Date:   Fri, 23 Apr 2021 10:18:12 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 15/17] selftests/bpf: add function linking selftest

On Thu, Apr 22, 2021 at 5:50 PM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 4/16/21 1:24 PM, Andrii Nakryiko wrote:
> > Add selftest validating various aspects of statically linking functions:
> >    - no conflicts and correct resolution for name-conflicting static funcs;
> >    - correct resolution of extern functions;
> >    - correct handling of weak functions, both resolution itself and libbpf's
> >      handling of unused weak function that "lost" (it leaves gaps in code with
> >      no ELF symbols);
> >    - correct handling of hidden visibility to turn global function into
> >      "static" for the purpose of BPF verification.
> >
> > Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
>
> Ack with a small nit below.
>
> Acked-by: Yonghong Song <yhs@...com>
>
> > ---
> >   tools/testing/selftests/bpf/Makefile          |  3 +-
> >   .../selftests/bpf/prog_tests/linked_funcs.c   | 42 +++++++++++
> >   .../selftests/bpf/progs/linked_funcs1.c       | 73 +++++++++++++++++++
> >   .../selftests/bpf/progs/linked_funcs2.c       | 73 +++++++++++++++++++
> >   4 files changed, 190 insertions(+), 1 deletion(-)
> >   create mode 100644 tools/testing/selftests/bpf/prog_tests/linked_funcs.c
> >   create mode 100644 tools/testing/selftests/bpf/progs/linked_funcs1.c
> >   create mode 100644 tools/testing/selftests/bpf/progs/linked_funcs2.c
> >
> > diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
> > index 666b462c1218..427ccfec1a6a 100644
> > --- a/tools/testing/selftests/bpf/Makefile
> > +++ b/tools/testing/selftests/bpf/Makefile
> > @@ -308,9 +308,10 @@ endef
> >
> >   SKEL_BLACKLIST := btf__% test_pinning_invalid.c test_sk_assign.c
> >
> > -LINKED_SKELS := test_static_linked.skel.h
> > +LINKED_SKELS := test_static_linked.skel.h linked_funcs.skel.h
> >
> >   test_static_linked.skel.h-deps := test_static_linked1.o test_static_linked2.o
> > +linked_funcs.skel.h-deps := linked_funcs1.o linked_funcs2.o
> >
> >   LINKED_BPF_SRCS := $(patsubst %.o,%.c,$(foreach skel,$(LINKED_SKELS),$($(skel)-deps)))
> >
> > diff --git a/tools/testing/selftests/bpf/prog_tests/linked_funcs.c b/tools/testing/selftests/bpf/prog_tests/linked_funcs.c
> > new file mode 100644
> > index 000000000000..03bf8ef131ce
> > --- /dev/null
> > +++ b/tools/testing/selftests/bpf/prog_tests/linked_funcs.c
> > @@ -0,0 +1,42 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/* Copyright (c) 2021 Facebook */
> > +
> > +#include <test_progs.h>
> > +#include <sys/syscall.h>
> > +#include "linked_funcs.skel.h"
> > +
> > +void test_linked_funcs(void)
> > +{
> > +     int err;
> > +     struct linked_funcs *skel;
> > +
> > +     skel = linked_funcs__open();
> > +     if (!ASSERT_OK_PTR(skel, "skel_open"))
> > +             return;
> > +
> > +     skel->rodata->my_tid = syscall(SYS_gettid);
> > +     skel->rodata->syscall_id = SYS_getpgid;
> > +
> > +     err = linked_funcs__load(skel);
> > +     if (!ASSERT_OK(err, "skel_load"))
> > +             goto cleanup;
> > +
> > +     err = linked_funcs__attach(skel);
> > +     if (!ASSERT_OK(err, "skel_attach"))
> > +             goto cleanup;
> > +
> > +     /* trigger */
> > +     syscall(SYS_getpgid);
> > +
> > +     ASSERT_EQ(skel->bss->output_val1, 2000 + 2000, "output_val1");
> > +     ASSERT_EQ(skel->bss->output_ctx1, SYS_getpgid, "output_ctx1");
> > +     ASSERT_EQ(skel->bss->output_weak1, 42, "output_weak1");
> > +
> > +     ASSERT_EQ(skel->bss->output_val2, 2 * 1000 + 2 * (2 * 1000), "output_val2");
> > +     ASSERT_EQ(skel->bss->output_ctx2, SYS_getpgid, "output_ctx2");
> > +     /* output_weak2 should never be updated */
> > +     ASSERT_EQ(skel->bss->output_weak2, 0, "output_weak2");
> > +
> > +cleanup:
> > +     linked_funcs__destroy(skel);
> > +}
> > diff --git a/tools/testing/selftests/bpf/progs/linked_funcs1.c b/tools/testing/selftests/bpf/progs/linked_funcs1.c
> > new file mode 100644
> > index 000000000000..cc621d4e4d82
> > --- /dev/null
> > +++ b/tools/testing/selftests/bpf/progs/linked_funcs1.c
> > @@ -0,0 +1,73 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/* Copyright (c) 2021 Facebook */
> > +
> > +#include "vmlinux.h"
> > +#include <bpf/bpf_helpers.h>
> > +#include <bpf/bpf_tracing.h>
> > +
> > +/* weak and shared between two files */
> > +const volatile int my_tid __weak = 0;
> > +const volatile long syscall_id __weak = 0;
>
> Since the new compiler (llvm13) is recommended for this patch set.
> We can simplify the above two definition with
>    int my_tid __weak;
>    long syscall_id __weak;
> The same for the other file.

This is not about old vs new compilers. I wanted to use .rodata
variables, but I'll switch to .bss, no problem.

>
> But I am also okay with the current form
> to *satisfy* llvm10 some people may still use.
>
> > +
> > +int output_val1 = 0;
> > +int output_ctx1 = 0;
> > +int output_weak1 = 0;
> > +
> > +/* same "subprog" name in all files, but it's ok because they all are static */
> > +static __noinline int subprog(int x)
> > +{
> > +     /* but different formula */
> > +     return x * 1;
> > +}
> > +
> > +/* Global functions can't be void */
> > +int set_output_val1(int x)
> > +{
> > +     output_val1 = x + subprog(x);
> > +     return x;
> > +}
> > +
> > +/* This function can't be verified as global, as it assumes raw_tp/sys_enter
> > + * context and accesses syscall id (second argument). So we mark it as
> > + * __hidden, so that libbpf will mark it as static in the final object file,
> > + * right before verifying it in the kernel.
> > + *
> > + * But we don't mark it as __hidden here, rather at extern site. __hidden is
> > + * "contaminating" visibility, so it will get propagated from either extern or
> > + * actual definition (including from the losing __weak definition).
> > + */
> > +void set_output_ctx1(__u64 *ctx)
> > +{
> > +     output_ctx1 = ctx[1]; /* long id, same as in BPF_PROG below */
> > +}
> > +
> > +/* this weak instance should win because it's the first one */
> > +__weak int set_output_weak(int x)
> > +{
> > +     output_weak1 = x;
> > +     return x;
> > +}
> > +
> > +extern int set_output_val2(int x);
> > +
> > +/* here we'll force set_output_ctx2() to be __hidden in the final obj file */
> > +__hidden extern void set_output_ctx2(__u64 *ctx);
> > +
> [...]

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