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
...
>> +}
>> +
>> +/* Fetch USDT argument *arg* (zero-indexed) and put its value into *res.
>> + * Returns 0 on success; negative error, otherwise.
>> + * On error *res is guaranteed to be set to zero.
>> + */
>> +__hidden __weak
>> +int bpf_usdt_arg(struct pt_regs *ctx, int arg, long *res)
>> +{
>> + struct __bpf_usdt_spec *spec;
>> + struct __bpf_usdt_arg_spec *arg_spec;
>> + unsigned long val;
>> + int err, spec_id;
>> +
>> + *res = 0;
>> +
>> + spec_id = __bpf_usdt_spec_id(ctx);
>> + if (spec_id < 0)
>> + return -ESRCH;
>> +
>> + spec = bpf_map_lookup_elem(&__bpf_usdt_specs, &spec_id);
>> + if (!spec)
>> + return -ESRCH;
>> +
>> + if (arg >= spec->arg_cnt)
>> + return -ENOENT;
>> +
>> + arg_spec = &spec->args[arg];
>> + switch (arg_spec->arg_type) {
>> + case BPF_USDT_ARG_CONST:
>> + val = arg_spec->val_off;
>> + break;
>> + case BPF_USDT_ARG_REG:
>> + err = bpf_probe_read_kernel(&val, sizeof(val), (void *)ctx + arg_spec->reg_off);
>> + if (err)
>> + return err;
>> + break;
>> + case BPF_USDT_ARG_REG_DEREF:
>> + err = bpf_probe_read_kernel(&val, sizeof(val), (void *)ctx + arg_spec->reg_off);
>> + if (err)
>> + return err;
>> + err = bpf_probe_read_user(&val, sizeof(val), (void *)val + arg_spec->val_off);
>> + if (err)
>> + return err;
Can you elaborate more on these two probe read call ?
I replace bpf_probe_read_kernel with bpf_probe_read_user, it also works.
Thanks.
>> + break;
>> + default:
>> + return -EINVAL;
>> + }
>> +
>> + val <<= arg_spec->arg_bitshift;
>> + if (arg_spec->arg_signed)
>> + val = ((long)val) >> arg_spec->arg_bitshift;
>> + * BPF_USDT serves the same purpose for USDT handlers as BPF_PROG for
>> + * tp_btf/fentry/fexit BPF programs and BPF_KPROBE for kprobes.
>> + * Original struct pt_regs * context is preserved as 'ctx' argument.
>> + */
>> +#define BPF_USDT(name, args...) \
>> +name(struct pt_regs *ctx); \
>> +static __attribute__((always_inline)) typeof(name(0)) \
>> +____##name(struct pt_regs *ctx, ##args); \
>> +typeof(name(0)) name(struct pt_regs *ctx) \
>> +{ \
>> + _Pragma("GCC diagnostic push") \
>> + _Pragma("GCC diagnostic ignored \"-Wint-conversion\"") \
>> + return ____##name(___bpf_usdt_args(args)); \
>> + _Pragma("GCC diagnostic pop") \
>> +} \
>> +static __attribute__((always_inline)) typeof(name(0)) \
>> +____##name(struct pt_regs *ctx, ##args)
>> +
>> +#endif /* __USDT_BPF_H__ */
