[PATCH v2 net 0/2] net: fix SO_RESERVE_MEM socket type restriction and value bounds

This series fixes two issues with SO_RESERVE_MEM and adds a kselftest suite:

- Patch 1 restricts sock_reserve_memory() to TCP sockets (sk_is_tcp(sk))
  instead of sk_has_account(sk), and caps the requested reservation to
  SZ_1G in sk_setsockopt(SO_RESERVE_MEM).

  Currently, sk_has_account(sk) allows SO_RESERVE_MEM on UDP sockets,
  where sk->sk_forward_alloc is protected by sk->sk_receive_queue.lock
  rather than the socket lock, and udp_rmem_release() does not account
  for sk_unused_reserved_mem(sk). Concurrent UDP packet processing and
  setsockopt(SO_RESERVE_MEM) corrupt sk_forward_alloc and memcg
  accounting. In addition, values near INT_MAX overflow 32-bit signed
  int in sk_mem_pages(delta) and (pages << PAGE_SHIFT).

- Patch 2 adds a kselftest (tools/testing/selftests/net/so_reserve_mem.c)
  covering non-TCP rejection (UDP, AF_UNIX, SOCK_RAW, MPTCP), bounds
  validation, reserve grow/shrink/rounding and SK_MEMINFO_FWD_ALLOC
  reporting, cgroup v2 memory.max enforcement, accept() child zero-reserve
  inheritance, and reserve retention across TCP traffic.

v2:
 - Patch 1: use page-aligned SZ_1G instead of INT_MAX >> 1 so values
   returned by getsockopt(SO_RESERVE_MEM) always round-trip through
   setsockopt(SO_RESERVE_MEM) (Sashiko).
 - Patch 2: enable CONFIG_CGROUPS=y and CONFIG_MEMCG=y in
   tools/testing/selftests/net/config, skip when memcg socket accounting
   is unavailable, restore cgroup.subtree_control on teardown, accept
   ENOPROTOOPT when MPTCP is disabled via sysctl, move the test process
   out of cg_child before lowering memory.max, wait for
   SK_MEMINFO_WMEM_QUEUED to drain before checking cfd forward alloc,
   and drop the arch-specific SO_RESERVE_MEM fallback define (Sashiko).
 - Link to v1: https://lore.kernel.org/netdev/20260924123602.1979090-1-edumazet@google.com/ (local)

Eric Dumazet (2):
  net: restrict SO_RESERVE_MEM to TCP sockets and cap max value
  selftests/net: add SO_RESERVE_MEM test

 net/core/sock.c                              |   4 +-
 tools/testing/selftests/net/.gitignore       |   1 +
 tools/testing/selftests/net/Makefile         |   1 +
 tools/testing/selftests/net/config           |   2 +
 tools/testing/selftests/net/so_reserve_mem.c | 404 +++++++++++++++++++
 5 files changed, 410 insertions(+), 2 deletions(-)
 create mode 100644 tools/testing/selftests/net/so_reserve_mem.c

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2.56.0.rc1.315.gc6ed9934b7-goog