Re: [RFC][arm64] possible infinite loop in btrfs search_ioctl()

[Al Viro]

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...