linux-kernel - Re: [RFC PATCH 1/2] tracing/user_events: Use remote writes for event enablement

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Message-Id: <20221031234703.e275650899728e64cd3fb6c3@kernel.org>
Date:   Mon, 31 Oct 2022 23:47:03 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Beau Belgrave <beaub@...ux.microsoft.com>
Cc:     rostedt@...dmis.org, mathieu.desnoyers@...icios.com,
        dcook@...ux.microsoft.com, alanau@...ux.microsoft.com,
        linux-trace-devel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] tracing/user_events: Use remote writes for
 event enablement

Hi,

I have some comments.

On Thu, 27 Oct 2022 15:40:10 -0700
Beau Belgrave <beaub@...ux.microsoft.com> wrote:

[...]
> @@ -1570,11 +1610,12 @@ static long user_reg_get(struct user_reg __user *ureg, struct user_reg *kreg)
>   * Registers a user_event on behalf of a user process.
>   */
>  static long user_events_ioctl_reg(struct user_event_file_info *info,
> -				  unsigned long uarg)
> +				  struct file *file, unsigned long uarg)
>  {
>  	struct user_reg __user *ureg = (struct user_reg __user *)uarg;
>  	struct user_reg reg;
>  	struct user_event *user;
> +	struct user_event_enabler *enabler;
>  	char *name;
>  	long ret;
>  
> @@ -1607,8 +1648,12 @@ static long user_events_ioctl_reg(struct user_event_file_info *info,
>  	if (ret < 0)
>  		return ret;
>  
> +	enabler = user_event_enabler_create(file, &reg, user);
> +
> +	if (!enabler)

Shouldn't we free the user_event if needed here?
(I found the similar memory leak pattern in the above failure case
 of the user_events_ref_add().)

> +		return -ENOMEM;
> +
>  	put_user((u32)ret, &ureg->write_index);
> -	put_user(user->index, &ureg->status_bit);
>  
>  	return 0;
>  }
[...]
> @@ -1849,7 +1863,6 @@ static int user_status_open(struct inode *node, struct file *file)
>  
>  static const struct file_operations user_status_fops = {
>  	.open = user_status_open,
> -	.mmap = user_status_mmap,

So, if this drops the mmap operation, can we drop the writable flag from
the status tracefs file?

static int create_user_tracefs(void)
{
[...]
        /* mmap with MAP_SHARED requires writable fd */
        emmap = tracefs_create_file("user_events_status", TRACE_MODE_WRITE,
                                    NULL, NULL, &user_status_fops);

Thank you,

-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>