[PATCH 0/4] nstree: Fix spurious ENOENT in listns pagination during grace period

[Yohei Kojima <yk@y-koj.net>]

This series fixes the spurious ENOENT set by listns when (1) pagination
is used and (2) listns tries to start enumeration from a destroyed or
inactive namespace.

The Cause of the Bug
====================
This bug was caused by lookup_ns_id_at(kls->last_ns_id + 1, ...), which
is called by do_listns(). This function returned NULL if the first
namespace after the given ns id was destroyed or inactivated before this
function is called:

A: active namespace
D: destroyed (or inactive) namespace

         +-----+-----+-----+-----+-----+-----+-----+-----+
state:   |  A  |  A  |  A  |  D  |  D  |  A  |  A  |  A  |
ns_id:   |  1  |  2  |  3  |  4  |  5  |  6  |  7  |  8  |
         +-----+-----+-----+-----+-----+-----+-----+-----+
	                |     |
		        |     +-- (kls->last_ns_id + 1)
		        +-- req.ns_id = 3

For listns(), there is no way to distinguish this case with the case
nstree is empty, therefore it returns -ENOENT although three namespaces
remains in the tree.

Solution
========
The bug is fixed by iterating over the nstree's internal list until it
reaches the first active namespace.

Patches Sequence
================
Patches 1 and Patch 2 fix the existing issues in namespace selftests.
Patch 3 fixes the spurious ENOENT bug. Patch 4 adds a regression test
for this bug.

Disclaimer on Reproduction
==========================
Unfortunately I couldn't reproduce this bug on VM environment, perhaps
because the test I added relies on timing-sensitive RCU behavior. At
least, I confirmed that this bug reproduces on my bare-metal machine
equipped with i7-14700K. Also, I confirmed that all namespaces tests
pass after applying this series.


Yohei Kojima (4):
  selftests/namespace: fix selftest hang-up caused by zombie processes
  selftests/namespace: fix unintentional skip in ns_active_ref_test.c
  nstree: Fix spurious ENOENT in listns pagination during grace period
  selftests/namespace: test spurious ENOENT bug in listns pagination

 kernel/nstree.c                               |  68 ++++--
 .../namespaces/listns_pagination_bug.c        | 200 ++++++++++++++++++
 .../selftests/namespaces/ns_active_ref_test.c |   4 +
 .../testing/selftests/namespaces/nsid_test.c  |   8 +
 4 files changed, 258 insertions(+), 22 deletions(-)

-- 
2.52.0