linux-kernel - Re: [PATCH] mm: Make failslab writable again

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Date:   Tue, 20 Sep 2022 11:29:40 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Alexander Atanasov <alexander.atanasov@...tuozzo.com>,
        Jonathan Corbet <corbet@....net>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     kernel@...nvz.org, Kees Cook <keescook@...omium.org>,
        Roman Gushchin <guro@...com>, Jann Horn <jannh@...gle.com>,
        Vijayanand Jitta <vjitta@...eaurora.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] mm: Make failslab writable again

On 9/20/22 11:17, Alexander Atanasov wrote:
> Hello,
> 
> On 20.09.22 11:42, Vlastimil Babka wrote:
>> On 9/20/22 10:20, Alexander Atanasov wrote:
>>> In (060807f841ac mm, slub: make remaining slub_debug related attributes
>>> read-only failslab) it was made RO.
>>
>> "read-only) failslab was made RO" ?
> 
> Yep.
> 
>>> I think it became a collateral victim to the other two options
>>> (sanity_checks and trace) for which the reasons are perfectly valid.
>>
>> The commit also mentioned that modifying the flags is not protected in any
>> way, see below.
> 
> Yes, indeed.
> 
>>> +static ssize_t failslab_store(struct kmem_cache *s, const char *buf,
>>> +                size_t length)
>>> +{
>>> +    if (s->refcount > 1)
>>> +        return -EINVAL;
>>> +
>>> +    s->flags &= ~SLAB_FAILSLAB;
>>> +    if (buf[0] == '1')
>>> +        s->flags |= SLAB_FAILSLAB;
>>
>> Could we at least use a temporary variable to set up the final value and
>> then do a WRITE_ONCE() to s->flags, so the compiler is not allowed to do
>> some funky stuff? Assuming this is really the only place where we modify
>> s->flags during runtime, so we can't miss other updates due to RMW.
> 
> Since it is set or clear - instead of temporary variable and potentially two
> writes and RMW issues i would suggest this:
> +    if (buf[0] == '1')
> +        s->flags |= SLAB_FAILSLAB;
> +       else
> +        s->flags &= ~SLAB_FAILSLAB;

This way also has RMW issues, and also the compiler is allowed to
temporarily modify s->flags any way it likes; with WRITE_ONCE() it can't.

> If at some point more places need to modify the flags at runtime they can
> switch to atomic bit ops.