BPF: The Status of BTF
Producers, Consumers
Arnaldo Carvalho de Melo
acme@redhat.com
Red Hat Inc.
What is this about?
- BPF Type Format
- Initially just data types
- Compact
- File/Number
- Global variables
Spreading
- Most new BPF features use it
- CO-RE
- BPF trampolines
- struct ops
- Dynamic re-linking
- KRSI
Always present
- (Kinda) Required by now
- Compact: 100s MBs (DWARF) to few MBs
- /sys/kernel/btf/vmlinux
Compact
$ uname -a Linux quaco 5.5.0-rc6+ #2 SMP Tue Jan 14 13:13:43 -03 2020 x86_64 GNU/Linux $ ls -la /sys/kernel/btf/vmlinux -r--r--r--. 1 3234516 /sys/kernel/btf/vmlinux $
pahole
- First producer
- Loads DWARF
- Encodes BTF
- CTF reading/encoding was there already
btfdiff
- pahole -F btf file.o
- pahole -F dwarf --flat_arrays file.o
- diff them
- Should produce the same results
- Regression tests
fullcircle
- pfunct --compile file.o
- Using DWARF
- BTF output needs some work, but feasible
- Build resulting file
- codiff debug info in both
- Should match
- Again: regression tests
pahole + kernel
- CONFIG_DEBUG_INFO_BTF=y
- Part of the kernel build process
- Fast enough so far
- Parallelization possible
- Recent work on making elfutils thread safe
- For the DWARF reading part
- One thread per CU (Compile Unit)
pahole + BTF
- Default to using /sys/kernel/btf/vmlinux
- Lots of other options
- To query various aspects of the kernel structs
$ pahole -s | sort -k2 -nr | head -10 cmp_data 290904 1 dec_data 274520 1 cpu_entry_area 217088 0 pglist_data 172928 4 saved_cmdlines_buffer 131104 1 debug_store_buffers 131072 0 hid_parser 110848 1 hid_local 110608 0 zonelist 81936 0 e820_table 64004 0 $
$ pahole -i list_head | head task_struct vm_area_struct rw_semaphore mutex device wait_queue_head work_struct address_space file deferred_split $
$ pahole -C task_struct --hex | grep 'struct list_head'
struct list_head rcu_tasks_holdout_list; /* 0x388 0x10 */
struct list_head tasks; /* 0x3b8 0x10 */
struct list_head children; /* 0x4d8 0x10 */
struct list_head sibling; /* 0x4e8 0x10 */
struct list_head ptraced; /* 0x500 0x10 */
struct list_head ptrace_entry; /* 0x510 0x10 */
struct list_head thread_group; /* 0x568 0x10 */
struct list_head thread_node; /* 0x578 0x10 */
struct list_head cg_list; /* 0x8d8 0x10 */
struct list_head pi_state_list; /* 0x900 0x10 */
struct list_head perf_event_list; /* 0x970 0x10 */
$
$ pahole -f bpf_prog perf_event: prog bpf_prog_aux: linked_prog bpf_prog_aux: prog bpf_prog_offload: prog bpf_prog_array_item: prog net: flow_dissector_prog net_device: xdp_prog sk_filter: prog seccomp_filter: prog bpf_raw_tracepoint: prog bpf_verifier_env: prog sock_reuseport: prog ns_get_path_bpf_prog_args: prog bpf_prog_list: prog xdp_attachment_info: prog sk_psock_progs: msg_parser sk_psock_progs: skb_parser sk_psock_progs: skb_verdict bpf_lwt_prog: prog $
$ pahole -C sk_filter
struct sk_filter {
refcount_t refcnt; /* 0 4 */
/* XXX 4 bytes hole, try to pack */
struct callback_head rcu; /* 8 16 */
struct bpf_prog * prog; /* 24 8 */
/* size: 32, cachelines: 1, members: 3 */
