Re: [BUG] fanotify: destroy/add race leaves a mark on a detached connector

[Daehyeon Ko <4ncienth@gmail.com>]

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Hi Amir,

I was able to make the handle-reopen sequence reliable on ext4 and drafted
an LTP test for it.

The test keeps 120 auxiliary inode marks on the victim to widen the interval
after the first FAN_DELETE_SELF, pins the unlink and handle-reopen workers to
different CPUs, and has the reopen worker read the event queue nonblocking
before calling open_by_handle_at().

I ran the exact final test for 100 iterations in each of two fresh 2-vCPU
KASAN boots per kernel:

  - unmodified v7.2: 200/200 iterations reopened the unlinked inode, and
    closing the reopened fd generated a second FAN_DELETE_SELF;
  - v7.2 with your patch: 200/200 iterations returned ESTALE from
    open_by_handle_at(), with no duplicate FAN_DELETE_SELF.

The test is based on LTP master commit
12724413534a6d4160ff9694ba6f09daa4ccb6bd. The attached patch adds
fanotify26 and the corresponding build and runtest entries.

Thanks,
Daehyeon

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