linux-kernel - Re: [PATCH v2] ftrace: Introduce PERMANENT ftrace

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Message-Id: <a39036e1-5235-9b5a-f847-12878538781e@linux.vnet.ibm.com>
Date:   Wed, 16 Oct 2019 10:32:20 +0530
From:   Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>
To:     Miroslav Benes <mbenes@...e.cz>, rostedt@...dmis.org,
        mingo@...hat.com, jpoimboe@...hat.com, jikos@...nel.org,
        pmladek@...e.com, joe.lawrence@...hat.com
Cc:     linux-kernel@...r.kernel.org, live-patching@...r.kernel.org
Subject: Re: [PATCH v2] ftrace: Introduce PERMANENT ftrace_ops flag

On 10/14/19 4:29 PM, Miroslav Benes wrote:
> Livepatch uses ftrace for redirection to new patched functions. It means
> that if ftrace is disabled, all live patched functions are disabled as
> well. Toggling global 'ftrace_enabled' sysctl thus affect it directly.
> It is not a problem per se, because only administrator can set sysctl
> values, but it still may be surprising.
> 
> Introduce PERMANENT ftrace_ops flag to amend this. If the
> FTRACE_OPS_FL_PERMANENT is set on any ftrace ops, the tracing cannot be
> disabled by disabling ftrace_enabled. Equally, a callback with the flag
> set cannot be registered if ftrace_enabled is disabled.
> 
> Signed-off-by: Miroslav Benes <mbenes@...e.cz>

The patch looks good to me. A minor typo in flag description below.

Reviewed-by: Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>

[...]
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index 8a8cb3c401b2..c2cad29dc557 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -142,6 +142,8 @@ ftrace_func_t ftrace_ops_get_func(struct ftrace_ops *ops);
>   * PID     - Is affected by set_ftrace_pid (allows filtering on those pids)
>   * RCU     - Set when the ops can only be called when RCU is watching.
>   * TRACE_ARRAY - The ops->private points to a trace_array descriptor.
> + * PERMAMENT - Set when the ops is permanent and should not be affected by
> + *             ftrace_enabled.
>   */

s/PERMAMENT/PERMANENT

> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 62a50bf399d6..d2992ea29fe1 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -6752,12 +6764,19 @@ ftrace_enable_sysctl(struct ctl_table *table, int write,
>  		ftrace_startup_sysctl();
> 
>  	} else {
> +		if (is_permanent_ops_registered()) {
> +			ftrace_enabled = last_ftrace_enabled;
> +			ret = -EBUSY;
> +			goto out;
> +		}
> +
>  		/* stopping ftrace calls (just send to ftrace_stub) */
>  		ftrace_trace_function = ftrace_stub;
> 
>  		ftrace_shutdown_sysctl();
>  	}
> 
> +	last_ftrace_enabled = !!ftrace_enabled;

No strong feelings on last_ftrace_enabled placement, leaving it to
your preference. 

>   out:
>  	mutex_unlock(&ftrace_lock);
>  	return ret;
> 


-- 
Kamalesh