/* sum members: 28, holes: 1, sum holes: 4 */
/* last cacheline: 32 bytes */
};
$ pahole -C xdp_attachment_info
struct xdp_attachment_info {
struct bpf_prog * prog; /* 0 8 */
u32 flags; /* 8 4 */
/* size: 16, cachelines: 1, members: 2 */
/* padding: 4 */
/* last cacheline: 16 bytes */
};
$ pahole bpf_insn
struct bpf_insn {
__u8 code; /* 0 1 */
__u8 dst_reg:4; /* 1: 0 1 */
__u8 src_reg:4; /* 1: 4 1 */
__s16 off; /* 2 2 */
__s32 imm; /* 4 4 */
/* size: 8, cachelines: 1, members: 5 */
/* last cacheline: 8 bytes */
};
$
bpftool
- BPF_BTF_GET_FD_BY_ID
- Pretty prints map keys/values
- Intermixes source code with bytecode/JITed code
# bpftool version bpftool v5.5.0-rc4
# bpftool btf help
Usage: bpftool btf { show | list } [id BTF_ID]
bpftool btf dump BTF_SRC [format FORMAT]
bpftool btf help
BTF_SRC := { id BTF_ID | prog PROG |
map MAP [{key | value | kv | all}] |
file FILE }
FORMAT := { raw | c }
MAP := { id MAP_ID | pinned FILE | name MAP_NAME }
PROG := { id PROG_ID | pinned FILE | tag PROG_TAG |
name PROG_NAME }
OPTIONS := { {-j|--json} [{-p|--pretty}] | {-f|--bpffs} |
{-m|--mapcompat} | {-n|--nomount} }
#
# bpftool prog | grep syscall_unaugme -A 100 314: tracepoint name syscall_unaugme tag 57cd311f2e27366b gpl loaded_at 2020-01-17T14:38:26-0300 uid 0 xlated 16B jited 40B memlock 4096B btf_id 81 315: tracepoint name sys_enter_conne tag f2131823a4275143 gpl loaded_at 2020-01-17T14:38:26-0300 uid 0 xlated 272B jited 168B memlock 4096B map_ids 228,223 btf_id 81 316: tracepoint name sys_enter_sendt tag 1962ce5cb9c415b3 gpl loaded_at 2020-01-17T14:38:26-0300 uid 0 xlated 272B jited 168B memlock 4096B map_ids 228,223 btf_id 81 317: tracepoint name sys_enter_open tag 0e59c3ac2bea5280 gpl loaded_at 2020-01-17T14:38:26-0300 uid 0 xlated 304B jited 185B memlock 4096B map_ids 228,223 btf_id 81 318: tracepoint name sys_enter_opena tag 0baf443610f59837 gpl loaded_at 2020-01-17T14:38:26-0300 uid 0 xlated 304B jited 185B memlock 4096B map_ids 228,223 btf_id 81 319: tracepoint name sys_enter_renam tag e6f565ab94fdb0b8 gpl loaded_at 2020-01-17T14:38:26-0300 uid 0 xlated 520B jited 300B memlock 4096B map_ids 228,223 btf_id 81 320: tracepoint name sys_enter_renam tag 01ad4488b2d5f42f gpl loaded_at 2020-01-17T14:38:26-0300 uid 0 xlated 520B jited 300B memlock 4096B map_ids 228,223 btf_id 81 321: tracepoint name sys_enter tag 0bc3fc9d11754ba1 gpl loaded_at 2020-01-17T14:38:26-0300 uid 0 xlated 272B jited 231B memlock 4096B map_ids 227,228,225 btf_id 81 322: tracepoint name sys_exit tag 29c7ae234d79bd5c gpl loaded_at 2020-01-17T14:38:26-0300 uid 0 xlated 184B jited 186B memlock 4096B map_ids 227,226 btf_id 81 #
# bpftool btf dump map id 168 key [2] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
# bpftool btf dump map id 168 value format c
typedef _Bool bool;
typedef unsigned short __u16;
typedef __u16 u16;
struct syscall {
bool enabled;
u16 string_args_len[6];
};
# bpftool btf dump file /sys/kernel/btf/vmlinux format c | \
grep 'struct fpu {' -A10
struct fpu {
unsigned int last_cpu;
