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Date:   Tue, 31 Aug 2021 15:28:57 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Catalin Marinas <catalin.marinas@arm.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Andreas Gruenbacher <agruenba@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        "Darrick J. Wong" <djwong@kernel.org>, Jan Kara <jack@suse.cz>,
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        cluster-devel <cluster-devel@redhat.com>,
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        "ocfs2-devel@oss.oracle.com" <ocfs2-devel@oss.oracle.com>,
        Josef Bacik <josef@toxicpanda.com>,
        Will Deacon <will@kernel.org>
Subject: Re: [RFC][arm64] possible infinite loop in btrfs search_ioctl()
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On Tue, Aug 31, 2021 at 02:54:50PM +0100, Catalin Marinas wrote:

> An arm64-specific workaround would be for pagefault_disable() to disable
> tag checking. It's a pretty big hammer, weakening the out of bounds
> access detection of MTE. My preference would be a fix in the btrfs code.
> 
> A btrfs option would be for copy_to_sk() to return an indication of
> where the fault occurred and get fault_in_pages_writeable() to check
> that location, even if the copying would restart from an earlier offset
> (this requires open-coding copy_to_user_nofault()). An attempt below,
> untested and does not cover read_extent_buffer_to_user_nofault():

Umm...  There's another copy_to_user_nofault() call in the same function
(same story, AFAICS).

Can't say I'm fond of their ABI, but then I guess it could've been worse -
iterating over btree, running a user-supplied chunk of INTERCAL over it,
with all details of internal representation cast in stone by that exposure...