Re: [PATCH bpf-next 1/7] libbpf: add BPF-side of USDT support

[Hengqi Chen <hengqi.chen@gmail.com>]

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__ */