x86/ftrace: Do not jump to direct code in created trampolines

When creating a trampoline based on the ftrace_regs_caller code, nop out the
jnz test that would jmup to the code that would return to a direct caller
(stored in the ORIG_RAX field) and not back to the function that called it.

Link: http://lkml.kernel.org/r/20200422162750.638839749@goodmis.org

Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

arch/x86/kernel/ftrace.c

@@ -286,6 +286,7 @@ extern void ftrace_regs_caller_ret(void);
 extern void ftrace_caller_end(void);
 extern void ftrace_caller_op_ptr(void);
 extern void ftrace_regs_caller_op_ptr(void);
+extern void ftrace_regs_caller_jmp(void);
 
 /* movq function_trace_op(%rip), %rdx */
 /* 0x48 0x8b 0x15 <offset-to-ftrace_trace_op (4 bytes)> */
@@ -316,6 +317,7 @@ create_trampoline(struct ftrace_ops *ops, unsigned int *tramp_size)
 	unsigned long end_offset;
 	unsigned long op_offset;
 	unsigned long call_offset;
+	unsigned long jmp_offset;
 	unsigned long offset;
 	unsigned long npages;
 	unsigned long size;
@@ -333,11 +335,13 @@ create_trampoline(struct ftrace_ops *ops, unsigned int *tramp_size)
 		end_offset = (unsigned long)ftrace_regs_caller_end;
 		op_offset = (unsigned long)ftrace_regs_caller_op_ptr;
 		call_offset = (unsigned long)ftrace_regs_call;
+		jmp_offset = (unsigned long)ftrace_regs_caller_jmp;
 	} else {
 		start_offset = (unsigned long)ftrace_caller;
 		end_offset = (unsigned long)ftrace_caller_end;
 		op_offset = (unsigned long)ftrace_caller_op_ptr;
 		call_offset = (unsigned long)ftrace_call;
+		jmp_offset = 0;
 	}
 
 	size = end_offset - start_offset;
@@ -367,6 +371,17 @@ create_trampoline(struct ftrace_ops *ops, unsigned int *tramp_size)
 	if (WARN_ON(ret < 0))
 		goto fail;
 
+	/* No need to test direct calls on created trampolines */
+	if (ops->flags & FTRACE_OPS_FL_SAVE_REGS) {
+		/* NOP the jnz 1f; but make sure it's a 2 byte jnz */
+		ip = trampoline + (jmp_offset - start_offset);
+		if (WARN_ON(*(char *)ip != 0x75))
+			goto fail;
+		ret = copy_from_kernel_nofault(ip, ideal_nops[2], 2);
+		if (ret < 0)
+			goto fail;
+	}
+
 	/*
 	 * The address of the ftrace_ops that is used for this trampoline
 	 * is stored at the end of the trampoline. This will be used to

arch/x86/kernel/ftrace_64.S

@@ -241,6 +241,7 @@ SYM_INNER_LABEL(ftrace_regs_call, SYM_L_GLOBAL)
 	 */
 	movq ORIG_RAX(%rsp), %rax
 	testq	%rax, %rax
+SYM_INNER_LABEL(ftrace_regs_caller_jmp, SYM_L_GLOBAL)
 	jnz	1f
 
 	restore_mcount_regs