BPF classifier and actions in tc(8)  Linux BPF classifier and actions in tc(8)

NAME
       BPF - BPF programmable classifier and actions for ingress/egress
       queueing disciplines

SYNOPSIS
   eBPF classifier (filter) or action:
       tc filter ... bpf [ object-file OBJ_FILE ] [ section CLS_NAME ] [
       export UDS_FILE ] [ verbose ] [ direct-action | da ] [ skip_hw |
       skip_sw ] [ police POLICE_SPEC ] [ action ACTION_SPEC ] [ classid
       CLASSID ]
       tc action ... bpf [ object-file OBJ_FILE ] [ section CLS_NAME ] [
       export UDS_FILE ] [ verbose ]


   cBPF classifier (filter) or action:
       tc filter ... bpf [ bytecode-file BPF_FILE | bytecode BPF_BYTECODE ] [
       police POLICE_SPEC ] [ action ACTION_SPEC ] [ classid CLASSID ]
       tc action ... bpf [ bytecode-file BPF_FILE | bytecode BPF_BYTECODE ]


DESCRIPTION
       Extended Berkeley Packet Filter ( eBPF ) and classic Berkeley Packet
       Filter (originally known as BPF, for better distinction referred to as
       cBPF here) are both available as a fully programmable and highly
       efficient classifier and actions. They both offer a minimal instruction
       set for implementing small programs which can safely be loaded into the
       kernel and thus executed in a tiny virtual machine from kernel space.
       An in-kernel verifier guarantees that a specified program always
       terminates and neither crashes nor leaks data from the kernel.

       In Linux, it's generally considered that eBPF is the successor of cBPF.
       The kernel internally transforms cBPF expressions into eBPF expressions
       and executes the latter. Execution of them can be performed in an
       interpreter or at setup time, they can be just-in-time compiled
       (JIT'ed) to run as native machine code.

       Currently, the eBPF JIT compiler is available for the following
       architectures:

       *   x86_64 (since Linux 3.18)
       *   arm64 (since Linux 3.18)
       *   s390 (since Linux 4.1)
       *   ppc64 (since Linux 4.8)
       *   sparc64 (since Linux 4.12)
       *   mips64 (since Linux 4.13)
       *   arm32 (since Linux 4.14)
       *   x86_32 (since Linux 4.18)

       Whereas the following architectures have cBPF, but did not (yet) switch
       to eBPF JIT support:

       *   ppc32
       *   sparc32
       *   mips32

       eBPF's instruction set has similar underlying principles as the cBPF
       instruction set, it however is modelled closer to the underlying
       architecture to better mimic native instruction sets with the aim to
       achieve a better run-time performance. It is designed to be JIT'ed with
       a one to one mapping, which can also open up the possibility for
       compilers to generate optimized eBPF code through an eBPF backend that
       performs almost as fast as natively compiled code. Given that LLVM
       provides such an eBPF backend, eBPF programs can therefore easily be
       programmed in a subset of the C language. Other than that, eBPF
       infrastructure also comes with a construct called "maps". eBPF maps are
       key/value stores that are shared between multiple eBPF programs, but
       also between eBPF programs and user space applications.

       For the traffic control subsystem, classifier and actions that can be
       attached to ingress and egress qdiscs can be written in eBPF or cBPF.
       The advantage over other classifier and actions is that eBPF/cBPF
       provides the generic framework, while users can implement their highly
       specialized use cases efficiently. This means that the classifier or
       action written that way will not suffer from feature bloat, and can
       therefore execute its task highly efficient. It allows for non-linear
       classification and even merging the action part into the
       classification. Combined with efficient eBPF map data structures, user
       space can push new policies like classids into the kernel without
       reloading a classifier, or it can gather statistics that are pushed
       into one map and use another one for dynamically load balancing traffic
       based on the determined load, just to provide a few examples.


