x86/fsgsbase/64: Make ptrace use the new FS/GS base helpers

Use the new FS/GS base helper functions in <asm/fsgsbase.h> in the platform
specific ptrace implementation of the following APIs:

  PTRACE_ARCH_PRCTL,
  PTRACE_SETREG,
  PTRACE_GETREG,
  etc.

The fsgsbase code is more abstracted out this way and the FS/GS-update
mechanism will be easier to change this way.

[ mingo: Wrote new changelog. ]

Based-on-code-from: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Markus T Metzger <markus.t.metzger@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Shankar <ravi.v.shankar@intel.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1537312139-5580-4-git-send-email-chang.seok.bae@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/ptrace.c

@@ -397,12 +397,11 @@ static int putreg(struct task_struct *child,
 		if (value >= TASK_SIZE_MAX)
 			return -EIO;
 		/*
-		 * When changing the segment base, use do_arch_prctl_64
-		 * to set either thread.fs or thread.fsindex and the
-		 * corresponding GDT slot.
+		 * When changing the FS base, use the same
+		 * mechanism as for do_arch_prctl_64().
 		 */
 		if (child->thread.fsbase != value)
-			return do_arch_prctl_64(child, ARCH_SET_FS, value);
+			return x86_fsbase_write_task(child, value);
 		return 0;
 	case offsetof(struct user_regs_struct,gs_base):
 		/*
@@ -411,7 +410,7 @@ static int putreg(struct task_struct *child,
 		if (value >= TASK_SIZE_MAX)
 			return -EIO;
 		if (child->thread.gsbase != value)
-			return do_arch_prctl_64(child, ARCH_SET_GS, value);
+			return x86_gsbase_write_task(child, value);
 		return 0;
 #endif
 	}
@@ -435,20 +434,10 @@ static unsigned long getreg(struct task_struct *task, unsigned long offset)
 		return get_flags(task);
 
 #ifdef CONFIG_X86_64
-	case offsetof(struct user_regs_struct, fs_base): {
-		if (task->thread.fsindex == 0)
-			return task->thread.fsbase;
-		else
-			return x86_fsgsbase_read_task(task,
-						      task->thread.fsindex);
-	}
-	case offsetof(struct user_regs_struct, gs_base): {
-		if (task->thread.gsindex == 0)
-			return task->thread.gsbase;
-		else
-			return x86_fsgsbase_read_task(task,
-						      task->thread.gsindex);
-	}
+	case offsetof(struct user_regs_struct, fs_base):
+		return x86_fsbase_read_task(task);
+	case offsetof(struct user_regs_struct, gs_base):
+		return x86_gsbase_read_task(task);
 #endif
 	}