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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 9 Aug 2022 11:27:22 -0700
Message-ID: <CAHk-=wiEAH+ojSpAgx_Ep=NKPWHU8AdO3V56BXcCsU97oYJ1EA@mail.gmail.com>
Subject: Re: [PATCH v1] mm/gup: fix FOLL_FORCE COW security issue and remove FOLL_COW
To:     David Hildenbrand <david@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        stable@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Peter Xu <peterx@redhat.com>, Hugh Dickins <hughd@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        John Hubbard <jhubbard@nvidia.com>,
        Jason Gunthorpe <jgg@nvidia.com>
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I'm still reading through this, but

 STOP DOING THIS

On Mon, Aug 8, 2022 at 12:32 AM David Hildenbrand <david@redhat.com> wrote:
>
> +       VM_BUG_ON(!is_cow_mapping(vma->vm_flags));

Using BUG_ON() for debugging is simply not ok.

And saying "but it's just a VM_BUG_ON()" does not change *anything*.
At least Fedora enables that unconditionally for normal people, it is
not some kind of "only VM people do this".

Really. BUG_ON() IS NOT FOR DEBUGGING.

Stop it. Now.

If you have a condition that must not happen, you either write that
condition into the code, or - if you are convinced it cannot happen -
you make it a WARN_ON_ONCE() so that people can report it to you.

The BUG_ON() will just make the machine die.

And for the facebooks and googles of the world, the WARN_ON() will be
sufficient.

               Linus