BPF selftests fixes for 6.1 branch

Recently Luiz Capitulino reported BPF test failure for kernel version 6.1.36 (see [7]). The following test_verifier test failed:
"precise: ST insn causing spi > allocated_stack".
After back-port of the following upstream commit:
ecdf985d7615 ("bpf: track immediate values written to stack by BPF_ST instruction")

Investigation in [8] shows that test failure is not a bug, but a difference in BPF verifier behavior between upstream, where commits [1,2,3] by Andrii Nakryiko are present, and 6.1.36, where these commits are absent. Both Luiz and Greg suggested back-porting [1,2,3] from upstream to avoid divergences.

Commits [1,2,3] break test_progs selftest "align/packet variable offset", commit [4] fixes this selftest.

I did some additional testing using the following compiler versions:
- Kernel compilation
- gcc version 11.3.0
- BPF tests compilation
- clang version 16.0.6
- clang version 17.0.0 (fa46feb31481)

And identified a few more failing BPF selftests:
- Tests failing with LLVM 16:
- test_verifier:
- precise: ST insn causing spi > allocated_stack FAIL (fixed by [1,2,3])
- test_progs:
- sk_assign (fixed by [6])
- Tests failing with LLVM 17:
- test_verifier:
- precise: ST insn causing spi > allocated_stack FAIL (fixed by [1,2,3])
- test_progs:

    - fexit_bpf2bpf/func_replace_verify                   (fixed by [5])
    - fexit_bpf2bpf/func_replace_return_code              (fixed by [5])
    - sk_assign                                           (fixed by [6])

Commits [4,5,6] only apply to BPF selftests and don't change verifier behavior.

After applying all of the listed commits I have test_verifier, test_progs, test_progs-no_alu32 and test_maps passing on my x86 setup, both for LLVM 16 and LLVM 17.

Upstream commits in chronological order:
[1] be2ef8161572 ("bpf: allow precision tracking for programs with subprogs")
[2] f63181b6ae79 ("bpf: stop setting precise in current state")
[3] 7a830b53c17b ("bpf: aggressively forget precise markings during state checkpointing")
[4] 4f999b767769 ("selftests/bpf: make test_align selftest more robust")
[5] 63d78b7e8ca2 ("selftests/bpf: Workaround verification failure for fexit_bpf2bpf/func_replace_return_code")
[6] 7ce878ca81bc ("selftests/bpf: Fix sk_assign on s390x")

Links:
[7] https://lore.kernel.org/stable/[email protected]/
[8] https://lore.kernel.org/stable/[email protected]/

Changelog:
V1 -> V2: added missing signed-off-by tags
V1: https://lore.kernel.org/stable/[email protected]/

Andrii Nakryiko (4):
bpf: allow precision tracking for programs with subprogs
bpf: stop setting precise in current state
bpf: aggressively forget precise markings during state checkpointing selftests/bpf: make test_align selftest more robust

Ilya Leoshkevich (1):
selftests/bpf: Fix sk_assign on s390x

Yonghong Song (1):
selftests/bpf: Workaround verification failure for
fexit_bpf2bpf/func_replace_return_code

 kernel/bpf/verifier.c                         | 202 ++++++++++++++++--
 .../testing/selftests/bpf/prog_tests/align.c  |  38 ++--
 .../selftests/bpf/prog_tests/sk_assign.c      |  25 ++-
 .../selftests/bpf/progs/connect4_prog.c       |   2 +-
 .../selftests/bpf/progs/test_sk_assign.c      |  11 +
 .../bpf/progs/test_sk_assign_libbpf.c         |   3 +
 6 files changed, 247 insertions(+), 34 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/test_sk_assign_libbpf.c

2.41.0