PARAMETERS
   object-file
       points to an object file that has an executable and linkable format
       (ELF) and contains eBPF opcodes and eBPF map definitions. The LLVM
       compiler infrastructure with clang(1) as a C language front end is one
       project that supports emitting eBPF object files that can be passed to
       the eBPF classifier (more details in the EXAMPLES section). This option
       is mandatory when an eBPF classifier or action is to be loaded.


   section
       is the name of the ELF section from the object file, where the eBPF
       classifier or action resides. By default the section name for the
       classifier is called "classifier", and for the action "action". Given
       that a single object file can contain multiple classifier and actions,
       the corresponding section name needs to be specified, if it differs
       from the defaults.


   export
       points to a Unix domain socket file. In case the eBPF object file also
       contains a section named "maps" with eBPF map specifications, then the
       map file descriptors can be handed off via the Unix domain socket to an
       eBPF "agent" herding all descriptors after tc lifetime. This can be
       some third party application implementing the IPC counterpart for the
       import, that uses them for calling into bpf(2) system call to read out
       or update eBPF map data from user space, for example, for monitoring
       purposes or to push down new policies.


   verbose
       if set, it will dump the eBPF verifier output, even if loading the eBPF
       program was successful. By default, only on error, the verifier log is
       being emitted to the user.


   direct-action | da
       instructs eBPF classifier to not invoke external TC actions, instead
       use the TC actions return codes (TC_ACT_OK, TC_ACT_SHOT etc.) for
       classifiers.


   skip_hw | skip_sw
       hardware offload control flags. By default TC will try to offload
       filters to hardware if possible.	 skip_hw explicitly disables the
       attempt to offload.  skip_sw forces the offload and disables running
       the eBPF program in the kernel.	If hardware offload is not possible
       and this flag was set kernel will report an error and filter will not
       be installed at all.


   police
       is an optional parameter for an eBPF/cBPF classifier that specifies a
       police in tc(1) which is attached to the classifier, for example, on an
       ingress qdisc.


   action
       is an optional parameter for an eBPF/cBPF classifier that specifies a
       subsequent action in tc(1) which is attached to a classifier.


   classid
   flowid
       provides the default traffic control class identifier for this
       eBPF/cBPF classifier. The default class identifier can also be
       overwritten by the return code of the eBPF/cBPF program. A default
       return code of -1 specifies the here provided default class identifier
       to be used. A return code of the eBPF/cBPF program of 0 implies that no
       match took place, and a return code other than these two will override
       the default classid. This allows for efficient, non-linear
       classification with only a single eBPF/cBPF program as opposed to
       having multiple individual programs for various class identifiers which
       would need to reparse packet contents.


   bytecode
       is being used for loading cBPF classifier and actions only. The cBPF
       bytecode is directly passed as a text string in the form of 's,c t f
       k,c t f k,c t f k,...' , where s denotes the number of subsequent
       4-tuples. One such 4-tuple consists of c t f k decimals, where c
       represents the cBPF opcode, t the jump true offset target, f the jump
       false offset target and k the immediate constant/literal. There are
       various tools that generate code in this loadable format, for example,
       bpf_asm that ships with the Linux kernel source tree under tools/net/ ,
       so it is certainly not expected to hack this by hand. The bytecode or
       bytecode-file option is mandatory when a cBPF classifier or action is
       to be loaded.


   bytecode-file
       also being used to load a cBPF classifier or action. It's effectively
       the same as bytecode only that the cBPF bytecode is not passed directly
       via command line, but rather resides in a text file.


