Date:   Fri, 23 Apr 2021 10:58:01 -0700
From:   Yonghong Song <yhs@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.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 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.

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