On Fri, Apr 26, 2024 at 2:15 AM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Wed, Apr 24, 2024 at 10:37 PM Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
> >
> > On Thu, Apr 25, 2024 at 8:34 AM Andrii Nakryiko
> > <andrii.nakryiko@xxxxxxxxx> wrote:
> > >
> > > On Thu, Apr 11, 2024 at 6:51 AM Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
> > > >
> > > > On Thu, Apr 11, 2024 at 9:11 PM Yafang Shao <laoar.shao@xxxxxxxxx> 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?
> >
>
> I might be missing something, but there is nothing wrong with
> bpf_probe_read_kernel() behavior. In "read 4" case you are overwriting
> only upper 4 bytes of u64, so lower 4 bytes are garbage. In "read 8"
> you are reading (upper) 4 bytes of garbage from uninitialized
> data_dst.
The issue doesn't lie with the dst but rather with the src. Even after
initializing the destination, the operation still fails. You can find
more details in the following link:
https://github.com/kernel-patches/bpf/pull/6882. It appears that
bpf_probe_read_kernel() encounters difficulties when dealing with
non-long-aligned source addresses.
>
> So getting back to iter implementation. Make sure you are
> zero-initializing that u64 value you are reading into?
>
It has been zero-initialized:
+ kit->nr_bits = 0;
+ kit->bits_copy = 0;
--
Regards
Yafang
