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Date: Fri, 23 Apr 2021 10:35:01 -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 10:18 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.
>
>>
>> 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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