linux-kernel - Re: [PATCH 4.19 03/89] cgroup, netclassid: periodically release file

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Date:   Wed, 18 Mar 2020 13:01:58 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     Pavel Machek <pavel@...x.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Dmitry Yakunin <zeil@...dex-team.ru>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 4.19 03/89] cgroup, netclassid: periodically release
 file_lock on classid updating

On 18/03/2020 12.02, Pavel Machek wrote:
> Hi!
> 
>> From: Dmitry Yakunin <zeil@...dex-team.ru>
>>
>> [ Upstream commit 018d26fcd12a75fb9b5fe233762aa3f2f0854b88 ]
> ...
>> Now update is non atomic and socket may be skipped using calls:
>>
>> dup2(oldfd, newfd);
>> close(oldfd);
>>
>> But this case is not typical. Moreover before this patch skip is possible
>> too by hiding socket fd in unix socket buffer.
> 
> Dunno. This makes interface even more interesting.

This is part of brilliant cgroup-v1 design. =)
Cgroup-v2 doesn't recolor sockets when task is moved.

> 
>> +
>>   static int update_classid_sock(const void *v, struct file *file, unsigned n)
>>   {
>>   	int err;
>> +	struct update_classid_context *ctx = (void *)v;
>>   	struct socket *sock = sock_from_file(file, &err);
>>
> ...
>> +	if (--ctx->batch == 0) {
>> +		ctx->batch = UPDATE_CLASSID_BATCH;
>> +		return n + 1;
>> +	}
>>   	return 0;
>>   }
> 
> We take "const void *" and then write to it. That's asking for
> trouble... right? Should the const annotation be removed, at least for
> sake of humans trying to understand the code?

Indeed, there is no much sense in opaque const void *.
This is how iterate_fd() is declared.

> 
> Best regards,
> 									Pavel
>