long unsigned int avx512_timestamp;
long: 64;
long: 64;
long: 64;
long: 64;
long: 64;
long: 64;
union fpregs_state state;
};
# pahole fpu
struct fpu {
unsigned int last_cpu; /* 0 4 */
/* XXX 4 bytes hole, try to pack */
long unsigned int avx512_timestamp; /* 8 8 */
/* XXX 48 bytes hole, try to pack */
/* --- cacheline 1 boundary (64 bytes) --- */
union fpregs_state state; /* 64 4096 */
/* size: 4160, cachelines: 65, members: 3 */
/* sum members: 4108, holes: 2, sum holes: 52 */
};
#
# pahole -F dwarf /lib/modules/5.5.0-rc6+/build/vmlinux -C fpu
struct fpu {
unsigned int last_cpu; /* 0 4 */
/* XXX 4 bytes hole, try to pack */
long unsigned int avx512_timestamp; /* 8 8 */
/* XXX 48 bytes hole, try to pack */
/* --- cacheline 1 boundary (64 bytes) --- */
union fpregs_state state __attribute__((__aligned__(64))); /* 64 4096 */
/* size: 4160, cachelines: 65, members: 3 */
/* sum members: 4108, holes: 2, sum holes: 52 */
/* forced alignments: 1, forced holes: 1, sum forced holes: 48 */
} __attribute__((__aligned__(64)));
kernel
- First consumer
- BPF_BTF_LOAD
- Validates BTF
- BPF_BTF_GET_FD_BY_ID
- Does not allow writes to BTF types
kernel validates BTF
- Validates header
- BTF_MAGIC
- BTF_VERSION
- flags
Some validations performed
$ grep btf_verifier_log kernel/bpf/btf.c
btf_verifier_log(env, "Exceeded max num of types");
btf_verifier_log_type(env, t, "nr_bits exceeds %zu",
btf_verifier_log_type(env, t, "nr_bits exceeds type_size");
btf_verifier_log_type(env, t, "Unsupported encoding");
btf_verifier_log_type(env, t, "Invalid type_id");
btf_verifier_log_type(env, t, "Invalid name");
btf_verifier_log_type(env, t, "Invalid btf_info kind_flag");
btf_verifier_log_type(env, t, "Expected size:%zu",
btf_verifier_log_type(env, t, "Loop detected");
btf_verifier_log(env, "Unaligned type_off");
btf_verifier_log(env, "No type found");
btf_verifier_log(env, "String section is not at the end");
btf_verifier_log(env, "Invalid string section");
btf_verifier_log(env, "Section overlap found");
btf_verifier_log(env, "Total section length too long");
btf_verifier_log(env, "Unsupported section found");
btf_verifier_log(env, "hdr_len not found");
btf_verifier_log(env, "btf_header not found");
btf_verifier_log(env, "Unsupported btf_header");
btf_verifier_log(env, "Invalid magic");
btf_verifier_log(env, "Unsupported version");
btf_verifier_log(env, "Unsupported flags");
$
kernel validating BTF (excerpts)
# perf ftrace -G '*btf*' perf trace -e *sleep sleep 1
7) | bpf_btf_load() {
7) | capable() {
7) 1.527 us | }
7) | btf_new_fd() {
7) 0.101 us | btf_sec_info_cmp();
7) | btf_struct_check_meta() {
7) 0.135 us | btf_name_valid_identifier.isra.12();
7) 0.109 us | __btf_verifier_log_type();
7) 0.107 us | btf_name_valid_identifier.isra.12();
7) 0.108 us | btf_verifier_log_member();
7) 3.642 us | }
7) | btf_int_check_meta() {
7) 0.100 us | __btf_verifier_log_type();
7) 0.315 us | }
7) | btf_ref_type_check_meta()
7) + 49.743 us | }
- Raw BTF
- Always available
- All types used in vmlinux
- All functions exported via kallsyms
