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Subject[PATCH 02/19] x86, fpu: Wrap get_xsave_addr() to make it safer
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From: Dave Hansen <dave.hansen@linux.intel.com>

The MPX code appears to be saving off the FPU in a potntially
unsafe way (if eagerfpu=off). 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.

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 | 32 ++++++++++++++++++++++++++++++++
2 files changed, 33 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 2015-05-28 08:49:45.191271502 -0700
+++ b/arch/x86/include/asm/fpu/xstate.h 2015-05-29 13:43:34.291184369 -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);
+const 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 2015-05-28 08:49:45.192271546 -0700
+++ b/arch/x86/kernel/fpu/xstate.c 2015-05-29 12:32:47.869662576 -0700
@@ -427,3 +427,35 @@ 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'.
+ */
+const void *get_xsave_field_ptr(int xsave_state)
+{
+ struct fpu *fpu = &current->thread.fpu;
+
+ if (!fpu->fpstate_active)
+ return NULL;
+ /*
+ * fpu__save() takes the CPU's xstate registers
+ * and saves them off to the 'fpu memory buffer.
+ */
+ fpu__save(fpu);
+
+ return get_xsave_addr(&fpu->xstate->xsave, xsave_state);
+}
_

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