR register through FP regset

[Ben Hutchings <ben@xxxxxxxxxxxxxxx>]

3.16.60-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: "Maciej W. Rozycki" <macro@xxxxxxxx>

commit 71e909c0cdad28a1df1fa14442929e68615dee45 upstream.

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@xxxxxxxxxx>
Signed-off-by: Maciej W. Rozycki <macro@xxxxxxxx>
Cc: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
Cc: linux-mips@xxxxxxxxxxxxxx
Patchwork: https://patchwork.linux-mips.org/patch/19273/
Signed-off-by: James Hogan <jhogan@xxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/mips/kernel/ptrace.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

--- a/arch/mips/kernel/ptrace.c
+++ b/arch/mips/kernel/ptrace.c
@@ -481,7 +481,7 @@ static int fpr_get_msa(struct task_struc
 /*
  * 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,
@@ -489,6 +489,7 @@ static int fpr_get(struct task_struct *t
 		   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))
@@ -501,6 +502,12 @@ static int fpr_get(struct task_struct *t
 	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;
 }
@@ -549,7 +556,8 @@ static int fpr_set_msa(struct task_struc
 /*
  * 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
@@ -563,6 +571,7 @@ static int fpr_set(struct task_struct *t
 		   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;
 
@@ -590,6 +599,11 @@ static int fpr_set(struct task_struct *t
 		ptrace_setfcr31(target, fcr31);
 	}
 
+	if (count > 0)
+		err = user_regset_copyin_ignore(&pos, &count, &kbuf, &ubuf,
+						fir_pos,
+						fir_pos + sizeof(u32));
+
 	return err;
 }