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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 21 Oct 2021 16:30:30 -1000
Message-ID: <CAHk-=wgvnU2PXFMpsNErdwE=tXGymLHe275jWkBhCbGiixWU5g@mail.gmail.com>
Subject: Re: [RFC][arm64] possible infinite loop in btrfs search_ioctl()
To:     Andreas Gruenbacher <agruenba@redhat.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        "Darrick J. Wong" <djwong@kernel.org>, Jan Kara <jack@suse.cz>,
        Matthew Wilcox <willy@infradead.org>,
        cluster-devel <cluster-devel@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "ocfs2-devel@oss.oracle.com" <ocfs2-devel@oss.oracle.com>,
        Josef Bacik <josef@toxicpanda.com>,
        Will Deacon <will@kernel.org>
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On Thu, Oct 21, 2021 at 4:42 AM Andreas Gruenbacher <agruenba@redhat.com> wrote:
>
> But probing the entire memory range in fault domain granularity in the
> page fault-in functions still doesn't actually make sense. Those
> functions really only need to guarantee that we'll be able to make
> progress eventually. From that point of view, it should be enough to
> probe the first byte of the requested memory range

That's probably fine.

Although it should be more than one byte - "copy_from_user()" might do
word-at-a-time optimizations, so you could have an infinite loop of

 (a) copy_from_user() fails because the chunk it tried to get failed partly

 (b) fault_in() probing succeeds, because the beginning part is fine

so I agree that the fault-in code doesn't need to do the whole area,
but it needs to at least do some <N bytes, up to length> thing, to
handle the situation where the copy_to/from_user requires more than a
single byte.

                 Linus