EXAMPLES
   eBPF TOOLING
       A full blown example including eBPF agent code can be found inside the
       iproute2 source package under: examples/bpf/

       As prerequisites, the kernel needs to have the eBPF system call namely
       bpf(2) enabled and ships with cls_bpf and act_bpf kernel modules for
       the traffic control subsystem. To enable eBPF/eBPF JIT support,
       depending which of the two the given architecture supports:

	   echo 1 > /proc/sys/net/core/bpf_jit_enable

       A given restricted C file can be compiled via LLVM as:

	   clang -O2 -emit-llvm -c bpf.c -o - | llc -march=bpf -filetype=obj
	   -o bpf.o

       The compiler invocation might still simplify in future, so for now,
       it's quite handy to alias this construct in one way or another, for
       example:

	   __bcc() {
		   clang -O2 -emit-llvm -c $1 -o - | \
		   llc -march=bpf -filetype=obj -o "`basename $1 .c`.o"
	   }

	   alias bcc=__bcc

       A minimal, stand-alone unit, which matches on all traffic with the
       default classid (return code of -1) looks like:


	   #include <linux/bpf.h>

	   #ifndef __section
	   # define __section(x)  __attribute__((section(x), used))
	   #endif

	   __section("classifier") int cls_main(struct __sk_buff *skb)
	   {
		   return -1;
	   }

	   char __license[] __section("license") = "GPL";

       More examples can be found further below in subsection eBPF PROGRAMMING
       as focus here will be on tooling.

       There can be various other sections, for example, also for actions.
       Thus, an object file in eBPF can contain multiple entrance points.
       Always a specific entrance point, however, must be specified when
       configuring with tc. A license must be part of the restricted C code
       and the license string syntax is the same as with Linux kernel modules.
       The kernel reserves its right that some eBPF helper functions can be
       restricted to GPL compatible licenses only, and thus may reject a
       program from loading into the kernel when such a license mismatch
       occurs.

       The resulting object file from the compilation can be inspected with
       the usual set of tools that also operate on normal object files, for
       example objdump(1) for inspecting ELF section headers:


	   objdump -h bpf.o
	   [...]
	   3 classifier	   000007f8  0000000000000000  0000000000000000	 00000040  2**3
			   CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
	   4 action-mark   00000088  0000000000000000  0000000000000000	 00000838  2**3
			   CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
	   5 action-rand   00000098  0000000000000000  0000000000000000	 000008c0  2**3
			   CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
	   6 maps	   00000030  0000000000000000  0000000000000000	 00000958  2**2
			   CONTENTS, ALLOC, LOAD, DATA
	   7 license	   00000004  0000000000000000  0000000000000000	 00000988  2**0
			   CONTENTS, ALLOC, LOAD, DATA
	   [...]

       Adding an eBPF classifier from an object file that contains a
       classifier in the default ELF section is trivial (note that instead of
       "object-file" also shortcuts such as "obj" can be used):

	   bcc bpf.c
	   tc filter add dev em1 parent 1: bpf obj bpf.o flowid 1:1

       In case the classifier resides in ELF section "mycls", then that same
       command needs to be invoked as:

	   tc filter add dev em1 parent 1: bpf obj bpf.o sec mycls flowid 1:1

       Dumping the classifier configuration will tell the location of the
       classifier, in other words that it's from object file "bpf.o" under
       section "mycls":

	   tc filter show dev em1
	   filter parent 1: protocol all pref 49152 bpf
	   filter parent 1: protocol all pref 49152 bpf handle 0x1 flowid 1:1
	   bpf.o:[mycls]

       The same program can also be installed on ingress qdisc side as opposed
       to egress ...

	   tc qdisc add dev em1 handle ffff: ingress
	   tc filter add dev em1 parent ffff: bpf obj bpf.o sec mycls flowid
	   ffff:1

       ... and again dumped from there:

	   tc filter show dev em1 parent ffff:
	   filter protocol all pref 49152 bpf
	   filter protocol all pref 49152 bpf handle 0x1 flowid ffff:1
	   bpf.o:[mycls]

       Attaching a classifier and action on ingress has the restriction that
       it doesn't have an actual underlying queueing discipline. What ingress
       can do is to classify, mangle, redirect or drop packets. When queueing
       is required on ingress side, then ingress must redirect packets to the
       ifb device, otherwise policing can be used. Moreover, ingress can be
       used to have an early drop point of unwanted packets before they hit
       upper layers of the networking stack, perform network accounting with
       eBPF maps that could be shared with egress, or have an early mangle
       and/or redirection point to different networking devices.

