Libbpf eBPF macro BPF_SNPRINTF
Note
This macro was moved from bpf_tracing.h to bpf_helpers.h in v0.5.0
The BPF_SNPRINTF macro is used to make in-BPF string formatting easier.
Definition
/*
* BPF_SNPRINTF wraps the bpf_snprintf helper with variadic arguments instead of
* an array of u64.
*/
#define BPF_SNPRINTF(out, out_size, fmt, args...) \
({ \
static const char ___fmt[] = fmt; \
unsigned long long ___param[___bpf_narg(args)]; \
\
_Pragma("GCC diagnostic push") \
_Pragma("GCC diagnostic ignored \"-Wint-conversion\"") \
___bpf_fill(___param, args); \
_Pragma("GCC diagnostic pop") \
\
bpf_snprintf(out, out_size, ___fmt, \
___param, sizeof(___param)); \
})
Usage
This macro is a wrapper around the bpf_snprintf helper. It places the literal format string in a global variable, this is necessary to get the compiler to emit code that will be accepted by the verifier.
Example
struct {
__uint(type, BPF_MAP_TYPE_RINGBUF);
__uint(max_entries, 4096);
} ringbuf SEC(".maps");
SEC("tc")
int example_prog(struct __sk_buff *ctx)
{
const int size = 100;
char *str_out = bpf_ringbuf_reserve(&ringbuf, size, 0);
if (!str_out)
return TC_ACT_OK;
// `len` is the length of the formatted string including NULL-termination char.
// if `len` > `size` then the string was truncated.
// `len` can also be -EBUSY if the per-CPU memory copy buffer is busy.
long len = BPF_SNPRINTF(str_out, size,
"Got a packet from interface %d, src: %pi4, dst: %pi4, src port: %d, dst port: %d\\n",
ctx->ingress_ifindex,
ctx->remote_ip4,
ctx->local_ip4,
ctx->remote_port,
ctx->local_port
);
bpf_ringbuf_submit(str_out, 0);
return TC_ACT_OK;
}