Re: [RFC PATCH bpf-next 3/8] selftests/bpf: add __ksym extern selftest

On Mon, Jun 15, 2020 at 9:45 AM Hao Luo [off-list ref] wrote:

Andrii, a couple of general comments on fixed_percpu_data.

I think it would be better to check the existence of fixed_percpu_data in kallsyms first. If it's not there, just skip, or maybe warn but not fail.

fixed_percpu_data is always there, but I missed the fact that it's
x86-specific one. I'll switch to some bpf-specific symbol (e.g., like
bpf_prog_fops or something along those lines).

Further, if we really want to be sure that  fixed_percpu_data is the first percpu var, we can read the value of __per_cpu_start, which marks the beginning address of the percpu section. Checking the address of fixed_percpu_data against __per_cpu_start rather than 0 should be more robust, I think, given that fixed_percpu_data exists.

There are assertions in Linux sources that fixed_percpu_data is 0, so
I don't think that it necessary. But it's a moot point, as I'll use
something less x86-specific.

Hao

On Fri, Jun 12, 2020 at 3:35 PM Andrii Nakryiko [off-list ref] wrote:

quoted

Validate libbpf is able to handle weak and strong kernel symbol externs in BPF
code correctly.

Signed-off-by: Andrii Nakryiko <redacted>
---
 .../testing/selftests/bpf/prog_tests/ksyms.c  | 71 +++++++++++++++++++
 .../testing/selftests/bpf/progs/test_ksyms.c  | 32 +++++++++
 2 files changed, 103 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/ksyms.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_ksyms.c

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