linux-kernel - Re: [PATCH v3 2/3] kernel/ucounts: expose count of inotify watches in use

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Message-Id: <20190531170046.ac2b52d8c4923fdeedf943cc@linux-foundation.org>
Date:   Fri, 31 May 2019 17:00:46 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Albert Vaca Cintora <albertvaka@...il.com>
Cc:     rdunlap@...radead.org, mingo@...nel.org, jack@...e.cz,
        ebiederm@...ssion.com, nsaenzjulienne@...e.de,
        linux-kernel@...r.kernel.org, corbet@....net,
        linux-doc@...r.kernel.org, mbrugger@...e.com
Subject: Re: [PATCH v3 2/3] kernel/ucounts: expose count of inotify watches
 in use

On Fri, 31 May 2019 21:50:15 +0200 Albert Vaca Cintora <albertvaka@...il.com> wrote:

> Adds a readonly 'current_inotify_watches' entry to the user sysctl table.
> The handler for this entry is a custom function that ends up calling
> proc_dointvec. Said sysctl table already contains 'max_inotify_watches'
> and it gets mounted under /proc/sys/user/.
> 
> Inotify watches are a finite resource, in a similar way to available file
> descriptors. The motivation for this patch is to be able to set up
> monitoring and alerting before an application starts failing because
> it runs out of inotify watches.
> 
> ...
>
> --- a/kernel/ucount.c
> +++ b/kernel/ucount.c
> @@ -118,6 +118,26 @@ static void put_ucounts(struct ucounts *ucounts)
>  	kfree(ucounts);
>  }
>  
> +#ifdef CONFIG_INOTIFY_USER
> +int proc_read_inotify_watches(struct ctl_table *table, int write,
> +		     void __user *buffer, size_t *lenp, loff_t *ppos)
> +{
> +	struct ucounts *ucounts;
> +	struct ctl_table fake_table;

hmm.

> +	int count = -1;
> +
> +	ucounts = get_ucounts(current_user_ns(), current_euid());
> +	if (ucounts != NULL) {
> +		count = atomic_read(&ucounts->ucount[UCOUNT_INOTIFY_WATCHES]);
> +		put_ucounts(ucounts);
> +	}
> +
> +	fake_table.data = &count;
> +	fake_table.maxlen = sizeof(count);
> +	return proc_dointvec(&fake_table, write, buffer, lenp, ppos);

proc_dointvec
->do_proc_dointvec
  ->__do_proc_dointvec
    ->proc_first_pos_non_zero_ignore
      ->warn_sysctl_write
        ->pr_warn_once(..., table->procname)

and I think ->procname is uninitialized.

That's from a cursory check.  Perhaps other uninitialized members of
fake_table are accessed, dunno.

we could do

	{
		struct ctl_table fake_table = {
			.data = &count,
			.maxlen = sizeof(count),
		};

		return proc_dointvec(&fake_table, write, buffer, lenp, ppos);
	}

or whatever.  That will cause the pr_warn_once to print "(null)" but
that's OK I guess.

Are there other places in the kernel which do this temp ctl_table
trick?  If so, what do they do?  If not, what is special about this
code?