arm64: ptrace: Fix VFP register dumping in compat coredumps

Currently, VFP registers are omitted from coredumps for compat
processes, due to a bug in the REGSET_COMPAT_VFP regset
implementation.

compat_vfp_get() needs to transfer non-contiguous data from
thread_struct.fpsimd_state, and uses put_user() to handle the
offending trailing word (FPSCR).  This fails when copying to a
kernel address (i.e., kbuf && !ubuf), which is what happens when
dumping core.  As a result, the ELF coredump core code silently
omits the NT_ARM_VFP note from the dump.

It would be possible to work around this with additional special
case code for the put_user(), but since user_regset_copyout() is
explicitly designed to handle this scenario it is cleaner to port
the put_user() to a user_regset_copyout() call, which this patch
does.
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/ptrace.c

@@ -894,7 +894,7 @@ static int compat_vfp_get(struct task_struct *target,
 {
 	struct user_fpsimd_state *uregs;
 	compat_ulong_t fpscr;
-	int ret;
+	int ret, vregs_end_pos;
 
 	uregs = &target->thread.fpsimd_state.user_fpsimd;
 
@@ -902,13 +902,16 @@ static int compat_vfp_get(struct task_struct *target,
 	 * The VFP registers are packed into the fpsimd_state, so they all sit
 	 * nicely together for us. We just need to create the fpscr separately.
 	 */
-	ret = user_regset_copyout(&pos, &count, &kbuf, &ubuf, uregs, 0,
-				  VFP_STATE_SIZE - sizeof(compat_ulong_t));
+	vregs_end_pos = VFP_STATE_SIZE - sizeof(compat_ulong_t);
+	ret = user_regset_copyout(&pos, &count, &kbuf, &ubuf, uregs,
+				  0, vregs_end_pos);
 
 	if (count && !ret) {
 		fpscr = (uregs->fpsr & VFP_FPSCR_STAT_MASK) |
 			(uregs->fpcr & VFP_FPSCR_CTRL_MASK);
-		ret = put_user(fpscr, (compat_ulong_t *)ubuf);
+
+		ret = user_regset_copyout(&pos, &count, &kbuf, &ubuf, &fpscr,
+					  vregs_end_pos, VFP_STATE_SIZE);
 	}
 
 	return ret;