static_call: Add some validation

Verify the text we're about to change is as we expect it to be.
Requested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200818135805.161974981@infradead.org

arch/x86/kernel/static_call.c

@@ -41,6 +41,26 @@ static void __static_call_transform(void *insn, enum insn_type type, void *func)
 	text_poke_bp(insn, code, size, NULL);
 }
 
+static void __static_call_validate(void *insn, bool tail)
+{
+	u8 opcode = *(u8 *)insn;
+
+	if (tail) {
+		if (opcode == JMP32_INSN_OPCODE ||
+		    opcode == RET_INSN_OPCODE)
+			return;
+	} else {
+		if (opcode == CALL_INSN_OPCODE ||
+		    !memcmp(insn, ideal_nops[NOP_ATOMIC5], 5))
+			return;
+	}
+
+	/*
+	 * If we ever trigger this, our text is corrupt, we'll probably not live long.
+	 */
+	WARN_ONCE(1, "unexpected static_call insn opcode 0x%x at %pS\n", opcode, insn);
+}
+
 static inline enum insn_type __sc_insn(bool null, bool tail)
 {
 	/*
@@ -60,11 +80,15 @@ void arch_static_call_transform(void *site, void *tramp, void *func, bool tail)
 {
 	mutex_lock(&text_mutex);
 
-	if (tramp)
+	if (tramp) {
+		__static_call_validate(tramp, true);
 		__static_call_transform(tramp, __sc_insn(!func, true), func);
+	}
 
-	if (IS_ENABLED(CONFIG_HAVE_STATIC_CALL_INLINE) && site)
+	if (IS_ENABLED(CONFIG_HAVE_STATIC_CALL_INLINE) && site) {
+		__static_call_validate(site, tail);
 		__static_call_transform(site, __sc_insn(!func, tail), func);
+	}
 
 	mutex_unlock(&text_mutex);
 }