Date:   Thu, 26 Jan 2023 11:52:24 +0000
From:   Mel Gorman <mgorman@techsingularity.net>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Suren Baghdasaryan <surenb@google.com>, michel@lespinasse.org,
        jglisse@google.com, mhocko@suse.com, vbabka@suse.cz,
        hannes@cmpxchg.org, dave@stgolabs.net, willy@infradead.org,
        liam.howlett@oracle.com, peterz@infradead.org,
        ldufour@linux.ibm.com, paulmck@kernel.org, mingo@redhat.com,
        will@kernel.org, luto@kernel.org, songliubraving@fb.com,
        peterx@redhat.com, david@redhat.com, dhowells@redhat.com,
        hughd@google.com, bigeasy@linutronix.de, kent.overstreet@linux.dev,
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        linuxppc-dev@lists.ozlabs.org, x86@kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@android.com
Subject: Re: [PATCH v3 1/7] kernel/fork: convert vma assignment to a memcpy
Message-ID: <20230126115224.3urhskf35eomk7xl@techsingularity.net>
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On Wed, Jan 25, 2023 at 05:34:49PM -0800, Andrew Morton wrote:
> On Wed, 25 Jan 2023 16:50:01 -0800 Suren Baghdasaryan <surenb@google.com> wrote:
> 
> > On Wed, Jan 25, 2023 at 4:22 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> > >
> > > On Wed, 25 Jan 2023 15:35:48 -0800 Suren Baghdasaryan <surenb@google.com> wrote:
> > >
> > > > Convert vma assignment in vm_area_dup() to a memcpy() to prevent compiler
> > > > errors when we add a const modifier to vma->vm_flags.
> > > >
> > > > ...
> > > >
> > > > --- a/kernel/fork.c
> > > > +++ b/kernel/fork.c
> > > > @@ -482,7 +482,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
> > > >                * orig->shared.rb may be modified concurrently, but the clone
> > > >                * will be reinitialized.
> > > >                */
> > > > -             *new = data_race(*orig);
> > > > +             memcpy(new, orig, sizeof(*new));
> > >
> > > The data_race() removal is unchangelogged?
> > 
> > True. I'll add a note in the changelog about that. Ideally I would
> > like to preserve it but I could not find a way to do that.
> > 
> 
> Perhaps Paul can comment?
> 
> I wonder if KCSAN knows how to detect this race, given that it's now in
> a memcpy.  I assume so.

data_race() is just wrapping an expression around
__kcsan_[en|dis]able_current and ensuring the expression is evaluated once
and returning the correct type. I believe the following should be sufficient.

diff --git a/kernel/fork.c b/kernel/fork.c
index 9f7fe3541897..1b30ee568e02 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -472,7 +472,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
 		 * orig->shared.rb may be modified concurrently, but the clone
 		 * will be reinitialized.
 		 */
-		*new = data_race(*orig);
+		data_race(memcpy(new, orig, sizeof(*new)));
 		INIT_LIST_HEAD(&new->anon_vma_chain);
 		dup_anon_vma_name(orig, new);
 	}

I don't see how memcpy could automagically figure out whether the memcpy
is prone to races or not in an arbitrary context.

Assuming using data_race this way is ok then

Acked-by: Mel Gorman <mgorman@techsingularity.net>

-- 
Mel Gorman
SUSE Labs