linux-kernel - Re: Re: [PATCH v2] mm: page_frag: Warn_on when frag_alloc size is bigger than PAGE

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Message-ID: <CAFL455=ZcU_fyM9kiuZUJeVmRv9Jx_FmURcweCrTXheRoKkSqg@mail.gmail.com>
Date:   Fri, 8 Jul 2022 10:06:53 +0200
From:   Maurizio Lombardi <mlombard@...hat.com>
To:     愚树 <chen45464546@....com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>
Subject: Re: Re: [PATCH v2] mm: page_frag: Warn_on when frag_alloc size is
 bigger than PAGE_SIZE

st 1. 6. 2022 v 14:49 odesílatel 愚树 <chen45464546@....com> napsal:
> Can we just add code to the relatively slow path to capture the mistake
> before it lead to memory corruption?
> Like:
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e6f211d..ac60a97 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5580,6 +5580,7 @@ void *page_frag_alloc_align(struct page_frag_cache *nc,
>                 /* reset page count bias and offset to start of new frag */
>                 nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
>                 offset = size - fragsz;
> +               BUG_ON(offset < 0);
>         }
>

Personally, I'm not really convinced this is the best solution.
The next time a driver abuses  the page_frag_alloc() interface, the
bug may go unnoticed for a long time...
until a server in production runs into OOM and crashes because it hits
the BUG_ON().

And why should the kernel panic? It's perfectly able to handle this
condition by failing
the allocation and returning NULL, and printing a warning maybe.

Maurizio