linux-kernel - Re: [PATCH 2/5] x86/intel_rdt: Add diagnostics when writing the schemata file

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Message-ID: <alpine.DEB.2.20.1709251555080.15252@nanos>
Date:   Mon, 25 Sep 2017 16:04:07 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Luck, Tony" <tony.luck@...el.com>
cc:     linux-kernel@...r.kernel.org, Boris Petkov <bp@...e.de>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Vikas Shivappa <vikas.shivappa@...el.com>
Subject: Re: [PATCH 2/5] x86/intel_rdt: Add diagnostics when writing the
 schemata file

On Mon, 18 Sep 2017, Luck, Tony wrote:
> @@ -208,14 +241,19 @@ ssize_t rdtgroup_schemata_write(struct kernfs_open_file *of,
>  	char *tok, *resname;
>  	int closid, ret = 0;
>  
> +	seq_buf_clear(&last_cmd_status);
> +
>  	/* Valid input requires a trailing newline */
> -	if (nbytes == 0 || buf[nbytes - 1] != '\n')
> +	if (nbytes == 0 || buf[nbytes - 1] != '\n') {
> +		seq_buf_puts(&last_cmd_status, "no trailing newline\n");
>  		return -EINVAL;
> +	}
>  	buf[nbytes - 1] = '\0';

In all other instances you access last_cmd_status within the
rdtgroup_kn_lock_live() protected section, which also serializes the show()
function via rdtgroup_mutex. Here you do it outside for obvious reasons,
but that opens a can of evil worms ...

Can you please provide and use two helpers - last_cmd_buf_clear() and
last_cmd_buf_puts() - which both have a
lockdep_assert_held(&rdtgroup_mutex) inside to make sure that we don't end
up with unprotected access accidentally?

Thanks,

	tglx