On 1/26/22 19:34, Pasha Tatashin wrote:
> The problems with page->_refcount are hard to debug, because usually
> when they are detected, the damage has occurred a long time ago. Yet,
> the problems with invalid page refcount may be catastrophic and lead to
> memory corruptions.
>
> Reduce the scope of when the _refcount problems manifest themselves by
> adding checks for underflows and overflows into functions that modify
> _refcount.
>
> Use atomic_fetch_* functions to get the old values of the _refcount,
> and use it to check for overflow/underflow.
>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
> ---
> include/linux/page_ref.h | 59 +++++++++++++++++++++++++++++-----------
> 1 file changed, 43 insertions(+), 16 deletions(-)
>
> diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
> index 2e677e6ad09f..fe4864f7f69c 100644
> --- a/include/linux/page_ref.h
> +++ b/include/linux/page_ref.h
> @@ -117,7 +117,10 @@ static inline void init_page_count(struct page *page)
>
> static inline void page_ref_add(struct page *page, int nr)
> {
> - atomic_add(nr, &page->_refcount);
> + int old_val = atomic_fetch_add(nr, &page->_refcount);
> + int new_val = old_val + nr;
> +
> + VM_BUG_ON_PAGE((unsigned int)new_val < (unsigned int)old_val, page);
This seems somewhat weird, as it will trigger not just on overflow, but also
if nr is negative. Which I think is valid usage, even though the function
has 'add' in name, because 'nr' is signed?
