Re: [PATCH bpf-next v6 0/2] bpf: Add a generic bits iterator

[Yonghong Song <yonghong.song@linux.dev>]

On 4/24/24 10:36 PM, Yafang Shao wrote:
> On Thu, Apr 25, 2024 at 8:34 AM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
>> On Thu, Apr 11, 2024 at 6:51 AM Yafang Shao <laoar.shao@gmail.com> wrote:
>>> On Thu, Apr 11, 2024 at 9:11 PM Yafang Shao <laoar.shao@gmail.com> wrote:
>>>> Three new kfuncs, namely bpf_iter_bits_{new,next,destroy}, have been
>>>> added for the new bpf_iter_bits functionality. These kfuncs enable the
>>>> iteration of the bits from a given address and a given number of bits.
>>>>
>>>> - bpf_iter_bits_new
>>>>    Initialize a new bits iterator for a given memory area. Due to the
>>>>    limitation of bpf memalloc, the max number of bits to be iterated
>>>>    over is (4096 * 8).
>>>> - bpf_iter_bits_next
>>>>    Get the next bit in a bpf_iter_bits
>>>> - bpf_iter_bits_destroy
>>>>    Destroy a bpf_iter_bits
>>>>
>>>> The bits iterator can be used in any context and on any address.
>>>>
>>>> Changes:
>>>> - v5->v6:
>>>>    - Add positive tests (Andrii)
>>>> - v4->v5:
>>>>    - Simplify test cases (Andrii)
>>>> - v3->v4:
>>>>    - Fix endianness error on s390x (Andrii)
>>>>    - zero-initialize kit->bits_copy and zero out nr_bits (Andrii)
>>>> - v2->v3:
>>>>    - Optimization for u64/u32 mask (Andrii)
>>>> - v1->v2:
>>>>    - Simplify the CPU number verification code to avoid the failure on s390x
>>>>      (Eduard)
>>>> - bpf: Add bpf_iter_cpumask
>>>>    https://lwn.net/Articles/961104/
>>>> - bpf: Add new bpf helper bpf_for_each_cpu
>>>>    https://lwn.net/Articles/939939/
>>>>
>>>> Yafang Shao (2):
>>>>    bpf: Add bits iterator
>>>>    selftests/bpf: Add selftest for bits iter
>>>>
>>>>   kernel/bpf/helpers.c                          | 120 +++++++++++++++++
>>>>   .../selftests/bpf/prog_tests/verifier.c       |   2 +
>>>>   .../selftests/bpf/progs/verifier_bits_iter.c  | 127 ++++++++++++++++++
>>>>   3 files changed, 249 insertions(+)
>>>>   create mode 100644 tools/testing/selftests/bpf/progs/verifier_bits_iter.c
>>>>
>>>> --
>>>> 2.39.1
>>>>
>>> It appears that the test case failed on s390x when the data is
>>> a u32 value because we need to set the higher 32 bits.
>>> will analyze it.
>>>
>> Hey Yafang, did you get a chance to debug and fix the issue?
>>
> Hi Andrii,
>
> Apologies for the delay; I recently returned from an extended holiday.
>
> The issue stems from the limitations of bpf_probe_read_kernel() on
> s390 architecture. The attachment provides a straightforward example
> to illustrate this issue. The observed results are as follows:
>
>      Error: #463/1 verifier_probe_read/probe read 4 bytes
>      8897 run_subtest:PASS:obj_open_mem 0 nsec
>      8898 run_subtest:PASS:unexpected_load_failure 0 nsec
>      8899 do_prog_test_run:PASS:bpf_prog_test_run 0 nsec
>      8900 run_subtest:FAIL:659 Unexpected retval: 2817064 != 512
>
>      Error: #463/2 verifier_probe_read/probe read 8 bytes
>      8903 run_subtest:PASS:obj_open_mem 0 nsec
>      8904 run_subtest:PASS:unexpected_load_failure 0 nsec
>      8905 do_prog_test_run:PASS:bpf_prog_test_run 0 nsec
>      8906 run_subtest:FAIL:659 Unexpected retval: 0 != 512
>
> More details can be found at:  https://github.com/kernel-patches/bpf/pull/6872
>
> Should we consider this behavior of bpf_probe_read_kernel() as
> expected, or is it something that requires fixing?

Maybe to guard the result with macros like __s390x__ to differentiate 
s390 vs. arm64/x86_64? There are some examples in prog_tests/* having 
such guards. probe_user.c:#if defined(__s390x__) 
test_bpf_syscall_macro.c:#if defined(__aarch64__) || defined(__s390__) 
xdp_adjust_tail.c:#if defined(__s390x__) xdp_do_redirect.c:#if 
defined(__s390x__)