linux-kernel - Re: [PATCH v4 0/3] mm, treewide: Rename kzfree() to kfree

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Date:   Wed, 17 Jun 2020 14:55:53 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     dsterba@...e.cz, Joe Perches <joe@...ches.com>,
        Waiman Long <longman@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        "Jason A . Donenfeld" <Jason@...c4.com>, linux-mm@...ck.org,
        keyrings@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-crypto@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-amlogic@...ts.infradead.org,
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Subject: Re: [PATCH v4 0/3] mm, treewide: Rename kzfree() to kfree_sensitive()

On Wed 17-06-20 05:23:21, Matthew Wilcox wrote:
> On Wed, Jun 17, 2020 at 01:31:57PM +0200, Michal Hocko wrote:
> > On Wed 17-06-20 04:08:20, Matthew Wilcox wrote:
> > > If you call vfree() under
> > > a spinlock, you're in trouble.  in_atomic() only knows if we hold a
> > > spinlock for CONFIG_PREEMPT, so it's not safe to check for in_atomic()
> > > in __vfree().  So we need the warning in order that preempt people can
> > > tell those without that there is a bug here.
> > 
> > ... Unless I am missing something in_interrupt depends on preempt_count() as
> > well so neither of the two is reliable without PREEMPT_COUNT configured.
> 
> preempt_count() always tracks whether we're in interrupt context,
> regardless of CONFIG_PREEMPT.  The difference is that CONFIG_PREEMPT
> will track spinlock acquisitions as well.

Right you are! Thanks for the clarification. I find the situation
around preempt_count quite confusing TBH. Looking at existing users
of in_atomic() (e.g. a random one zd_usb_iowrite16v_async which check
in_atomic and then does GFP_KERNEL allocation which would be obviously
broken on !PREEMPT if the function can be called from an atomic
context), I am wondering whether it would make sense to track atomic
context also for !PREEMPT. This check is just terribly error prone.

-- 
Michal Hocko
SUSE Labs