On Tue, 18 Feb 2020 at 13:40, Qian Cai <cai@xxxxxx> wrote: > > > > > On Feb 18, 2020, at 5:29 AM, Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> 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
