[PATCH 0/3] mm/mlock: make zero length requests a no-op and reject wrapping ranges

["Jose A. Perez de Azpillaga" <azpijr@gmail.com>]

Follow-up to Park Tae-sun's patch [1], which was dropped. David asked
which of the cases it raised can actually be triggered, and for selftests
that show them. Two can.

Patch 1 makes a zero length mlock()/munlock() a no-op before the address
is rounded. Today mlock(addr + 5, 0) locks a page and munlock(addr + 5, 0)
unlocks one.

Patch 2 rejects ranges where start + len overflows, or where the end
overflows when rounded to a page. Today these get a silent success, act
on a single page, or get -ENOMEM/-EINVAL depending on CAP_IPC_LOCK.

Patch 3 adds both cases to mlock2-tests.

I could not find the commit that introduced either behaviour in the git
history. Both are already present in the earliest history I could find,
so they may predate the imported git history. There is therefore no
commit to use for a Fixes tag. I have not added Cc: stable since these
changes intentionally alter UAPI-visible behaviour. A man-pages patch
for the zero length case will follow.

The mlock-random-test failure CI reported on [1] came from that patch,
not the test: it assigned 0 to do_mlock()'s error, which has to stay
-ENOMEM for the over-limit case, so mlock() over RLIMIT_MEMLOCK without
CAP_IPC_LOCK returned 0 without locking anything. With [1] applied the
test fails 40 runs out of 40; with error set back to -ENOMEM after the
check_mlock_range() call it passes all 40, as it does with this series.

Tested on x86_64 (KASAN, lockdep) in QEMU: mlock2-tests 31/31 as root and
as nobody, 24/31 and 23/31 on mm-unstable; mlock-random-test and
on-fault-limit pass.

[1] https://lore.kernel.org/all/179066531783.50175.13377521828379196177@dgu.ac.kr/

Jose A. Perez de Azpillaga (3):
  mm/mlock: make a zero length request a no-op whatever the alignment
  mm/mlock: reject ranges that cannot be represented
  selftests/mm: test mlock() and munlock() range normalisation

 mm/mlock.c                                |  29 +++-
 tools/testing/selftests/mm/mlock2-tests.c | 191 +++++++++++++++++++++-
 2 files changed, 215 insertions(+), 5 deletions(-)


base-commit: 33eb75fed9eef7a57e3f77c37c160d3e9ec55f2e
--
2.55.0