From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Hengqi Chen <hengqi.chen@gmail.com>
Cc: Andrii Nakryiko <andrii@kernel.org>, bpf <bpf@vger.kernel.org>,
Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Kernel Team <kernel-team@fb.com>,
Alan Maguire <alan.maguire@oracle.com>,
Dave Marchevsky <davemarchevsky@fb.com>
Subject: Re: [PATCH bpf-next 1/7] libbpf: add BPF-side of USDT support
Date: Wed, 30 Mar 2022 22:48:14 -0700 [thread overview]
Message-ID: <CAEf4Bza-=c=d-aDz0Ruv2AZrn5fh+5HqL-gvUWa7c9=o+PuWYA@mail.gmail.com> (raw)
In-Reply-To: <605dc1f0-2c66-25f0-ef76-a3c052fcc2d8@gmail.com>
On Wed, Mar 30, 2022 at 8:36 AM Hengqi Chen <hengqi.chen@gmail.com> wrote:
>
>
>
> On 2022/3/30 11:10 AM, Hengqi Chen wrote:
> > On 2022/3/25 1:29 PM, Andrii Nakryiko wrote:
> >> Add BPF-side implementation of libbpf-provided USDT support. This
> >> consists of single header library, usdt.bpf.h, which is meant to be used
> >> from user's BPF-side source code. This header is added to the list of
> >> installed libbpf header, along bpf_helpers.h and others.
> >>
> >> BPF-side implementation consists of two BPF maps:
> >> - spec map, which contains "a USDT spec" which encodes information
> >> necessary to be able to fetch USDT arguments and other information
> >> (argument count, user-provided cookie value, etc) at runtime;
> >> - IP-to-spec-ID map, which is only used on kernels that don't support
> >> BPF cookie feature. It allows to lookup spec ID based on the place
> >> in user application that triggers USDT program.
> >>
> >> These maps have default sizes, 256 and 1024, which are chosen
> >> conservatively to not waste a lot of space, but handling a lot of common
> >> cases. But there could be cases when user application needs to either
> >> trace a lot of different USDTs, or USDTs are heavily inlined and their
> >> arguments are located in a lot of differing locations. For such cases it
> >> might be necessary to size those maps up, which libbpf allows to do by
> >> overriding BPF_USDT_MAX_SPEC_CNT and BPF_USDT_MAX_IP_CNT macros.
>
> >> +
> >> +__weak struct {
> >> + __uint(type, BPF_MAP_TYPE_ARRAY);
> >> + __uint(max_entries, BPF_USDT_MAX_SPEC_CNT);
> >> + __type(key, int);
> >> + __type(value, struct __bpf_usdt_spec);
> >> +} __bpf_usdt_specs SEC(".maps");
> >> +
> >> +__weak struct {
> >> + __uint(type, BPF_MAP_TYPE_HASH);
> >> + __uint(max_entries, BPF_USDT_MAX_IP_CNT);
> >> + __type(key, long);
> >> + __type(value, struct __bpf_usdt_spec);
> >
> > type should be int.
> >
> >> +} __bpf_usdt_specs_ip_to_id SEC(".maps");
>
> These weak symbols make BPF object open failed:
>
> libbpf: No offset found in symbol table for VAR __bpf_usdt_specs
> libbpf: Error finalizing .BTF: -2.
>
> bpf_object_open
> bpf_object__finalize_btf
> btf_finalize_data
> btf_fixup_datasec
> find_elf_var_offset
>
> This is because during BTF fixup, we only allow GLOBAL VAR.
>
> Applying the following diff can workaround the issue:
>
> + unsigned char bind = ELF64_ST_BIND(sym->st_info);
>
> - if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL ||
> + if ((bind != STB_GLOBAL && bind != STB_WEAK) ||
>
>
Interesting that selftests don't run into this bug, probably because
BPF linker converts STB_WEAK into STB_GLOBAL? I'll check that, thanks
for catching!
> >> +#endif /* __USDT_BPF_H__ */
next prev parent reply other threads:[~2022-03-31 5:48 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-25 5:29 [PATCH bpf-next 0/7] Add libbpf support for USDTs Andrii Nakryiko
2022-03-25 5:29 ` [PATCH bpf-next 1/7] libbpf: add BPF-side of USDT support Andrii Nakryiko
2022-03-30 3:10 ` Hengqi Chen
2022-03-30 15:22 ` Hengqi Chen
2022-03-31 5:44 ` Andrii Nakryiko
2022-03-30 15:36 ` Hengqi Chen
2022-03-31 5:48 ` Andrii Nakryiko [this message]
2022-03-31 5:44 ` Andrii Nakryiko
2022-03-31 11:30 ` Alan Maguire
2022-03-31 18:49 ` Andrii Nakryiko
2022-03-31 20:52 ` Andrii Nakryiko
2022-03-31 18:34 ` program local storage. Was: " Alexei Starovoitov
2022-03-31 20:13 ` Andrii Nakryiko
2022-04-01 0:38 ` Alexei Starovoitov
2022-04-01 16:56 ` Andrii Nakryiko
2022-03-25 5:29 ` [PATCH bpf-next 2/7] libbpf: wire up USDT API and bpf_link integration Andrii Nakryiko
2022-03-30 3:24 ` Hengqi Chen
2022-03-31 5:56 ` Andrii Nakryiko
2022-03-31 12:13 ` Alan Maguire
2022-03-31 19:02 ` Andrii Nakryiko
2022-03-25 5:29 ` [PATCH bpf-next 3/7] libbpf: add USDT notes parsing and resolution logic Andrii Nakryiko
2022-03-31 13:37 ` Alan Maguire
2022-03-31 19:13 ` Andrii Nakryiko
2022-03-25 5:29 ` [PATCH bpf-next 4/7] libbpf: wire up spec management and other arch-independent USDT logic Andrii Nakryiko
2022-03-31 14:49 ` Alan Maguire
2022-03-31 19:16 ` Andrii Nakryiko
2022-03-25 5:29 ` [PATCH bpf-next 5/7] libbpf: add x86-specific USDT arg spec parsing logic Andrii Nakryiko
2022-03-31 15:13 ` Alan Maguire
2022-03-31 19:20 ` Andrii Nakryiko
2022-03-25 5:29 ` [PATCH bpf-next 6/7] selftests/bpf: add basic USDT selftests Andrii Nakryiko
2022-03-31 15:54 ` Alan Maguire
2022-03-31 19:28 ` Andrii Nakryiko
2022-03-25 5:29 ` [PATCH bpf-next 7/7] selftests/bpf: add urandom_read shared lib and USDTs Andrii Nakryiko
2022-03-31 22:13 ` Alan Maguire
2022-04-01 16:59 ` Andrii Nakryiko
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