x86/fpu: Rename and sanitize fpu__save/copy()

Both function names are a misnomer.

fpu__save() is actually about synchronizing the hardware register state
into the task's memory state so that either coredump or a math exception
handler can inspect the state at the time where the problem happens.

The function guarantees to preserve the register state, while "save" is a
common terminology for saving the current state so it can be modified and
restored later. This is clearly not the case here.

Rename it to fpu_sync_fpstate().

fpu__copy() is used to clone the current task's FPU state when duplicating
task_struct. While the register state is a copy the rest of the FPU state
is not.

Name it accordingly and remove the really pointless @src argument along
with the warning which comes along with it.

Nothing can ever copy the FPU state of a non-current task. It's clearly
just a consequence of arch_dup_task_struct(), but it makes no sense to
proliferate that further.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210623121455.196727450@linutronix.de

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

@@ -26,14 +26,16 @@
 /*
  * High level FPU state handling functions:
  */
-extern void fpu__save(struct fpu *fpu);
 extern int  fpu__restore_sig(void __user *buf, int ia32_frame);
 extern void fpu__drop(struct fpu *fpu);
-extern int  fpu__copy(struct task_struct *dst, struct task_struct *src);
 extern void fpu__clear_user_states(struct fpu *fpu);
 extern void fpu__clear_all(struct fpu *fpu);
 extern int  fpu__exception_code(struct fpu *fpu, int trap_nr);
 
+extern void fpu_sync_fpstate(struct fpu *fpu);
+
+extern int  fpu_clone(struct task_struct *dst);
+
 /*
  * Boot time FPU initialization functions:
  */

arch/x86/kernel/fpu/core.c

@@ -159,11 +159,10 @@ void kernel_fpu_end(void)
 EXPORT_SYMBOL_GPL(kernel_fpu_end);
 
 /*
- * Save the FPU state (mark it for reload if necessary):
- *
- * This only ever gets called for the current task.
+ * Sync the FPU register state to current's memory register state when the
+ * current task owns the FPU. The hardware register state is preserved.
  */
-void fpu__save(struct fpu *fpu)
+void fpu_sync_fpstate(struct fpu *fpu)
 {
 	WARN_ON_FPU(fpu != &current->thread.fpu);
 
@@ -221,18 +220,18 @@ void fpstate_init(union fpregs_state *state)
 }
 EXPORT_SYMBOL_GPL(fpstate_init);
 
-int fpu__copy(struct task_struct *dst, struct task_struct *src)
+/* Clone current's FPU state on fork */
+int fpu_clone(struct task_struct *dst)
 {
+	struct fpu *src_fpu = &current->thread.fpu;
 	struct fpu *dst_fpu = &dst->thread.fpu;
-	struct fpu *src_fpu = &src->thread.fpu;
 
+	/* The new task's FPU state cannot be valid in the hardware. */
 	dst_fpu->last_cpu = -1;
 
-	if (!static_cpu_has(X86_FEATURE_FPU))
+	if (!cpu_feature_enabled(X86_FEATURE_FPU))
 		return 0;
 
-	WARN_ON_FPU(src_fpu != &current->thread.fpu);
-
 	/*
 	 * Don't let 'init optimized' areas of the XSAVE area
 	 * leak into the child task:

arch/x86/kernel/fpu/regset.c

@@ -41,7 +41,7 @@ int regset_xregset_fpregs_active(struct task_struct *target, const struct user_r
 static void sync_fpstate(struct fpu *fpu)
 {
 	if (fpu == &current->thread.fpu)
-		fpu__save(fpu);
+		fpu_sync_fpstate(fpu);
 }
 
 /*

arch/x86/kernel/process.c

@@ -87,8 +87,7 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
 #ifdef CONFIG_VM86
 	dst->thread.vm86 = NULL;
 #endif
-
-	return fpu__copy(dst, src);
+	return fpu_clone(dst);
 }
 
 /*

arch/x86/kernel/traps.c

@@ -1046,9 +1046,10 @@ static void math_error(struct pt_regs *regs, int trapnr)
 	}
 
 	/*
-	 * Save the info for the exception handler and clear the error.
+	 * Synchronize the FPU register state to the memory register state
+	 * if necessary. This allows the exception handler to inspect it.
 	 */
-	fpu__save(fpu);
+	fpu_sync_fpstate(fpu);
 
 	task->thread.trap_nr	= trapnr;
 	task->thread.error_code = 0;