On Wed, Sep 9, 2020 at 6:59 PM Ilya Leoshkevich <iii@xxxxxxxxxxxxx> wrote:
>
> Hi!
>
> On Fri, 2020-06-19 at 16:16 -0700, Andrii Nakryiko wrote:
> > Validate libbpf is able to handle weak and strong kernel symbol
> > externs in BPF
> > code correctly.
> >
> > Signed-off-by: Andrii Nakryiko <andriin@xxxxxx>
> > ---
> > .../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
> >
> > diff --git a/tools/testing/selftests/bpf/prog_tests/ksyms.c
> > b/tools/testing/selftests/bpf/prog_tests/ksyms.c
> > new file mode 100644
> > index 000000000000..e3d6777226a8
> > --- /dev/null
> > +++ b/tools/testing/selftests/bpf/prog_tests/ksyms.c
> > @@ -0,0 +1,71 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/* Copyright (c) 2019 Facebook */
> > +
> > +#include <test_progs.h>
> > +#include "test_ksyms.skel.h"
> > +#include <sys/stat.h>
> > +
> > +static int duration;
> > +
> > +static __u64 kallsyms_find(const char *sym)
> > +{
> > + char type, name[500];
> > + __u64 addr, res = 0;
> > + FILE *f;
> > +
> > + f = fopen("/proc/kallsyms", "r");
> > + if (CHECK(!f, "kallsyms_fopen", "failed to open: %d\n", errno))
> > + return 0;
> > +
> > + while (fscanf(f, "%llx %c %499s%*[^\n]\n", &addr, &type, name)
> > > 0) {
> > + if (strcmp(name, sym) == 0) {
> > + res = addr;
> > + goto out;
> > + }
> > + }
> > +
> > + CHECK(false, "not_found", "symbol %s not found\n", sym);
> > +out:
> > + fclose(f);
> > + return res;
> > +}
> > +
> > +void test_ksyms(void)
> > +{
> > + __u64 link_fops_addr = kallsyms_find("bpf_link_fops");
> > + const char *btf_path = "/sys/kernel/btf/vmlinux";
> > + struct test_ksyms *skel;
> > + struct test_ksyms__data *data;
> > + struct stat st;
> > + __u64 btf_size;
> > + int err;
> > +
> > + if (CHECK(stat(btf_path, &st), "stat_btf", "err %d\n", errno))
> > + return;
> > + btf_size = st.st_size;
> > +
> > + skel = test_ksyms__open_and_load();
> > + if (CHECK(!skel, "skel_open", "failed to open and load
> > skeleton\n"))
> > + return;
> > +
> > + err = test_ksyms__attach(skel);
> > + if (CHECK(err, "skel_attach", "skeleton attach failed: %d\n",
> > err))
> > + goto cleanup;
> > +
> > + /* trigger tracepoint */
> > + usleep(1);
> > +
> > + data = skel->data;
> > + CHECK(data->out__bpf_link_fops != link_fops_addr,
> > "bpf_link_fops",
> > + "got 0x%llx, exp 0x%llx\n",
> > + data->out__bpf_link_fops, link_fops_addr);
> > + CHECK(data->out__bpf_link_fops1 != 0, "bpf_link_fops1",
> > + "got %llu, exp %llu\n", data->out__bpf_link_fops1,
> > (__u64)0);
> > + CHECK(data->out__btf_size != btf_size, "btf_size",
> > + "got %llu, exp %llu\n", data->out__btf_size, btf_size);
> > + CHECK(data->out__per_cpu_start != 0, "__per_cpu_start",
> > + "got %llu, exp %llu\n", data->out__per_cpu_start,
> > (__u64)0);
> > +
> > +cleanup:
> > + test_ksyms__destroy(skel);
> > +}
>
> Why is __per_cpu_start expected to be 0? On my x86_64 Debian VM it is
> something like ffffffffxxxxxxxx, and this test fails. Wouldn't
> it be better to take the value from kallsyms, like it's done with
> bpf_link_fops, or am I missing something in my setup?
>
Hm... those per-CPU symbols are not real addresses, they are relative
offsets, so I thought that __per_cpu_start always got to be 0. Strange
that you see a real kernel address instead. I guess looking up in
kallsyms would work either way, please feel free to send a fix.
Thanks!
> Best regards,
> Ilya
>
