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 <sur...@google.com>
> > wrote:
> >
> > > On Wed, Jan 25, 2023 at 4:22 PM Andrew Morton <a...@linux-foundation.org>
> > > wrote:
> > > >
> > > > On Wed, 25 Jan 2023 15:35:48 -0800 Suren Baghdasaryan
> > > > <sur...@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.
>
> 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,
-- Marco
