[PATCH 3/7] x86/fpu: Extract restore_from_ia32_fxstate() and clean up fpu__restore_sig()

[Andrei Vagin <avagin@google.com>]

Improve readability of the signal frame restoration code. Previously,
most of __fpu_restore_sig() was dedicated to handling the 32-bit compat
fpstate, while the native direct path lived in restore_fpregs_from_user().
Having the compat handling intermixed with the main flow made it tricky
to quickly see what code was doing what.

Extract the 32-bit legacy/compat FPU restore handling into a separate
helper function, restore_from_ia32_fxstate(), and inline the remainder
of __fpu_restore_sig() directly into fpu__restore_sig().

The legacy 32-bit FP frame duplicates the FP state portion of the
FX/XSAVE frame. For backward compatibility, the legacy FP frame is
treated as the source of truth, and its state is folded into the
FX/XSAVE state before restoring the registers.

Reviewed-by: Alexander Mikhalitsyn <alexander@mihalicyn.com>
Signed-off-by: Andrei Vagin <avagin@google.com>
---
 arch/x86/kernel/fpu/signal.c | 67 +++++++++++++++++++++++-------------
 1 file changed, 43 insertions(+), 24 deletions(-)

diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index cd7db6dc819b..60063a7a44f8 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -325,32 +325,24 @@ static bool restore_fpregs_from_user(void __user *buf, u64 xrestore_mask, bool f
 	return true;
 }
 
-static bool __fpu_restore_sig(void __user *buf_f, void __user *buf_fx,
-			      bool ia32_fxstate)
+#if defined(CONFIG_X86_32) || defined(CONFIG_IA32_EMULATION)
+/*
+ * Restore FPU state from a signal frame when a legacy 32-bit FP frame
+ * (buf_f) is present.
+ *
+ * The legacy FP frame duplicates the FP state portion of the FX/XSAVE
+ * frame (buf_fx). For backward compatibility, the legacy FP frame is
+ * treated as the source of truth, and its state is folded into the
+ * FX/XSAVE state before restoring the registers.
+ */
+static bool restore_from_ia32_fxstate(void __user *buf_f, void __user *buf_fx,
+				      u64 xrestore_mask, bool fx_only)
 {
 	struct task_struct *tsk = current;
 	struct fpu *fpu = x86_task_fpu(tsk);
 	struct user_i387_ia32_struct env;
-	bool success, fx_only = false;
 	union fpregs_state *fpregs;
-	u64 xrestore_mask = 0;
-
-	if (use_xsave()) {
-		struct _fpx_sw_bytes fx_sw_user;
-
-		if (!check_xstate_in_sigframe(buf_fx, &fx_sw_user))
-			return false;
-
-		fx_only = !fx_sw_user.magic1;
-		xrestore_mask = fx_sw_user.xfeatures;
-	} else {
-		xrestore_mask = XFEATURE_MASK_FPSSE;
-	}
-
-	if (likely(!ia32_fxstate)) {
-		/* Restore the FPU registers directly from user memory. */
-		return restore_fpregs_from_user(buf_fx, xrestore_mask, fx_only);
-	}
+	bool success;
 
 	/*
 	 * Copy the legacy state because the FP portion of the FX frame has
@@ -436,6 +428,13 @@ static bool __fpu_restore_sig(void __user *buf_f, void __user *buf_fx,
 	fpregs_unlock();
 	return success;
 }
+#else
+static inline bool restore_from_ia32_fxstate(void __user *buf_f, void __user *buf_fx,
+					     u64 xrestore_mask, bool fx_only)
+{
+	return false;
+}
+#endif
 
 static inline unsigned int xstate_sigframe_size(struct fpstate *fpstate)
 {
@@ -450,10 +449,11 @@ static inline unsigned int xstate_sigframe_size(struct fpstate *fpstate)
 bool fpu__restore_sig(void __user *buf, int ia32_frame)
 {
 	struct fpu *fpu = x86_task_fpu(current);
-	void __user *buf_fx = buf;
+	bool success = false, fx_only = false;
 	bool ia32_fxstate = false;
-	bool success = false;
+	void __user *buf_fx = buf;
 	unsigned int size;
+	u64 xrestore_mask;
 
 	if (unlikely(!buf)) {
 		fpu__clear_user_states(fpu);
@@ -482,10 +482,29 @@ bool fpu__restore_sig(void __user *buf, int ia32_frame)
 		success = !fpregs_soft_set(current, NULL, 0,
 					   sizeof(struct user_i387_ia32_struct),
 					   NULL, buf);
+		goto out;
+	}
+
+	if (use_xsave()) {
+		struct _fpx_sw_bytes fx_sw_user;
+
+		if (!check_xstate_in_sigframe(buf_fx, &fx_sw_user))
+			goto out;
+
+		fx_only = !fx_sw_user.magic1;
+		xrestore_mask = fx_sw_user.xfeatures;
 	} else {
-		success = __fpu_restore_sig(buf, buf_fx, ia32_fxstate);
+		xrestore_mask = XFEATURE_MASK_FPSSE;
+	}
+
+	if (ia32_fxstate) {
+		success = restore_from_ia32_fxstate(buf, buf_fx,
+						    xrestore_mask, fx_only);
+		goto out;
 	}
 
+	/* Restore the FPU registers directly from user memory. */
+	success = restore_fpregs_from_user(buf_fx, xrestore_mask, fx_only);
 out:
 	if (unlikely(!success))
 		fpu__clear_user_states(fpu);
-- 
2.55.0.979.g7e5102b832-goog