When BPF_PROG_TEST_RUN returns non-linear output and userspace provides a
short data_out buffer, bpf_test_finish() can return -ENOSPC before copying
the packet prefix or updating data_size_out.

Fix this by deriving the linear copy length from the packet layout rather
than from the already-clamped copy_size. Add selftest coverage for both
non-linear skb and XDP frags paths.

---

Changes in v2:

* Fix the Fixes tag to point to the commit that introduced the shared
  non-linear copy-out logic.
* Drop skb-specific wording from the fix commit.
* Move the selftest from skb_load_bytes.c to prog_run_opts.c.
* Add XDP frags coverage in addition to non-linear skb coverage.

v1:
https://lore.kernel.org/bpf/20260615073856.152479-1-sun.jian.kdev@gmail.com/ (local)

Tested with:
  ./test_progs -t prog_run_opts -v
  ./test_progs -t skb_load_bytes -v
  ./test_progs -t xdp_pull_data -v

Sun Jian (2):
  bpf: Fix partial copy of non-linear test_run output
  selftests/bpf: Cover partial copy of non-linear test_run output

 net/bpf/test_run.c                            | 11 ++-
 .../selftests/bpf/prog_tests/prog_run_opts.c  | 72 +++++++++++++++++++
 .../selftests/bpf/progs/test_pkt_access.c     | 12 ++++
 3 files changed, 88 insertions(+), 7 deletions(-)

Range-diff:
1:  3691b07aa440 ! 1:  e5a0c426d4cb bpf: Fix partial copy of non-linear skb test_run output
    @@ Metadata
     Author: Sun Jian [off-list ref]
     
      ## Commit message ##
    -    bpf: Fix partial copy of non-linear skb test_run output
    +    bpf: Fix partial copy of non-linear test_run output
     
    -    For non-linear skbs, bpf_test_finish() derives the linear head copy
    -    length from copy_size - frag_size. This only matches the skb head length
    -    when copy_size is the full packet size.
    +    For non-linear test_run output, bpf_test_finish() derives the linear
    +    data copy length from copy_size - frag_size. This only matches the
    +    linear data length when copy_size is the full packet size.
     
         When userspace provides a short data_out buffer, copy_size is clamped to
         that buffer size. If copy_size is smaller than frag_size, the computed
         length becomes negative and bpf_test_finish() returns -ENOSPC before
         copying the packet prefix or updating data_size_out.
     
    -    Compute the linear head length from the skb layout instead, and clamp the
    -    head copy length to copy_size. This preserves the expected partial-copy
    -    semantics: return -ENOSPC, copy the packet prefix that fits in data_out,
    -    and report the full packet length through data_size_out.
    +    Compute the linear data length from the packet layout instead, and clamp
    +    the linear copy length to copy_size. This preserves the expected
    +    partial-copy semantics: return -ENOSPC, copy the packet prefix that fits
    +    in data_out, and report the full packet length through data_size_out.
     
    -    Fixes: 838baa351cee ("bpf: Craft non-linear skbs in BPF_PROG_TEST_RUN")
    +    Fixes: 7855e0db150ad ("bpf: test_run: add xdp_shared_info pointer in bpf_test_finish signature")
         Signed-off-by: Sun Jian [off-list ref]
     
      ## net/bpf/test_run.c ##
2:  663847520f0b < -:  ------------ selftests/bpf: Cover partial copy of non-linear skb test_run output
-:  ------------ > 2:  680506532d97 selftests/bpf: Cover partial copy of non-linear test_run output
-- 
2.43.0