Re: [PATCH v5 1/3] net: introduce helper sendpage_ok() in include/linux/net.h

[Eric Dumazet]

On 8/16/20 12:08 AM, Coly Li wrote:
> The original problem was from nvme-over-tcp code, who mistakenly uses
> kernel_sendpage() to send pages allocated by __get_free_pages() without
> __GFP_COMP flag. Such pages don't have refcount (page_count is 0) on
> tail pages, sending them by kernel_sendpage() may trigger a kernel panic
> from a corrupted kernel heap, because these pages are incorrectly freed
> in network stack as page_count 0 pages.
> 
> This patch introduces a helper sendpage_ok(), it returns true if the
> checking page,
> - is not slab page: PageSlab(page) is false.
> - has page refcount: page_count(page) is not zero
> 
> All drivers who want to send page to remote end by kernel_sendpage()
> may use this helper to check whether the page is OK. If the helper does
> not return true, the driver should try other non sendpage method (e.g.
> sock_no_sendpage()) to handle the page.
> 
>
> 
> diff --git a/include/linux/net.h b/include/linux/net.h
> index d48ff1180879..a807fad31958 100644
> --- a/include/linux/net.h
> +++ b/include/linux/net.h
> @@ -21,6 +21,7 @@
>  #include <linux/rcupdate.h>
>  #include <linux/once.h>
>  #include <linux/fs.h>
> +#include <linux/mm.h>
>  #include <linux/sockptr.h>
>  
>  #include <uapi/linux/net.h>
> @@ -286,6 +287,21 @@ do {									\
>  #define net_get_random_once_wait(buf, nbytes)			\
>  	get_random_once_wait((buf), (nbytes))
>  
> +/*
> + * E.g. XFS meta- & log-data is in slab pages, or bcache meta
> + * data pages, or other high order pages allocated by
> + * __get_free_pages() without __GFP_COMP, which have a page_count
> + * of 0 and/or have PageSlab() set. We cannot use send_page for
> + * those, as that does get_page(); put_page(); and would cause
> + * either a VM_BUG directly, or __page_cache_release a page that
> + * would actually still be referenced by someone, leading to some
> + * obscure delayed Oops somewhere else.
> + */
> +static inline bool sendpage_ok(struct page *page)
> +{
> +	return  (!PageSlab(page) && page_count(page) >= 1);
> +}
>

return (A);

Can simply be written :

return A;

In this case :

return !PageSlab(page) && page_count(page) >= 1;

BTW, do you have plans to refine code added with commit a10674bf2406afc2554f9c7d31b2dc65d6a27fd9
("tcp: detecting the misuse of .sendpage for Slab objects")