On 09.08.22 22:00, Linus Torvalds wrote:
> On Mon, Aug 8, 2022 at 12:32 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>
>
> So I've read through the patch several times, and it seems fine, but
> this function (and the pmd version of it) just read oddly to me.
>
>> +static inline bool can_follow_write_pte(pte_t pte, struct page *page,
>> + struct vm_area_struct *vma,
>> + unsigned int flags)
>> +{
>> + if (pte_write(pte))
>> + return true;
>> + if (!(flags & FOLL_FORCE))
>> + return false;
>> +
>> + /*
>> + * See check_vma_flags(): only COW mappings need that special
>> + * "force" handling when they lack VM_WRITE.
>> + */
>> + if (vma->vm_flags & VM_WRITE)
>> + return false;
>> + VM_BUG_ON(!is_cow_mapping(vma->vm_flags));
>
> So apart from the VM_BUG_ON(), this code just looks really strange -
> even despite the comment. Just conceptually, the whole "if it's
> writable, return that you cannot follow it for a write" just looks so
> very very strange.
>
> That doesn't make the code _wrong_, but considering how many times
> this has had subtle bugs, let's not write code that looks strange.
>
> So I would suggest that to protect against future bugs, we try to make
> it be fairly clear and straightforward, and maybe even a bit overly
> protective.
>
> For example, let's kill the "shared mapping that you don't have write
> permissions to" very explicitly and without any subtle code at all.
> The vm_flags tests are cheap and easy, and we could very easily just
> add some core ones to make any mistakes much less critical.
>
> Now, making that 'is_cow_mapping()' check explicit at the very top of
> this would already go a long way:
>
> /* FOLL_FORCE for writability only affects COW mappings */
> if (!is_cow_mapping(vma->vm_flags))
> return false;
I actually put the is_cow_mapping() mapping check in there because
check_vma_flags() should make sure that we cannot possibly end up here
in that case. But we can spell it out with comments, doesn't hurt.
>
> but I'd actually go even further: in this case that "is_cow_mapping()"
> helper to some degree actually hides what is going on.
>
> So I'd actually prefer for that function to be written something like
>
> /* If the pte is writable, we can write to the page */
> if (pte_write(pte))
> return true;
>
> /* Maybe FOLL_FORCE is set to override it? */
> if (flags & FOLL_FORCE)
> return false;
>
> /* But FOLL_FORCE has no effect on shared mappings */
> if (vma->vm_flags & MAP_SHARED)
> return false;
>
> /* .. or read-only private ones */
> if (!(vma->vm_flags & MAP_MAYWRITE))
> return false;
>
> /* .. or already writable ones that just need to take a write fault */
> if (vma->vm_flags & MAP_WRITE)
> return false;
>
> and the two first vm_flags tests above are basically doing tat
> "is_cow_mapping()", and maybe we could even have a comment to that
> effect, but wouldn't it be nice to just write it out that way?
>
> And after you've written it out like the above, now that
>
> if (!page || !PageAnon(page) || !PageAnonExclusive(page))
> return false;
>
> makes you pretty safe from a data sharing perspective: it's most
> definitely not a shared page at that point.
>
> So if you write it that way, the only remaining issues are the magic
> special soft-dirty and uffd ones, but at that point it's purely about
> the semantics of those features, no longer about any possible "oh, we
> fooled some shared page to be writable".
>
> And I think the above is fairly legible without any subtle cases, and
> the one-liner comments make it all fairly clear that it's testing.
>
> Is any of this in any _technical_ way different from what your patch
> did? No. It's literally just rewriting it to be a bit more explicit in
> what it is doing, I think, and it makes that odd "it's not writable if
> VM_WRITE is set" case a bit more explicit.
>
> Hmm?
No strong opinion. I'm happy as long as it's fixed, and the fix is robust.
--
Thanks,
David / dhildenb
