Support non-linear skbs for BPF_PROG_TEST_RUN

This patchset adds support for non-linear skbs when running tc programs
with BPF_PROG_TEST_RUN.

We've had multiple bugs in the past few years in Cilium caused by
missing calls to bpf_skb_pull_data(). Daniel suggested this new
BPF_PROG_TEST_RUN flag as a way to uncover these bugs in our BPF tests.

Changes in v7:
  - Refactor use of 'size' variable as suggested by Martin.
  - Support copying back the non-linear area to data_out.
  - Minor code changes for readability, suggested by Martin.
Changes in v6:
  - Disallow non-linear skb in prog_run_skb only for LWT programs
    instead of all non-L2 program types, on suggestion from Martin.
  - Reject non-null ctx->data and ctx->data_meta, as suggested by Amery.
  - Bound linear_size to 'PAGE_SIZE - headroom - tailroom' to be
    consistent with prog_run_xdp, as suggested by Martin.
  - Allocate exactly linear_size bytes in bpf_test_init, spotted by
    Martin.
  - Fix wrong conflict resolution on double-free fix, spotted by Amery.
  - Rebased.
Changes in v5:
  - Fix double free on data in first patch.
Changes in v4:
  - Per Martin's suggestion, follow the XDP code pattern and use
    bpf_test_init only to initialize the linear area. That way data is
    directly copied to the right areas and we avoid the call to
    __pskb_pull_tail.
  - Fixed outdated commit descriptions.
  - Rebased.
Changes in v3:
  - Dropped BPF_F_TEST_SKB_NON_LINEAR and used the ctx->data_end to
    determine if the user wants non-linear skb, as suggested by Amery.
  - Introduced a second commit with a bit of refactoring to allow for
    the above requested change.
  - Fix bug found by syzkaller on third commit.
  - Rebased.
Changes in v2:
  - Made the linear size configurable via ctx->data_end, as suggested
    by Amery.
  - Reworked the selftests to allow testing the configurable linear
    size.
  - Fix warnings reported by kernel test robot on first commit.
  - Rebased.

Paul Chaignon (5):
  bpf: Refactor cleanup of bpf_prog_test_run_skb
  bpf: Reorder bpf_prog_test_run_skb initialization
  bpf: Craft non-linear skbs in BPF_PROG_TEST_RUN
  selftests/bpf: Support non-linear flag in test loader
  selftests/bpf: Test direct packet access on non-linear skbs

 net/bpf/test_run.c                            | 141 +++++++++++++-----
 tools/testing/selftests/bpf/progs/bpf_misc.h  |   4 +
 .../bpf/progs/verifier_direct_packet_access.c |  59 ++++++++
 tools/testing/selftests/bpf/test_loader.c     |  19 ++-
 4 files changed, 182 insertions(+), 41 deletions(-)

-- 
2.43.0