On Fri, Aug 13, 2021 at 4:01 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Wed, Aug 11, 2021 at 5:40 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
> >
> > Currently weak typeless ksyms have default value zero, when they don't
> > exist in the kernel. However, weak typed ksyms are rejected by libbpf
> > if they can not be resolved. This means that if a bpf object contains
> > the declaration of a nonexistent weak typed ksym, it will be rejected
> > even if there is no program that references the symbol.
> >
> > Nonexistent weak typed ksyms can also default to zero just like
> > typeless ones. This allows programs that access weak typed ksyms to be
> > accepted by verifier, if the accesses are guarded. For example,
> >
> > extern const int bpf_link_fops3 __ksym __weak;
> >
> > /* then in BPF program */
> >
> > if (&bpf_link_fops3) {
> > /* use bpf_link_fops3 */
> > }
> >
> > If actual use of nonexistent typed ksym is not guarded properly,
> > verifier would see that register is not PTR_TO_BTF_ID and wouldn't
> > allow to use it for direct memory reads or passing it to BPF helpers.
> >
> > Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
> > ---
>
> Looks good, applied to bpf-next. For the future, please split libbpf
> and selftests changes into separate patches, it's nicer to have those
> logically separate.
>
> At some point we should probably also improve libbpf error reporting
> for such situations, for better user experience. We have a similar
> problem with CO-RE relocation, verifier doesn't know about those
> concepts, so verifier log is not very helpful, but libbpf can make
> sense out of it with some extra BPF verifier log parsing.
>
Thanks. Will split the tests in future.
> > Changes since v2:
> > - Move special handling and warning from find_ksym_btf_id() to
> > bpf_object__resolve_ksym_var_btf_id().
> > - Removed bpf_link_fops3 from tests since it's not used.
> > - Separated variable declaration and statements.
> >
> > Changes since v1:
> > - Weak typed symbols default to zero, as suggested by Andrii.
> > - Use ASSERT_XXX() for tests.
> >
> > tools/lib/bpf/libbpf.c | 16 +++---
> > .../selftests/bpf/prog_tests/ksyms_btf.c | 31 ++++++++++
> > .../selftests/bpf/progs/test_ksyms_weak.c | 56 +++++++++++++++++++
> > 3 files changed, 96 insertions(+), 7 deletions(-)
> > create mode 100644 tools/testing/selftests/bpf/progs/test_ksyms_weak.c
> >
>
> [...]
