x86/fpu/regset: Move fpu__read_begin() into regset

The function can only be used from the regset get() callbacks safely. So
there is no reason to have it globally exposed.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Signed-off-by: NBorislav Petkov <bp@suse.de>
Reviewed-by: NBorislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210623121453.234942936@linutronix.de

arch/x86/include/asm/fpu/internal.h

@@ -26,7 +26,6 @@
 /*
  * High level FPU state handling functions:
  */
-extern void fpu__prepare_read(struct fpu *fpu);
 extern void fpu__prepare_write(struct fpu *fpu);
 extern void fpu__save(struct fpu *fpu);
 extern int  fpu__restore_sig(void __user *buf, int ia32_frame);

arch/x86/kernel/fpu/core.c

@@ -281,26 +281,6 @@ static void fpu__initialize(struct fpu *fpu)
 	trace_x86_fpu_init_state(fpu);
 }
 
-/*
- * This function must be called before we read a task's fpstate.
- *
- * There's two cases where this gets called:
- *
- * - for the current task (when coredumping), in which case we have
- *   to save the latest FPU registers into the fpstate,
- *
- * - or it's called for stopped tasks (ptrace), in which case the
- *   registers were already saved by the context-switch code when
- *   the task scheduled out.
- *
- * If the task has used the FPU before then save it.
- */
-void fpu__prepare_read(struct fpu *fpu)
-{
-	if (fpu == &current->thread.fpu)
-		fpu__save(fpu);
-}
-
 /*
  * This function must be called before we write a task's fpstate.
  *

arch/x86/kernel/fpu/regset.c

@@ -28,6 +28,22 @@ int regset_xregset_fpregs_active(struct task_struct *target, const struct user_r
 		return 0;
 }
 
+/*
+ * The regset get() functions are invoked from:
+ *
+ *   - coredump to dump the current task's fpstate. If the current task
+ *     owns the FPU then the memory state has to be synchronized and the
+ *     FPU register state preserved. Otherwise fpstate is already in sync.
+ *
+ *   - ptrace to dump fpstate of a stopped task, in which case the registers
+ *     have already been saved to fpstate on context switch.
+ */
+static void sync_fpstate(struct fpu *fpu)
+{
+	if (fpu == &current->thread.fpu)
+		fpu__save(fpu);
+}
+
 int xfpregs_get(struct task_struct *target, const struct user_regset *regset,
 		struct membuf to)
 {
@@ -36,7 +52,7 @@ int xfpregs_get(struct task_struct *target, const struct user_regset *regset,
 	if (!cpu_feature_enabled(X86_FEATURE_FXSR))
 		return -ENODEV;
 
-	fpu__prepare_read(fpu);
+	sync_fpstate(fpu);
 
 	if (!use_xsave()) {
 		return membuf_write(&to, &fpu->state.fxsave,
@@ -96,7 +112,7 @@ int xstateregs_get(struct task_struct *target, const struct user_regset *regset,
 	if (!cpu_feature_enabled(X86_FEATURE_XSAVE))
 		return -ENODEV;
 
-	fpu__prepare_read(fpu);
+	sync_fpstate(fpu);
 
 	copy_xstate_to_uabi_buf(to, &fpu->state.xsave, XSTATE_COPY_XSAVE);
 	return 0;
@@ -287,7 +303,7 @@ int fpregs_get(struct task_struct *target, const struct user_regset *regset,
 	struct user_i387_ia32_struct env;
 	struct fxregs_state fxsave, *fx;
 
-	fpu__prepare_read(fpu);
+	sync_fpstate(fpu);
 
 	if (!cpu_feature_enabled(X86_FEATURE_FPU))
 		return fpregs_soft_get(target, regset, to);