powerpc/ptrace: Simplify vr_get/set() to avoid GCC warning

GCC 8 warns about the logic in vr_get/set(), which with -Werror breaks
the build:

  In function ‘user_regset_copyin’,
      inlined from ‘vr_set’ at arch/powerpc/kernel/ptrace.c:628:9:
  include/linux/regset.h:295:4: error: ‘memcpy’ offset [-527, -529] is
  out of the bounds [0, 16] of object ‘vrsave’ with type ‘union
  <anonymous>’ [-Werror=array-bounds]
  arch/powerpc/kernel/ptrace.c: In function ‘vr_set’:
  arch/powerpc/kernel/ptrace.c:623:5: note: ‘vrsave’ declared here
     } vrsave;

This has been identified as a regression in GCC, see GCC bug 88273.

However we can avoid the warning and also simplify the logic and make
it more robust.

Currently we pass -1 as end_pos to user_regset_copyout(). This says
"copy up to the end of the regset".

The definition of the regset is:
	[REGSET_VMX] = {
		.core_note_type = NT_PPC_VMX, .n = 34,
		.size = sizeof(vector128), .align = sizeof(vector128),
		.active = vr_active, .get = vr_get, .set = vr_set
	},

The end is calculated as (n * size), ie. 34 * sizeof(vector128).

In vr_get/set() we pass start_pos as 33 * sizeof(vector128), meaning
we can copy up to sizeof(vector128) into/out-of vrsave.

The on-stack vrsave is defined as:
  union {
	  elf_vrreg_t reg;
	  u32 word;
  } vrsave;

And elf_vrreg_t is:
  typedef __vector128 elf_vrreg_t;

So there is no bug, but we rely on all those sizes lining up,
otherwise we would have a kernel stack exposure/overwrite on our
hands.

Rather than relying on that we can pass an explict end_pos based on
the sizeof(vrsave). The result should be exactly the same but it's
more obviously not over-reading/writing the stack and it avoids the
compiler warning.
Reported-by: Meelis Roos <mroos@linux.ee>
Reported-by: Mathieu Malaterre <malat@debian.org>
Cc: stable@vger.kernel.org
Tested-by: Mathieu Malaterre <malat@debian.org>
Tested-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/ptrace.c

@@ -567,6 +567,7 @@ static int vr_get(struct task_struct *target, const struct user_regset *regset,
 		/*
 		 * Copy out only the low-order word of vrsave.
 		 */
+		int start, end;
 		union {
 			elf_vrreg_t reg;
 			u32 word;
@@ -575,8 +576,10 @@ static int vr_get(struct task_struct *target, const struct user_regset *regset,
 
 		vrsave.word = target->thread.vrsave;
 
+		start = 33 * sizeof(vector128);
+		end = start + sizeof(vrsave);
 		ret = user_regset_copyout(&pos, &count, &kbuf, &ubuf, &vrsave,
-					  33 * sizeof(vector128), -1);
+					  start, end);
 	}
 
 	return ret;
@@ -614,6 +617,7 @@ static int vr_set(struct task_struct *target, const struct user_regset *regset,
 		/*
 		 * We use only the first word of vrsave.
 		 */
+		int start, end;
 		union {
 			elf_vrreg_t reg;
 			u32 word;
@@ -622,8 +626,10 @@ static int vr_set(struct task_struct *target, const struct user_regset *regset,
 
 		vrsave.word = target->thread.vrsave;
 
+		start = 33 * sizeof(vector128);
+		end = start + sizeof(vrsave);
 		ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, &vrsave,
-					 33 * sizeof(vector128), -1);
+					 start, end);
 		if (!ret)
 			target->thread.vrsave = vrsave.word;
 	}