Re: [PATCH 3/3] x86/ftrace: Do not jump to direct code in created trampolines

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Apr 22, 2020 at 12:25:42PM -0400, Steven Rostedt wrote:

> @@ -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 = probe_kernel_read(ip, ideal_nops[2], 2);

Now you're just being silly, are you really, actually worried you can't
read ideal_nops[] ?

> +		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
> diff --git a/arch/x86/kernel/ftrace_64.S b/arch/x86/kernel/ftrace_64.S
> index 0882758d165a..f72ef157feb3 100644
> --- a/arch/x86/kernel/ftrace_64.S
> +++ b/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
>  

I you worry about performance, it would make more sense to do something
like so:

SYM_INNER_LABEL(ftrace_regs_caller_from, SYM_L_GLOBAL)
	movq ORIG_RAX(%rsp), %rax
	testq	%rax, %rax
	jnz	1f
SYM_INNER_LABEL(ftrace_regs_caller_to, SYM_L_GLOBAL)

	if (ops->flags & FTRACE_OPS_FL_SAVE_REGS) {
		ip = trampoline + (ftrace_regs_caller_from - start_offset);
		((u8[])ip)[0] = JMP8_INSN_OPCODE;
		((u8[])ip)[1] = ftrace_regs_caller_to - ftrace_regs_caller_from - JMP8_INSN_SIZE;
	}

Or nop the whole range, but it's like 10 bytes so I'm not sure that's
actually faster.