Message-ID: <CAEf4BzaH4Sr5HenM1+-j4d=r=juw8pacPpXZM3YBxJu2S_uPwg@mail.gmail.com>
Date:   Fri, 23 Apr 2021 10:59:58 -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 Fri, Apr 23, 2021 at 10:58 AM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 4/23/21 10:55 AM, Andrii Nakryiko wrote:
> > On Fri, Apr 23, 2021 at 10:35 AM Yonghong Song <yhs@...com> wrote:
> >>
> >>
> >>
> >> On 4/23/21 10:18 AM, Andrii Nakryiko wrote:
> >>> 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.
> >>
> >> I see. You can actually hone one "const volatile ing my_tid __weak = 0"
> >> and another "long syscall_id __weak". This way, you will be able to
> >> test both .rodata and .bss section.
> >
> > I wonder if you meant to have one my_tid __weak in .bss and another
> > my_tid __weak in .rodata. Or just my_tid in .bss and syscall_id in
> > .rodata?
> >
> > If the former (mixing ELF sections across definitions of the same
> > symbol), then it's disallowed right now. libbpf will error out on
> > mismatched sections. I tested this with normal compilation, it does
> > work and the final section is the section of the winner.
> >
> > But I think that's quite confusing, actually, so I'm going to leave it
> > disallowed for now. E.g., if one file expects a read-write variable
> > and another expects that same variable to be read-only, and the winner
> > ends up being read-only one, then the file expecting read-write will
> > essentially have incorrect code (and will be rejected by BPF verifier,
> > if anything attempts to write). So I think it's better to reject it at
> > the linking time.
> >
> > But I'll do one (my_tid) as .bss, and another (syscall_id) as .rodata.
>
> I mean this one. Permitting the same variable in both .bss and .rodata
> sections is never a good practice.

Ok, cool, that's what we do right now. I wonder why it is allowed by
user-space linkers, it seems dangerous.

>
> >
> >>
> >>>
> >>>>
> >>>> 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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