public inbox for [email protected]
 help / color / mirror / Atom feed
From: [email protected] (Eric W. Biederman)
To: Alexey Gladkov <[email protected]>
Cc: Linus Torvalds <[email protected]>,
	LKML <[email protected]>,
	io-uring <[email protected]>,
	Kernel Hardening <[email protected]>,
	Linux Containers <[email protected]>,
	Linux-MM <[email protected]>,
	Andrew Morton <[email protected]>,
	Christian Brauner <[email protected]>,
	Jann Horn <[email protected]>, Jens Axboe <[email protected]>,
	Kees Cook <[email protected]>,
	Oleg Nesterov <[email protected]>
Subject: Re: [RFC PATCH v3 1/8] Use refcount_t for ucounts reference counting
Date: Thu, 21 Jan 2021 09:50:34 -0600	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]> (Alexey Gladkov's message of "Thu, 21 Jan 2021 13:04:27 +0100")

Alexey Gladkov <[email protected]> writes:

> On Tue, Jan 19, 2021 at 07:57:36PM -0600, Eric W. Biederman wrote:
>> Alexey Gladkov <[email protected]> writes:
>> 
>> > On Mon, Jan 18, 2021 at 12:34:29PM -0800, Linus Torvalds wrote:
>> >> On Mon, Jan 18, 2021 at 11:46 AM Alexey Gladkov
>> >> <[email protected]> wrote:
>> >> >
>> >> > Sorry about that. I thought that this code is not needed when switching
>> >> > from int to refcount_t. I was wrong.
>> >> 
>> >> Well, you _may_ be right. I personally didn't check how the return
>> >> value is used.
>> >> 
>> >> I only reacted to "it certainly _may_ be used, and there is absolutely
>> >> no comment anywhere about why it wouldn't matter".
>> >
>> > I have not found examples where checked the overflow after calling
>> > refcount_inc/refcount_add.
>> >
>> > For example in kernel/fork.c:2298 :
>> >
>> >    current->signal->nr_threads++;                           
>> >    atomic_inc(&current->signal->live);                      
>> >    refcount_inc(&current->signal->sigcnt);  
>> >
>> > $ semind search signal_struct.sigcnt
>> > def include/linux/sched/signal.h:83  		refcount_t		sigcnt;
>> > m-- kernel/fork.c:723 put_signal_struct 		if (refcount_dec_and_test(&sig->sigcnt))
>> > m-- kernel/fork.c:1571 copy_signal 		refcount_set(&sig->sigcnt, 1);
>> > m-- kernel/fork.c:2298 copy_process 				refcount_inc(&current->signal->sigcnt);
>> >
>> > It seems to me that the only way is to use __refcount_inc and then compare
>> > the old value with REFCOUNT_MAX
>> >
>> > Since I have not seen examples of such checks, I thought that this is
>> > acceptable. Sorry once again. I have not tried to hide these changes.
>> 
>> The current ucount code does check for overflow and fails the increment
>> in every case.
>> 
>> So arguably it will be a regression and inferior error handling behavior
>> if the code switches to the ``better'' refcount_t data structure.
>> 
>> I originally didn't use refcount_t because silently saturating and not
>> bothering to handle the error makes me uncomfortable.
>> 
>> Not having to acquire the ucounts_lock every time seems nice.  Perhaps
>> the path forward would be to start with stupid/correct code that always
>> takes the ucounts_lock for every increment of ucounts->count, that is
>> later replaced with something more optimal.
>> 
>> Not impacting performance in the non-namespace cases and having good
>> performance in the other cases is a fundamental requirement of merging
>> code like this.
>
> Did I understand your suggestion correctly that you suggest to use
> spin_lock for atomic_read and atomic_inc ?
>
> If so, then we are already incrementing the counter under ucounts_lock.
>
> 	...
> 	if (atomic_read(&ucounts->count) == INT_MAX)
> 		ucounts = NULL;
> 	else
> 		atomic_inc(&ucounts->count);
> 	spin_unlock_irq(&ucounts_lock);
> 	return ucounts;
>
> something like this ?

Yes.  But without atomics.  Something a bit more like:
> 	...
> 	if (ucounts->count == INT_MAX)
> 		ucounts = NULL;
> 	else
> 		ucounts->count++;
> 	spin_unlock_irq(&ucounts_lock);
> 	return ucounts;

I do believe at some point we will want to say using the spin_lock for
ucounts->count is cumbersome, and suboptimal and we want to change it to
get a better performing implementation.

Just for getting the semantics correct we should be able to use just
ucounts_lock for locking.  Then when everything is working we can
profile and optimize the code.

I just don't want figuring out what is needed to get hung up over little
details that we can change later.

Eric


  reply	other threads:[~2021-01-21 15:54 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-15 14:57 [RFC PATCH v3 0/8] Count rlimits in each user namespace Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 1/8] Use refcount_t for ucounts reference counting Alexey Gladkov
2021-01-18 19:14   ` Linus Torvalds
2021-01-18 19:45     ` Alexey Gladkov
2021-01-18 20:34       ` Linus Torvalds
2021-01-18 20:56         ` Alexey Gladkov
2021-01-19  4:35           ` Kaiwan N Billimoria
2021-01-20  1:57           ` Eric W. Biederman
2021-01-20  1:58             ` Eric W. Biederman
2021-01-21 12:04             ` Alexey Gladkov
2021-01-21 15:50               ` Eric W. Biederman [this message]
2021-01-21 16:07                 ` Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 2/8] Add a reference to ucounts for each cred Alexey Gladkov
2021-01-18  8:31   ` [PATCH v4 " Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 3/8] Move RLIMIT_NPROC counter to ucounts Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 4/8] Move RLIMIT_MSGQUEUE " Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 5/8] Move RLIMIT_SIGPENDING " Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 6/8] Move RLIMIT_MEMLOCK " Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 7/8] Move RLIMIT_NPROC check to the place where we increment the counter Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 8/8] kselftests: Add test to check for rlimit changes in different user namespaces Alexey Gladkov

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox