[PATCH 02/19] x86, fpu: Wrap get_xsave_addr() to make it safer

[Dave Hansen <dave@sr71.net>]

From: Dave Hansen <dave.hansen@linux.intel.com>

The MPX code appears to be saving off the FPU in an unsafe
way.  It does not disable preemption or ensure that the
FPU state has been allocated.  All of the preemption safety
comes from the unfortunatley-named 'unlazy_fpu()'.

This patch introduces a new helper which will do both of
those things internally.

Note that this requires a patch from Oleg in order to work
properly.  It is currently in tip/x86/fpu.

> commit f893959b0898bd876673adbeb6798bdf25c034d7
> Author: Oleg Nesterov <oleg@redhat.com>
> Date:   Fri Mar 13 18:30:30 2015 +0100
>
>    x86/fpu: Don't abuse drop_init_fpu() in flush_thread()

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: bp@alien8.de
Cc: Rik van Riel <riel@redhat.com>
Cc: Suresh Siddha <sbsiddha@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: the arch/x86 maintainers <x86@kernel.org>

---

Changes from v21:
 * add comments about preemption
 * rename helper to get_xsave_field_ptr()

Changes from "v19":
 * remove 'tsk' argument to get_xsave_addr() since the code
   can only realistically work on 'current', and fix up the
   comment a bit to match.

Changes from "v17":
 * fix s/xstate/xsave_field/ in the function comment
 * remove EXPORT_SYMBOL_GPL()

---

 b/arch/x86/include/asm/fpu/xstate.h |    1 +
 b/arch/x86/kernel/fpu/xstate.c      |   33 +++++++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff -puN arch/x86/include/asm/fpu/xstate.h~tsk_get_xsave_addr arch/x86/include/asm/fpu/xstate.h
--- a/arch/x86/include/asm/fpu/xstate.h~tsk_get_xsave_addr	2015-05-27 09:32:14.928463071 -0700
+++ b/arch/x86/include/asm/fpu/xstate.h	2015-05-27 09:32:14.934463342 -0700
@@ -41,5 +41,6 @@ extern u64 xstate_fx_sw_bytes[USER_XSTAT
 extern void update_regset_xstate_info(unsigned int size, u64 xstate_mask);
 
 void *get_xsave_addr(struct xregs_state *xsave, int xstate);
+void *get_xsave_field_ptr(int xstate_field);
 
 #endif
diff -puN arch/x86/kernel/fpu/xstate.c~tsk_get_xsave_addr arch/x86/kernel/fpu/xstate.c
--- a/arch/x86/kernel/fpu/xstate.c~tsk_get_xsave_addr	2015-05-27 09:32:14.930463161 -0700
+++ b/arch/x86/kernel/fpu/xstate.c	2015-05-27 09:32:14.934463342 -0700
@@ -427,3 +427,36 @@ void *get_xsave_addr(struct xregs_state
 	return (void *)xsave + xstate_comp_offsets[feature_nr];
 }
 EXPORT_SYMBOL_GPL(get_xsave_addr);
+
+/*
+ * This wraps up the common operations that need to occur when retrieving
+ * data from xsave state.  It first ensures that the current task was
+ * using the FPU and retrieves the data in to a buffer.  It then calculates
+ * the offset of the requested field in the buffer.
+ *
+ * This function is safe to call whether the FPU is in use or not.
+ *
+ * Note that this only works on the current task.
+ *
+ * Inputs:
+ *	@xsave_state: state which is defined in xsave.h (e.g. XSTATE_FP,
+ *	XSTATE_SSE, etc...)
+ * Output:
+ *	address of the state in the xsave area or NULL if the state
+ *	is not present or is in its 'init state'.
+ */
+void *get_xsave_field_ptr(int xsave_state)
+{
+	union fpregs_state *xstate;
+
+	if (!current->thread.fpu.fpstate_active)
+		return NULL;
+	/*
+	 * fpu__save() takes the CPU's xstate registers
+	 * and saves them off to the 'fpu memory buffer.
+	 */
+	fpu__save(&current->thread.fpu);
+	xstate = &current->thread.fpu.state;
+
+	return get_xsave_addr(&xstate->xsave, xsave_state);
+}
_