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

On Tue, 7 Feb 2023 at 18:24, Suren Baghdasaryan <[email protected]> wrote:
>
> On Tue, Feb 7, 2023 at 9:16 AM Marco Elver <[email protected]> wrote:
> >
> > On Thu, Jan 26, 2023 at 09:27AM -0800, Paul E. McKenney wrote:
> > > On Wed, Jan 25, 2023 at 05:34:49PM -0800, Andrew Morton wrote:
> > > > On Wed, 25 Jan 2023 16:50:01 -0800 Suren Baghdasaryan <[email protected]> wrote:
> > > >
> > > > > On Wed, Jan 25, 2023 at 4:22 PM Andrew Morton <[email protected]> wrote:
> > > > > >
> > > > > > On Wed, 25 Jan 2023 15:35:48 -0800 Suren Baghdasaryan <[email protected]> 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.
> > >
> > > I ran an experiment memcpy()ing between a static array and an onstack
> > > array, and KCSAN did not complain. But maybe I was setting it up wrong.
> > >
> > > This is what I did:
> > >
> > > long myid = (long)arg; /* different value for each task */
> > > static unsigned long z1[10] = { 0 };
> > > unsigned long z2[10];
> > >
> > > ...
> > >
> > > memcpy(z1, z2, ARRAY_SIZE(z1) * sizeof(z1[0]));
> > > for (zi = 0; zi < ARRAY_SIZE(z1); zi++)
> > > z2[zi] += myid;
> > > memcpy(z2, z1, ARRAY_SIZE(z1) * sizeof(z1[0]));
> > >
> > > Adding Marco on CC for his thoughts.
> >
> > ( Sorry for not seeing it earlier - just saw this by chance. )
> >
> > memcpy() data races will be detected as of (given a relatively recent
> > Clang compiler):
> >
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7c201739beef
> >
> > Also beware that the compiler is free to "optimize" things by either
> > inlining memcpy() (turning an explicit memcpy() into just a bunch of
> > loads/stores), or outline plain assignments into memcpy() calls. So the
> > only way to be sure what ends up there is to look at the disassembled
> > code.
> >
> > The data_race() was introduced by:
> >
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cda099b37d716
> >
> > It says:
> > "vm_area_dup() blindly copies all fields of original VMA to the new one.
> > This includes coping vm_area_struct::shared.rb which is normally
> > protected by i_mmap_lock. But this is fine because the read value will
> > be overwritten on the following __vma_link_file() under proper
> > protection. Thus, mark it as an intentional data race and insert a few
> > assertions for the fields that should not be modified concurrently."
> >
> > And as far as I can tell this hasn't changed.
>
> Thanks for the feedback, Marco!
> So, IIUC Mel's proposal to use data_race(memcpy(new, orig,
> sizeof(*new))); is fine in this case, right?

Yes, that'd work.