Libbpf eBPF macro BPF_CORE_READ_BITFIELD
The BPF_CORE_READ_BITFIELD macro extracts a bitfield from a given structure in a CO-RE relocatable way.
Definition
#define BPF_CORE_READ_BITFIELD(s, field) ({ \
const void *p = (const void *)s + __CORE_RELO(s, field, BYTE_OFFSET); \
unsigned long long val; \
\
/* This is a so-called barrier_var() operation that makes specified \
* variable "a black box" for optimizing compiler. \
* It forces compiler to perform BYTE_OFFSET relocation on p and use \
* its calculated value in the switch below, instead of applying \
* the same relocation 4 times for each individual memory load. \
*/ \
asm volatile("" : "=r"(p) : "0"(p)); \
\
switch (__CORE_RELO(s, field, BYTE_SIZE)) { \
case 1: val = *(const unsigned char *)p; break; \
case 2: val = *(const unsigned short *)p; break; \
case 4: val = *(const unsigned int *)p; break; \
case 8: val = *(const unsigned long long *)p; break; \
default: val = 0; break; \
} \
val <<= __CORE_RELO(s, field, LSHIFT_U64); \
if (__CORE_RELO(s, field, SIGNED)) \
val = ((long long)val) >> __CORE_RELO(s, field, RSHIFT_U64); \
else \
val = val >> __CORE_RELO(s, field, RSHIFT_U64); \
val; \
})
Usage
BPF_CORE_READ_BITFIELD extract bitfield, identified by s->field, and return its value as u64. This is a variant of the BPF_CORE_READ_BITFIELD_PROBED macro. This macro is using direct memory reads and should be used from BPF program types that support such functionality (e.g., typed raw tracepoints).
Example
// SPDX-License-Identifier: GPL-2.0
/* Copyright (c) 2019 Facebook */
struct tcp_sock {
u32 snd_cwnd; /* Sending congestion window */
/* [...] */
u8 chrono_type : 2, /* current chronograph type */
repair : 1,
tcp_usec_ts : 1, /* TSval values in usec */
is_sack_reneg:1, /* in recovery from loss with SACK reneg? */
is_cwnd_limited:1;/* forward progress limited by snd_cwnd? */
/* [...] */
u32 max_packets_out; /* max packets_out in last window */
}
static inline bool tcp_is_cwnd_limited(const struct sock *sk)
{
const struct tcp_sock *tp = tcp_sk(sk);
/* If in slow start, ensure cwnd grows to twice what was ACKed. */
if (tcp_in_slow_start(tp))
return tp->snd_cwnd < 2 * tp->max_packets_out;
return !!BPF_CORE_READ_BITFIELD(tp, is_cwnd_limited);
}