linux-kernel - Re: [PATCH v2] mm: mmap_lock: fix use-after-free race and css ref leak in tracepoints

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Date:   Tue, 1 Dec 2020 16:31:04 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Chinwen Chang <chinwen.chang@...iatek.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        David Rientjes <rientjes@...gle.com>,
        Davidlohr Bueso <dbueso@...e.de>,
        Ingo Molnar <mingo@...hat.com>, Jann Horn <jannh@...gle.com>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Michel Lespinasse <walken@...gle.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Vlastimil Babka <vbabka@...e.cz>,
        Yafang Shao <laoar.shao@...il.com>, davem@...emloft.net,
        dsahern@...nel.org, gregkh@...uxfoundation.org, kuba@...nel.org,
        liuhangbin@...il.com, tj@...nel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v2] mm: mmap_lock: fix use-after-free race and css ref
 leak in tracepoints

On Tue, 1 Dec 2020 16:28:47 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Tue,  1 Dec 2020 12:32:49 -0800
> Axel Rasmussen <axelrasmussen@...gle.com> wrote:
> 
> > +/* Called with reg_lock held. */  
> 
> The above comment is reduntant, as the lockdep_is_held() below also suggest
> that it is ;-)
>


> 
> >  static inline char *get_memcg_path_buf(void)
> >  {
> > +	char *buf;
> >  	int idx;
> >  
> > +	rcu_read_lock();  
> 
> The caller of get_mm_memcg_path() has preemption disabled, which is also
> now an RCU lock. So the rcu_read_lock() is somewhat redundant.


BTW, both of these comments are FYI. You can keep the comment and keep the
rcu_read_lock(). I was just stating that they are redundant, but keeping
them may make the code a bit more robust.

-- Steve