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) ||
>> +#endif /* __USDT_BPF_H__ */
