lkml.org 
[lkml]   [2013]   [Dec]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Subject[RFC PATCH tip 0/5] tracing filters with BPF
Date
Hi All,

the following set of patches adds BPF support to trace filters.

Trace filters can be written in C and allow safe read-only access to any
kernel data structure. Like systemtap but with safety guaranteed by kernel.

The user can do:
cat bpf_program > /sys/kernel/debug/tracing/.../filter
if tracing event is either static or dynamic via kprobe_events.

The filter program may look like:
void filter(struct bpf_context *ctx)
{
char devname[4] = "eth5";
struct net_device *dev;
struct sk_buff *skb = 0;

dev = (struct net_device *)ctx->regs.si;
if (bpf_memcmp(dev->name, devname, 4) == 0) {
char fmt[] = "skb %p dev %p eth5\n";
bpf_trace_printk(fmt, skb, dev, 0, 0);
}
}

The kernel will do static analysis of bpf program to make sure that it cannot
crash the kernel (doesn't have loops, valid memory/register accesses, etc).
Then kernel will map bpf instructions to x86 instructions and let it
run in the place of trace filter.

To demonstrate performance I did a synthetic test:
dev = init_net.loopback_dev;
do_gettimeofday(&start_tv);
for (i = 0; i < 1000000; i++) {
struct sk_buff *skb;
skb = netdev_alloc_skb(dev, 128);
kfree_skb(skb);
}
do_gettimeofday(&end_tv);
time = end_tv.tv_sec - start_tv.tv_sec;
time *= USEC_PER_SEC;
time += (long long)((long)end_tv.tv_usec - (long)start_tv.tv_usec);

printk("1M skb alloc/free %lld (usecs)\n", time);

no tracing
[ 33.450966] 1M skb alloc/free 145179 (usecs)

echo 1 > enable
[ 97.186379] 1M skb alloc/free 240419 (usecs)
(tracing slows down kfree_skb() due to event_buffer_lock/buffer_unlock_commit)

echo 'name==eth5' > filter
[ 139.644161] 1M skb alloc/free 302552 (usecs)
(running filter_match_preds() for every skb and discarding
event_buffer is even slower)

cat bpf_prog > filter
[ 171.150566] 1M skb alloc/free 199463 (usecs)
(JITed bpf program is safely checking dev->name == eth5 and discarding)

echo 0 > enable
[ 258.073593] 1M skb alloc/free 144919 (usecs)
(tracing is disabled, performance is back to original)

The C program compiled into BPF and then JITed into x86 is faster than
filter_match_preds() approach (199-145 msec vs 302-145 msec)

tracing+bpf is a tool for safe read-only access to variables without recompiling
the kernel and without affecting running programs.

BPF filters can be written manually (see tools/bpf/trace/filter_ex1.c)
or better compiled from restricted C via GCC or LLVM

Q: What is the difference between existing BPF and extended BPF?
A:
Existing BPF insn from uapi/linux/filter.h
struct sock_filter {
__u16 code; /* Actual filter code */
__u8 jt; /* Jump true */
__u8 jf; /* Jump false */
__u32 k; /* Generic multiuse field */
};

Extended BPF insn from linux/bpf.h
struct bpf_insn {
__u8 code; /* opcode */
__u8 a_reg:4; /* dest register*/
__u8 x_reg:4; /* source register */
__s16 off; /* signed offset */
__s32 imm; /* signed immediate constant */
};

opcode encoding is the same between old BPF and extended BPF.
Original BPF has two 32-bit registers.
Extended BPF has ten 64-bit registers.
That is the main difference.

Old BPF was using jt/jf fields for jump-insn only.
New BPF combines them into generic 'off' field for jump and non-jump insns.
k==imm field has the same meaning.

Thanks

Alexei Starovoitov (5):
Extended BPF core framework
Extended BPF JIT for x86-64
Extended BPF (64-bit BPF) design document
use BPF in tracing filters
tracing filter examples in BPF

Documentation/bpf_jit.txt | 204 +++++++
arch/x86/Kconfig | 1 +
arch/x86/net/Makefile | 1 +
arch/x86/net/bpf64_jit_comp.c | 625 ++++++++++++++++++++
arch/x86/net/bpf_jit_comp.c | 23 +-
arch/x86/net/bpf_jit_comp.h | 35 ++
include/linux/bpf.h | 149 +++++
include/linux/bpf_jit.h | 129 +++++
include/linux/ftrace_event.h | 3 +
include/trace/bpf_trace.h | 27 +
include/trace/ftrace.h | 14 +
kernel/Makefile | 1 +
kernel/bpf_jit/Makefile | 3 +
kernel/bpf_jit/bpf_check.c | 1054 ++++++++++++++++++++++++++++++++++
kernel/bpf_jit/bpf_run.c | 452 +++++++++++++++
kernel/trace/Kconfig | 1 +
kernel/trace/Makefile | 1 +
kernel/trace/bpf_trace_callbacks.c | 191 ++++++
kernel/trace/trace.c | 7 +
kernel/trace/trace.h | 11 +-
kernel/trace/trace_events.c | 9 +-
kernel/trace/trace_events_filter.c | 61 +-
kernel/trace/trace_kprobe.c | 6 +
lib/Kconfig.debug | 15 +
tools/bpf/llvm/README.txt | 6 +
tools/bpf/trace/Makefile | 34 ++
tools/bpf/trace/README.txt | 15 +
tools/bpf/trace/filter_ex1.c | 52 ++
tools/bpf/trace/filter_ex1_orig.c | 23 +
tools/bpf/trace/filter_ex2.c | 74 +++
tools/bpf/trace/filter_ex2_orig.c | 47 ++
tools/bpf/trace/trace_filter_check.c | 82 +++
32 files changed, 3332 insertions(+), 24 deletions(-)
create mode 100644 Documentation/bpf_jit.txt
create mode 100644 arch/x86/net/bpf64_jit_comp.c
create mode 100644 arch/x86/net/bpf_jit_comp.h
create mode 100644 include/linux/bpf.h
create mode 100644 include/linux/bpf_jit.h
create mode 100644 include/trace/bpf_trace.h
create mode 100644 kernel/bpf_jit/Makefile
create mode 100644 kernel/bpf_jit/bpf_check.c
create mode 100644 kernel/bpf_jit/bpf_run.c
create mode 100644 kernel/trace/bpf_trace_callbacks.c
create mode 100644 tools/bpf/llvm/README.txt
create mode 100644 tools/bpf/trace/Makefile
create mode 100644 tools/bpf/trace/README.txt
create mode 100644 tools/bpf/trace/filter_ex1.c
create mode 100644 tools/bpf/trace/filter_ex1_orig.c
create mode 100644 tools/bpf/trace/filter_ex2.c
create mode 100644 tools/bpf/trace/filter_ex2_orig.c
create mode 100644 tools/bpf/trace/trace_filter_check.c

--
1.7.9.5



\
 
 \ /
  Last update: 2013-12-03 05:41    [W:0.174 / U:0.208 seconds]
©2003-2017 Jasper Spaans. hosted at Digital OceanAdvertise on this site