linux-kernel - Re: [PATCH 2/6] riscv/ftrace: Add dynamic function tracer support

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Date:   Wed, 10 Jan 2018 11:36:43 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Alan Kao <nonerkao@...il.com>
Cc:     Palmer Dabbelt <palmer@...ive.com>, Albert Ou <albert@...ive.com>,
        Ingo Molnar <mingo@...hat.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Kamil Rytarowski <n54@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        patches@...ups.riscv.org, linux-kernel@...r.kernel.org,
        Alan Kao <alankao@...estech.com>,
        Greentime Hu <greentime@...estech.com>
Subject: Re: [PATCH 2/6] riscv/ftrace: Add dynamic function tracer support

On Wed, 10 Jan 2018 15:38:10 +0800
Alan Kao <nonerkao@...il.com> wrote:

> +static int __ftrace_modify_call(unsigned long hook_pos, unsigned long target,
> +				bool enable)
> +{
> +	unsigned int offset = (unsigned int)(target - hook_pos);
> +	unsigned int auipc_call = to_auipc_insn(offset);
> +	unsigned int jalr_call = to_jalr_insn(offset);
> +	unsigned int calls[2] = {auipc_call, jalr_call};
> +	unsigned int nops[2] = {NOP4, NOP4};
> +	int ret = 0;
> +
> +	/* when ftrace_make_nop is called */
> +	if (!enable)
> +		ret = ftrace_check_current_call(hook_pos, calls);
> +
> +	if (ret)
> +		goto out;
> +
> +	/* replace the auipc-jalr pair at once */
> +	ret = probe_kernel_write((void *)hook_pos, enable ? calls : nops,
> +				 MCOUNT_INSN_SIZE);
> +	if (ret)

You don't want to return the return of probe_kernel_write() here. For
ftrace bug to work properly, if a fail to write occurs, you need to
return -EPERM.

-- Steve

> +		goto out;
> +
> +	smp_mb();
> +	flush_icache_range((void *)hook_pos, (void *)hook_pos + MCOUNT_INSN_SIZE);
> +
> +out:
> +	return ret;
> +}
> +
> +int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
> +{
> +	int ret = ftrace_check_current_call(rec->ip, NULL);
> +
> +	if (ret)
> +		return ret;
> +
> +	return __ftrace_modify_call(rec->ip, addr, true);
> +}
> +
> +int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
> +		    unsigned long addr)
> +{
> +	return __ftrace_modify_call(rec->ip, addr, false);
> +}
> +
> +int ftrace_update_ftrace_func(ftrace_func_t func)
> +{
> +	int ret = __ftrace_modify_call((unsigned long)&ftrace_call,
> +				       (unsigned long)func, true);
> +	if (!ret) {
> +		ret = __ftrace_modify_call((unsigned long)&ftrace_regs_call,
> +					   (unsigned long)func, true);
> +	}
> +
> +	return ret;
> +}
> +
> +int __init ftrace_dyn_arch_init(void)
> +{
> +	return 0;
> +}
> +#endif