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From: Rasmus Villemoes <linux@rasmusvillemoes.dk>
To: Peter Zijlstra <peterz@infradead.org>, Greg KH <greg@kroah.com>
Cc: mingo@kernel.org, mgorman@suse.de, juri.lelli@redhat.com,
	vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
	rostedt@goodmis.org, bsegall@google.com, bristot@redhat.com,
	joshdon@google.com, valentin.schneider@arm.com,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 6/9] debugfs: Implement debugfs_create_str()
Date: Fri, 26 Mar 2021 13:53:59 +0100	[thread overview]
Message-ID: <70594935-18e6-0791-52f9-0448adf37155@rasmusvillemoes.dk> (raw)
In-Reply-To: <YF3Hv5zXb/6lauzs@hirez.programming.kicks-ass.net>

On 26/03/2021 12.38, Peter Zijlstra wrote:

> Implement debugfs_create_str() to easily display names and such in
> debugfs.

Patches 7-9 don't seem to add any users of this. What's it for precisely?

> +
> +again:
> +	rcu_read_lock();
> +	str = rcu_dereference(*(char **)file->private_data);
> +	len = strlen(str) + 1;
> +
> +	if (!copy || copy_len < len) {
> +		rcu_read_unlock();
> +		kfree(copy);
> +		copy = kmalloc(len + 1, GFP_KERNEL);
> +		if (!copy) {
> +			debugfs_file_put(dentry);
> +			return -ENOMEM;
> +		}
> +		copy_len = len;
> +		goto again;
> +	}
> +
> +	strncpy(copy, str, len);
> +	copy[len] = '\n';
> +	copy[len+1] = '\0';
> +	rcu_read_unlock();

As noted (accidentally off-list), this is broken. I think you want this
on top

- len = strlen(str) + 1;
+ len = strlen(str);

- strncpy(copy, str, len);
+ memcpy(copy, str, len);
  copy[len] = '\n';
- copy[len+1] = '\0';

> +EXPORT_SYMBOL_GPL(debugfs_read_file_str);

Why?

> +
> +ssize_t debugfs_write_file_str(struct file *file, const char __user *user_buf,
> +			       size_t count, loff_t *ppos)
> +{
> +	struct dentry *dentry = F_DENTRY(file);
> +	char *old, *new = NULL;
> +	int pos = *ppos;
> +	int r;
> +
> +	r = debugfs_file_get(dentry);
> +	if (unlikely(r))
> +		return r;
> +
> +	old = *(char **)file->private_data;
> +
> +	/* only allow strict concattenation */
> +	r = -EINVAL;
> +	if (pos && pos != strlen(old))
> +		goto error;

Other than the synchronize_rcu() below, I don't see any rcu stuff in
here. What prevents one task from kfree'ing old while another computes
its strlen()? Or two tasks from getting the same value of old and both
kfree'ing the same pointer?

And what part of the kernel periodically looks at some string and
decides something needs to be done? Shouldn't a write imply some
notification or callback? I can see the usefulness of the read part to
expose some otherwise-maintained string, but what good does allowing
writes do?

Rasmus

  parent reply	other threads:[~2021-03-26 12:55 UTC|newest]

Thread overview: 41+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-26 10:33 [PATCH 0/9] sched: Clean up SCHED_DEBUG Peter Zijlstra
2021-03-26 10:33 ` [PATCH 1/9] sched/numa: Allow runtime enabling/disabling of NUMA balance without SCHED_DEBUG Peter Zijlstra
2021-03-26 10:33 ` [PATCH 2/9] sched: Remove sched_schedstats sysctl out from under SCHED_DEBUG Peter Zijlstra
2021-03-26 10:33 ` [PATCH 3/9] sched: Dont make LATENCYTOP select SCHED_DEBUG Peter Zijlstra
2021-03-26 10:33 ` [PATCH 4/9] sched: Move SCHED_DEBUG to debugfs Peter Zijlstra
2021-03-26 11:06   ` Greg KH
2021-04-07 10:46   ` Valentin Schneider
2021-04-07 12:26     ` Peter Zijlstra
2021-04-07 12:57       ` Valentin Schneider
2021-03-26 10:33 ` [PATCH 5/9] sched,preempt: Move preempt_dynamic to debug.c Peter Zijlstra
2021-03-26 10:33 ` [PATCH 6/9] debugfs: Implement debugfs_create_str() Peter Zijlstra
2021-03-26 11:05   ` Greg KH
2021-03-26 11:18     ` Peter Zijlstra
2021-03-26 11:30       ` Greg KH
2021-03-26 11:38         ` [PATCH v2 " Peter Zijlstra
2021-03-26 12:18           ` Greg KH
2021-03-26 12:53           ` Rasmus Villemoes [this message]
2021-03-26 12:57             ` Greg KH
2021-03-26 13:10               ` Rasmus Villemoes
2021-03-26 14:12                 ` Peter Zijlstra
2021-03-26 14:19                   ` Greg KH
2021-03-26 14:22             ` Peter Zijlstra
2021-03-26 14:58               ` Rasmus Villemoes
2021-03-26 15:19                 ` Peter Zijlstra
2021-03-27 10:41                   ` Greg KH
2021-03-26 14:50   ` [PATCH v3 " Peter Zijlstra
2021-03-27 10:42     ` Greg KH
2021-03-27 22:24   ` [PATCH " Al Viro
2021-03-28  0:33     ` Steven Rostedt
2021-03-26 10:33 ` [PATCH 7/9] sched,debug: Convert sysctl sched_domains to debugfs Peter Zijlstra
2021-03-26 13:11   ` Dietmar Eggemann
2021-04-07 10:46   ` Valentin Schneider
2021-04-07 12:18     ` Peter Zijlstra
2021-03-26 10:34 ` [PATCH 8/9] sched: Move /proc/sched_debug " Peter Zijlstra
2021-03-26 11:05   ` Greg KH
2021-03-26 10:34 ` [PATCH 9/9] sched,fair: Alternative sched_slice() Peter Zijlstra
2021-03-26 12:08   ` Dietmar Eggemann
2021-03-26 14:07     ` Peter Zijlstra
2021-03-26 15:37   ` Vincent Guittot
2021-03-26 18:30     ` Peter Zijlstra
2021-04-12 10:14 [PATCH v2 0/9] sched: Clean up SCHED_DEBUG Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 6/9] debugfs: Implement debugfs_create_str() Peter Zijlstra

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