- CONFIG_DEBUG_INFO_BTF=y
- Uses pahole to convert from DWARF
$ uname -a Linux quaco 5.5.0-rc6+ #2 SMP Tue Jan 14 13:13:43 -03 2020 x86_64 GNU/Linux $ ls -la /sys/kernel/btf/vmlinux -r--r--r--. 1 3234516 /sys/kernel/btf/vmlinux $
$ size -Ad ~/git/build/v5.5-rc6+/vmlinux | grep -E 'BTF|debug' .BTF 3234488 .debug_aranges 184000 .debug_info 219842532 .debug_abbrev 6409858 .debug_line 23369539 .debug_frame 2693968 .debug_str 3800630 .debug_loc 14871978 .debug_ranges 15582016 $
BPF CO-RE
- Compile Once, Run Everywhere
- Relocation field offset records
- __builtin_preserve_access_index(x)
- Compare kernel types
- With the ones in the BPF prog
- Fixup offsets
- Check for field existence in a kernel
- Records bitfield accesses
- LINUX_KERNEL_VERSION
- CONFIG_ kconfig entries
- Constant propagation
- Relocation records where it is used
- From /boot/config-$(uname -r)
- Fallback to /proc/config.gz
- Or via bpf_object_open_opts.kconfig
- tools/testing/selftests/bpf/prog_tests/core_extern.c
- tools/testing/selftests/bpf/progs/test_core_extern.c
$ bpftool btf dump file /sys/kernel/btf/vmlinux format c | head #ifndef BPF_NO_PRESERVE_ACCESS_INDEX #pragma clang attribute push (__attribute__((preserve_access_index)), apply_to = record) #endif typedef signed char __s8; typedef unsigned char __u8; typedef short int __s16; $
First struct
$ bpftool btf dump file /sys/kernel/btf/vmlinux format c | \
grep '^struct ' -m 1 -A3 -B10
typedef phys_addr_t resource_size_t;
typedef struct {
int counter;
} atomic_t;
typedef struct {
s64 counter;
} atomic64_t;
struct list_head {
struct list_head *next;
struct list_head *prev;
};
$
runqslower
- Reimplementation of BCC's runqslower
- Traces high scheduling delays
- BPF skeleton
- auto-generated from BPF object file
- memory-mapped interface to global data
- Auto-generation of "relocatable" vmlinux.h
- Necessary for BTF-typed raw tracepoints with direct memory access
$ make -C tools/bpf/runqslower/In file included from runqslower.bpf.c:3: .output/vmlinux.h:2:15: error: attribute 'preserve_access_index' is not supported by '#pragma clang attribute' #pragma clang attribute push (__attribute__((preserve_access_index)), apply_to = record) ^ .output/vmlinux.h:111541:15: error: '#pragma clang attribute pop' with no matching '#pragma clang attribute push' #pragma clang attribute pop ^ 2 errors generated. make: *** [Makefile:58: .output/runqslower.bpf.o] Error 1 make: Leaving directory '/home/acme/git/bpf/tools/bpf/runqslower' $ clang -v clang version 10.0.0 (https://git.llvm.org/git/clang.git/ 65acf43270ea2894dffa0d0b292b92402f80c8cb) (https://git.llvm.org/git/llvm.git/ a461b7a03cde32cd6560dcbcea23ec51dfd1e522)
- SVN mirror abandoned after move to MONOREPO/github
- https://llvm.org/docs/Proposals/GitHubMove.html
- Use single repo llvm
- https://github.com/llvm/llvm-project
- build it to get close to llvm 11
- To get pragma attribute preserve_access_index
- & llvm to generate datasec and variables type info
- Needed for ruqslower args
