Re: [PATCH -next] fork: annotate a data race in vm_area_dup()

[Marco Elver <elver@google.com>]

On Tue, 18 Feb 2020 at 13:40, Qian Cai <cai@lca.pw> wrote:
>
>
>
> > On Feb 18, 2020, at 5:29 AM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
> >
> > I think I've got this:
> >
> > vm_area_dup() blindly copies all fields of orignal 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 protectection.
>
> Right, multiple processes could share the same file-based address space where those vma have been linked into address_space::i_mmap via vm_area_struct::shared.rb. Thus, the reader could see its shared.rb linkage pointers got updated by other processes.
>
> >
> > So the fix is correct, but justificaiton is lacking.
> >
> > Also, I would like to more fine-grained annotation: marking with
> > data_race() 200 bytes copy may hide other issues.
>
> That is the harder part where I don’t think we have anything for that today. Macro, any suggestions? ASSERT_IGNORE_FIELD()?

There is no nice interface I can think of. All options will just cause
more problems, inconsistencies, or annoyances.

Ideally, to not introduce more types of macros and keep it consistent,
ASSERT_EXCLUSIVE_FIELDS_EXCEPT(var, ...) maybe what you're after:
"Check no concurrent writers to struct, except ignore the provided
fields".

This option doesn't quite work, unless you just restrict it to 1 field
(we can't use ranges, because e.g. vm_area_struct has
__randomize_layout). The next time around, you'll want 2 fields, and
it won't work. Also, do we know that 'shared.rb' is the only thing we
want to ignore?

If you wanted something similar to ASSERT_EXCLUSIVE_BITS, it'd have to
be ASSERT_EXCLUSIVE_FIELDS(var, ...), however, this is quite annoying
for structs with many fields as you'd have to list all of them. It's
similar to what you can already do currently (but I also don't
recommend because it's unmaintainable):

ASSERT_EXCLUSIVE_WRITER(orig->vm_start);
ASSERT_EXCLUSIVE_WRITER(orig->vm_end);
ASSERT_EXCLUSIVE_WRITER(orig->vm_next);
ASSERT_EXCLUSIVE_WRITER(orig->vm_prev);
... and so on ...
*new = data_race(*orig);

Also note that vm_area_struct has __randomize_layout, which makes
using ranges impossible. All in all, I don't see a terribly nice
option.

If, however, you knew that there are 1 or 2 fields only that you want
to make sure are not modified concurrently, ASSERT_EXCLUSIVE_WRITER +
data_race() would probably work well (or even ASSERT_EXCLUSIVE_ACCESS
if you want to make sure there are no writers nor _readers_).

Thanks,
-- Marco