x86/mpx: Remove MPX APIs

MPX is being removed from the kernel due to a lack of support in the
toolchain going forward (gcc).

The first step is to remove the userspace-visible ABIs so that applications
will stop using it.  The most visible one are the enable/disable prctl()s.
Remove them first.

This is the most minimal and least invasive change needed to ensure that
apps stop using MPX with new kernels.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190705175321.DB42F0AD@viggo.jf.intel.com

Author: hansendc

include/uapi/linux/prctl.h

@@ -181,7 +181,7 @@ struct prctl_mm_map {
 #define PR_GET_THP_DISABLE	42
 
 /*
- * Tell the kernel to start/stop helping userspace manage bounds tables.
+ * No longer implemented, but left here to ensure the numbers stay reserved:
  */
 #define PR_MPX_ENABLE_MANAGEMENT  43
 #define PR_MPX_DISABLE_MANAGEMENT 44

kernel/sys.c

@@ -103,12 +103,6 @@
 #ifndef SET_TSC_CTL
 # define SET_TSC_CTL(a)		(-EINVAL)
 #endif
-#ifndef MPX_ENABLE_MANAGEMENT
-# define MPX_ENABLE_MANAGEMENT()	(-EINVAL)
-#endif
-#ifndef MPX_DISABLE_MANAGEMENT
-# define MPX_DISABLE_MANAGEMENT()	(-EINVAL)
-#endif
 #ifndef GET_FP_MODE
 # define GET_FP_MODE(a)		(-EINVAL)
 #endif
@@ -2456,15 +2450,9 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 		up_write(&me->mm->mmap_sem);
 		break;
 	case PR_MPX_ENABLE_MANAGEMENT:
-		if (arg2 || arg3 || arg4 || arg5)
-			return -EINVAL;
-		error = MPX_ENABLE_MANAGEMENT();
-		break;
 	case PR_MPX_DISABLE_MANAGEMENT:
-		if (arg2 || arg3 || arg4 || arg5)
-			return -EINVAL;
-		error = MPX_DISABLE_MANAGEMENT();
-		break;
+		/* No longer implemented: */
+		return -EINVAL;
 	case PR_SET_FP_MODE:
 		error = SET_FP_MODE(me, arg2);
 		break;