$ make -C tools/bpf/runqslower/ make: Entering directory '/home/acme/git/bpf/tools/bpf/runqslower' LINK /home/acme/git/bpf/tools/bpf/runqslower/.output//libbpf/libbpf.a LINK /home/acme/git/bpf/tools/bpf/runqslower/.output/bpftool INSTALL bpftool GEN vmlinux.h BPF runqslower.bpf.o GEN-SKEL runqslower.skel.h CC runqslower.o BINARY runqslower make: Leaving directory '/home/acme/git/bpf/tools/bpf/runqslower'
$ file tools/bpf/runqslower/.output/runqslower tools/bpf/runqslower/.output/runqslower: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 3.2.0, BuildID[sha1]=e7986148fa9629ef9ca8ea1fd76c3a0bbdf70b06, with debug_info, not stripped
$ ls -la tools/bpf/runqslower/.output/runqslower -rwxrwxr-x. 1 507016 tools/bpf/runqslower/.output/runqslower $
$ strip tools/bpf/runqslower/.output/runqslower $ ls -la tools/bpf/runqslower/.output/runqslower -rwxrwxr-x. 1 249208 tools/bpf/runqslower/.output/runqslower $
$ sudo tools/bpf/runqslower/.output/runqslower 500 Tracing run queue latency higher than 500 us TIME COMM PID LAT(us) 19:00:11 swapper/3 22111 1904 19:00:30 weechat 41 1244 19:00:30 weechat 41 1316 ^C $
- Boilerplate being reduced
- Everything needed in one binary
- Small
- Runs in any kernel
- With subset of fields used
- Similar to BCC
- Userspace part
- BPF/kernel part
- Natural struct field deref
- No bpf_probe_read
$ cat tools/bpf/runqslower/runqslower.h
/* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
#ifndef __RUNQSLOWER_H
#define __RUNQSLOWER_H
#define TASK_COMM_LEN 16
struct event {
char task[TASK_COMM_LEN];
__u64 delta_us;
pid_t pid;
};
#endif /* __RUNQSLOWER_H */
$
- tools/bpf/runqslower/runqslower.c
- Uses argp.h for command line options
- Initializes global data with those options
- Sends the BPF bytecode to the kernel
- Sets up perf ring buffer
- Reads events
- All using tools/lib/bpf (libbpf)
#include <bpf.h>
#include "runqslower.h"
#include "runqslower.skel.h"
void handle_event(void *ctx, int cpu, void *data, __u32 data_sz)
{
const struct event *e = data;
strftime(ts, sizeof(ts), "%H:%M:%S", tm);
printf("%-8s %-16s %-6d %14llu\n", ts, e->task, e->pid, e->delta_us);
}
int main(int argc, char **argv)
{
err = argp_parse(&argp, argc, argv, 0, NULL, NULL);
obj = runqslower_bpf__open();
obj->rodata->min_us = env.min_us; // arg, BPF global var
err = runqslower_bpf__load(obj);
err = runqslower_bpf__attach(obj);
printf("%-8s %-16s %-6s %14s\n", "TIME", "COMM", "PID", "LAT(us)");
pb_opts.sample_cb = handle_event;
pb_opts.lost_cb = handle_lost_events;
pb = perf_buffer__new(bpf_map__fd(obj->maps.events), 64, &pb_opts);
while ((err = perf_buffer__poll(pb, 100)) >= 0);
}
- tools/bpf/runqslower/runqslower.bpf.c
- Uses per pid has map for timestamps
- To calculate the deltas/latencies
- Connects to BTF tracepoints
- To use normal pointer dereference
- Sets up events
- Pushes to userspace via perf ring buffer
$ cat tools/bpf/runqslower/runqslower.bpf.c #include "vmlinux.h" #include <bpf_helpers.h> #include "runqslower.h"
const volatile __u64 min_us = 0; const volatile pid_t targ_pid = 0;
