linux-kernel - Re: [PATCH v1] mm/gup: fix FOLL_FORCE COW security issue and remove FOLL

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Date:   Tue, 9 Aug 2022 16:07:50 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, stable@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Peter Xu <peterx@...hat.com>, Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH v1] mm/gup: fix FOLL_FORCE COW security issue and remove
 FOLL_COW

On Tue, Aug 09, 2022 at 11:59:45AM -0700, Linus Torvalds wrote:

> But as a very good approximation, the rule is "absolutely no new
> BUG_ON() calls _ever_". Because I really cannot see a single case
> where "proper error handling and WARN_ON_ONCE()" isn't the right
> thing.

Parallel to this discussion I've had ones where people more or less
say

 Since BUG_ON crashes the machine and Linus says that crashing the
 machine is bad, WARN_ON will also crash the machine if you set the
 panic_on_warn parameter, so it is also bad, thus we shouldn't use
 anything.

I've generally maintained that people who set the panic_on_warn *want*
these crashes, because that is the entire point of it. So we should
use WARN_ON with an error recovery for "can't happen" assertions like
these. I think it is what you are saying here.

Jason