linux-kernel - Re: [PATCH v3 1/7] kernel/fork: convert vma assignment to a memcpy

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Message-ID: <20230126115224.3urhskf35eomk7xl@techsingularity.net>
Date:   Thu, 26 Jan 2023 11:52:24 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, michel@...pinasse.org,
        jglisse@...gle.com, mhocko@...e.com, vbabka@...e.cz,
        hannes@...xchg.org, dave@...olabs.net, willy@...radead.org,
        liam.howlett@...cle.com, peterz@...radead.org,
        ldufour@...ux.ibm.com, paulmck@...nel.org, mingo@...hat.com,
        will@...nel.org, luto@...nel.org, songliubraving@...com,
        peterx@...hat.com, david@...hat.com, dhowells@...hat.com,
        hughd@...gle.com, bigeasy@...utronix.de, kent.overstreet@...ux.dev,
        punit.agrawal@...edance.com, lstoakes@...il.com,
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        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
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Subject: Re: [PATCH v3 1/7] kernel/fork: convert vma assignment to a memcpy

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@...gle.com> wrote:
> 
> > On Wed, Jan 25, 2023 at 4:22 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
> > >
> > > On Wed, 25 Jan 2023 15:35:48 -0800 Suren Baghdasaryan <surenb@...gle.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@...hsingularity.net>

-- 
Mel Gorman
SUSE Labs