struct {
__uint(type, BPF_MAP_TYPE_HASH);
__uint(max_entries, 10240);
__type(key, u32);
__type(value, u64);
} start SEC(".maps");
struct {
__uint(type, BPF_MAP_TYPE_PERF_EVENT_ARRAY);
__uint(key_size, sizeof(u32));
__uint(value_size, sizeof(u32));
} events SEC(".maps");
$ readelf -sW tools/bpf/runqslower/.output/runqslower.bpf.o | \
egrep min_us\|targ_pid
Num: Size Type Bind Vis Ndx Name
18: 8 OBJECT GLOBAL DEFAULT 5 min_us
20: 4 OBJECT GLOBAL DEFAULT 5 targ_pid
$
__always_inline static int trace_enqueue(u32 tgid, u32 pid)
{
if (!pid || (targ_pid && targ_pid != pid))
return 0;
u64 ts = bpf_ktime_get_ns();
bpf_map_update_elem(&start, &pid, &ts, 0);
return 0;
}
SEC("tp_btf/sched_wakeup") int handle__sched_wakeup(u64 *ctx)
{
// TP_PROTO(struct task_struct *p)
struct task_struct *p = (void *)ctx[0];
return trace_enqueue(p->tgid, p->pid);
}
SEC("tp_btf/sched_switch") int handle__sched_switch(u64 *ctx)
{
// TP_PROTO(bool preempt, struct task_struct *prev,
// struct task_struct *next)
struct task_struct *prev = (struct task_struct *)ctx[1];
struct task_struct *next = (struct task_struct *)ctx[2];
struct event event = {};
u32 pid = next->pid;
u64 *tstamp = bpf_map_lookup_elem(&start, &pid); if (!tstamp) return 0; /* missed enqueue */
u64 delta_us = (bpf_ktime_get_ns() - *tstamp) / 1000; if (min_us && delta_us <= min_us) return 0;
event.pid = pid; event.delta_us = delta_us; bpf_get_current_comm(&event.task, sizeof(event.task));
bpf_perf_event_output(ctx, &events, BPF_F_CURRENT_CPU, &event, sizeof(event));
bpf_map_delete_elem(&start, &pid); return 0; }
$ ls -la tools/bpf/runqslower/.output/runqslower.bpf.o -rw-rw-r--. 1 31688 tools/bpf/runqslower/.output/runqslower.bpf.o
$ file tools/bpf/runqslower/.output/runqslower.bpf.o
tools/bpf/runqslower/.output/runqslower.bpf.o: ELF 64-bit LSB relocatable,
eBPF, version 1 (SYSV), not stripped
$
$ eu-readelf -SW tools/bpf/runqslower/.output/runqslower.bpf.o | grep \.rel Section Headers: [Nr] Name Type Size [11] .reltp_btf/sched_wakeup REL 20 [13] .reltp_btf/sched_switch REL 60 [14] .rel.BTF REL 50 [15] .rel.BTF.ext REL 460 $
BPF_PROG_TYPE_STRUCT_OPS
- TCP Congestion control algorithms
- Pointer table
- Faster turnaround for testing new algos
- Builds on top of CO-RE infra
- General mechanism for any kernel struct ops
$ cd net/ipv4/ $ grep -l 'tcp_register_congestion_control(&' tcp_*.c tcp_bbr.c tcp_bic.c tcp_cdg.c tcp_cubic.c tcp_dctcp.c tcp_highspeed.c tcp_htcp.c tcp_hybla.c tcp_illinois.c tcp_lp.c tcp_nv.c tcp_scalable.c tcp_vegas.c tcp_veno.c tcp_westwood.c tcp_yeah.c $
- Data Center TCP
- tools/testing/selftests/bpf/progs/bpf_dctcp.c
- Just for testing
- Not the same as net/ipv4/tcp_dctcp.c
- Helpers for tcp_sock/inet_connection_sock structs/logic
- structs with same name as in kernel
- But with just the fields needed for DCTCP
static unsigned int dctcp_shift_g = 4; /* g = 1/2^4 */ static unsigned int dctcp_alpha_on_init = DCTCP_MAX_ALPHA;
// tools/testing/selftests/bpf/bpf_tcp_helpers.h
struct tcp_sock {
struct inet_connection_sock inet_conn;
