Re: [PATCH v3] proc,fcntl: introduce F_SET_DESCRIPTION

From: Alexey Dobriyan <adobriyan@gmail.com>
To: Pascal Bouchareine <kalou@tfz.net>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-api@vger.kernel.org, Al Viro <viro@zeniv.linux.org.uk>,
	Jeff Layton <jlayton@poochiereds.net>,
	"J. Bruce Fields" <bfields@fieldses.org>
Subject: Re: [PATCH v3] proc,fcntl: introduce F_SET_DESCRIPTION
Date: Mon, 27 Jul 2020 17:21:40 +0300	[thread overview]
Message-ID: <20200727142140.GA116567@localhost.localdomain> (raw)
In-Reply-To: <20200725052236.4062-1-kalou@tfz.net>

On Fri, Jul 24, 2020 at 10:22:36PM -0700, Pascal Bouchareine wrote:
> This command attaches a description to a file descriptor for
> troubleshooting purposes. The free string is displayed in the
> process fdinfo file for that fd /proc/pid/fdinfo/fd.
> 
> One intended usage is to allow processes to self-document sockets
> for netstat and friends to report

> +static long fcntl_set_description(struct file *file, char __user *desc)
> +{
> +	char *d;
> +
> +	d = strndup_user(desc, MAX_FILE_DESC_SIZE);

This should be kmem accounted because allocation is persistent.
To make things more entertaining, strndup_user() doesn't have gfp_t argument.

> +	if (IS_ERR(d))
> +		return PTR_ERR(d);
> +
> +	spin_lock(&file->f_lock);
> +	kfree(file->f_description);
> +	file->f_description = d;
> +	spin_unlock(&file->f_lock);

Generally kfree under spinlock is not good idea.
You can replace the pointer and free without spinlock.

> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -980,6 +980,9 @@ struct file {
>  	struct address_space	*f_mapping;
>  	errseq_t		f_wb_err;
>  	errseq_t		f_sb_err; /* for syncfs */
> +
> +#define MAX_FILE_DESC_SIZE 256
> +	char                    *f_description;

struct file is nicely aligned to 256 bytes on distro configs.
Will this break everything?

	$ cat /sys/kernel/slab/filp/object_size