Spurious wakeups in tcp_msg_wait_data() aren't being handled by
tcp_bpf_recvmsg() and tcp_bpf_recvmsg_parser(), leading to unexpected
EAGAIN errors returned by recvfrom()/recv().  Adding handling for the
wakeup and a selftest.

This issue was first discovered in an application that adds sockets to
a sockmap as a way to view the received data.  No redirects or any
other operations are performed.

Sashiko has noted a few other pre-existing issues in the same area
(https://patch.msgid.link/20260714205118.17DB11F000E9@smtp.kernel.org)
that can lead to EAGAIN in tcp_bpf_recvmsg() and
tcp_bpf_recvmsg_parser().  A local run of Sashiko also identified a
potential issue in tcp_bpf_recvmsg() handling zero-data FIN packets when
the MSG_PEEK flag is set.  To prevent this patchset from growing too
large, I intend to submit follow up patches to address these once this
one has been accepted.

Changes in v7:
- In tcp_bpf_recvmsg check for data in the TCP_CLOSE, !timeo and
  signal_pending branches.
- Refactored changes to tcp_bpf_recvmsg to reduce duplication.
- Increase the number of iterations in the selftests.
- Documented selftests numeric constants.
- Link to v6: https://patch.msgid.link/20260720171535.67867-1-nnamdio@gmail.com

Changes in v6:
- Closing the file descriptor in the selftest worker thread.
- Ensuring the selftest loop breaks early for an error in the worker
  thread.
- Added comments to selftest regarding focus on EAGAIN error.
- Updated selftest commit message to imperative mood.
- Link to v5: https://patch.msgid.link/20260717155348.54975-1-nnamdio@gmail.com

Changes in v5:
- Move selftest from net into bpf/prog_tests/sockmap_basic.c
- Link to v4: https://patch.msgid.link/20260715213538.37229-1-nnamdio@gmail.com

Changes in v4:
- Fix potential data loss in tcp_bpf_recvmsg() when a FIN or RST has
  been received.
- Check the return code of pthread_create() in the selftest.
- Fix race caused by using EXPECT macros in a thread in selftest.
- Link to v3: https://patch.msgid.link/20260714203927.32289-1-nnamdio@gmail.com

Changes in v3:
- Added the sockmap_recvfrom selftest.
- Link to v2: https://patch.msgid.link/alFRK66z45eDNZA7@localhost.localdomain

Changes in v2:
- In tcp_bpf_recvmsg, handle signals and the socket closing in the loop.
- Fix spurious wakeups when SO_RCVTIMEO has been set on the socket.
- Link to v1: https://patch.msgid.link/ak_rR-Skd8Mvn4mH@localhost.localdomain

Signed-off-by: Nnamdi Onyeyiri <nnamdio@gmail.com>
---
Nnamdi Onyeyiri (2):
  bpf, sockmap: handle spurious tcp_msg_wait_data() wakeup
  selftests/bpf: add sockmap recvfrom EAGAIN selftest

 net/ipv4/tcp_bpf.c                            |  72 ++++++++--
 .../selftests/bpf/prog_tests/sockmap_basic.c  | 136 ++++++++++++++++++
 2 files changed, 199 insertions(+), 9 deletions(-)

-- 
2.52.0