__u32 rcv_nxt, snd_nxt, snd_una;
__u8 ecn_flags;
__u32 delivered;
__u32 delivered_ce;
__u32 snd_cwnd, snd_cwnd_cnt, snd_cwnd_clamp;
__u32 snd_ssthresh;
__u8 syn_data:1, /* SYN includes data */
syn_fastopen:1, /* SYN includes Fast Open option */
syn_fastopen_exp:1,/* SYN includes Fast Open exp. option */
syn_fastopen_ch:1, /* Active TFO re-enabling probe */
syn_data_acked:1,/* data in SYN is acked by SYN-ACK */
save_syn:1, /* Save headers of SYN packet */
is_cwnd_limited:1,/* forward progress limited by snd_cwnd? */
syn_smc:1; /* SYN includes SMC */
__u32 max_packets_out;
__u32 lsndtime;
__u32 prior_cwnd;
} __attribute__((preserve_access_index));
$ pahole tcp_sock | grep size: -A4 /* size: 2192, cachelines: 35, members: 135 */ /* sum members: 2181, holes: 3, sum holes: 8 */ /* sum bitfield members: 24 bits (3 bytes) */ /* paddings: 3, sum paddings: 10 */ /* last cacheline: 16 bytes */ $
- To register/unregister/introspect struct ops
- Receives tcp_congestion_ops pointer
- Populates map
- bpf map dump
- Shows how many users (refcnt)
SEC("struct_ops/dctcp_init")
void BPF_PROG(dctcp_init, struct sock *sk)
{
const struct tcp_sock *tp = tcp_sk(sk);
struct dctcp *ca = inet_csk_ca(sk);
ca->prior_rcv_nxt = tp->rcv_nxt;
ca->dctcp_alpha = min(dctcp_alpha_on_init, DCTCP_MAX_ALPHA);
ca->loss_cwnd = 0;
ca->ce_state = 0;
dctcp_reset(tp, ca);
}
SEC("struct_ops/dctcp_ssthresh")
__u32 BPF_PROG(dctcp_ssthresh, struct sock *sk)
{
struct dctcp *ca = inet_csk_ca(sk);
struct tcp_sock *tp = tcp_sk(sk);
ca->loss_cwnd = tp->snd_cwnd;
return max(tp->snd_cwnd - ((tp->snd_cwnd * ca->dctcp_alpha) > 11U), 2U);
}
BPF trampolines
- Kernel code call into BPF, nearly zero overhead
- Arch dependent trampoline generation
- Converts native calling convention
- Into BPF calling convention
- kernel_to_BPF converts kernel args
- btf_distill_func_proto()
- Uses BTF to cast args into u64 array for BPF's ctx arg
- BPF_TRACE_ENTRY/EXIT replaces kprobe/kretprobe
- More than tracing: XDP minus retpoline
- http://git.kernel.org/torvalds/c/fec56f5890d93fc2e
Dynamic re-linking
- Original motivation: XDP chaining
- BPF_PROG_TYPE_EXT
- Replace placeholder BPF functions
- In rootlet XDP program
- While they are running
- BTF used to verify function signatures
- New function must match replaced sig
perf
- BPF_BTF_GET_FD_BY_ID
- PERF_RECORD_BPF_EVENT
- Annotates source code with JITed code
gcc
- Jose Marchesi
- BPF target in trunk since Sep/9/2019
- BTF not yet being produced
- Patch in the works
- Will use pahole for testing
gcc helper builtins
$ grep __builtin_bpf testsuite/gcc.target/bpf/*.c | \ sed -r 's/.*(__builtin_bpf[a-z0-9_]+).*/\1/g' | sort -u | head __builtin_bpf_helper_bind __builtin_bpf_helper_clone_redirect __builtin_bpf_helper_csum_diff __builtin_bpf_helper_csum_update __builtin_bpf_helper_current_task_under_cgroup __builtin_bpf_helper_fib_lookup __builtin_bpf_helper_get_cgroup_classid __builtin_bpf_helper_get_current_cgroup_id __builtin_bpf_helper_get_current_comm __builtin_bpf_helper_get_current_pid_tgid $