       Multiple eBPF actions and classifier can be placed into a single object
       file within various sections. In that case, non-default section names
       must be provided, which is the case for both actions in this example:

	   tc filter add dev em1 parent 1: bpf obj bpf.o flowid 1:1 \
				    action bpf obj bpf.o sec action-mark \
				    action bpf obj bpf.o sec action-rand ok

       The advantage of this is that the classifier and the two actions can
       then share eBPF maps with each other, if implemented in the programs.

       In order to access eBPF maps from user space beyond tc(8) setup
       lifetime, the ownership can be transferred to an eBPF agent via Unix
       domain sockets. There are two possibilities for implementing this:

       1) implementation of an own eBPF agent that takes care of setting up
       the Unix domain socket and implementing the protocol that tc(8)
       dictates. A code example of this can be found inside the iproute2
       source package under: examples/bpf/

       2) use tc exec for transferring the eBPF map file descriptors through a
       Unix domain socket, and spawning an application such as sh(1) . This
       approach's advantage is that tc will place the file descriptors into
       the environment and thus make them available just like stdin, stdout,
       stderr file descriptors, meaning, in case user applications run from
       within this fd-owner shell, they can terminate and restart without
       losing eBPF maps file descriptors. Example invocation with the previous
       classifier and action mixture:

	   tc exec bpf imp /tmp/bpf
	   tc filter add dev em1 parent 1: bpf obj bpf.o exp /tmp/bpf flowid
	   1:1 \
				    action bpf obj bpf.o sec action-mark \
				    action bpf obj bpf.o sec action-rand ok

       Assuming that eBPF maps are shared with classifier and actions, it's
       enough to export them once, for example, from within the classifier or
       action command. tc will setup all eBPF map file descriptors at the time
       when the object file is first parsed.

       When a shell has been spawned, the environment will have a couple of
       eBPF related variables. BPF_NUM_MAPS provides the total number of maps
       that have been transferred over the Unix domain socket. BPF_MAP<X>'s
       value is the file descriptor number that can be accessed in eBPF agent
       applications, in other words, it can directly be used as the file
       descriptor value for the bpf(2) system call to retrieve or alter eBPF
       map values. <X> denotes the identifier of the eBPF map. It corresponds
       to the id member of struct bpf_elf_map  from the tc eBPF map
       specification.

       The environment in this example looks as follows:


	   sh# env | grep BPF
	       BPF_NUM_MAPS=3
	       BPF_MAP1=6
	       BPF_MAP0=5
	       BPF_MAP2=7
	   sh# ls -la /proc/self/fd
	       [...]
	       lrwx------. 1 root root 64 Apr 14 16:46 5 -> anon_inode:bpf-map
	       lrwx------. 1 root root 64 Apr 14 16:46 6 -> anon_inode:bpf-map
	       lrwx------. 1 root root 64 Apr 14 16:46 7 -> anon_inode:bpf-map
	   sh# my_bpf_agent

       eBPF agents are very useful in that they can prepopulate eBPF maps from
       user space, monitor statistics via maps and based on that feedback, for
       example, rewrite classids in eBPF map values during runtime. Given that
       eBPF agents are implemented as normal applications, they can also
       dynamically receive traffic control policies from external controllers
       and thus push them down into eBPF maps to dynamically adapt to network
       conditions. Moreover, eBPF maps can also be shared with other eBPF
       program types (e.g. tracing), thus very powerful combination can
       therefore be implemented.


   eBPF PROGRAMMING
       eBPF classifier and actions are being implemented in restricted C
       syntax (in future, there could additionally be new language frontends
       supported).

       The header file linux/bpf.h provides eBPF helper functions that can be
       called from an eBPF program.  This man page will only provide two
       minimal, stand-alone examples, have a look at examples/bpf from the
       iproute2 source package for a fully fledged flow dissector example to
       better demonstrate some of the possibilities with eBPF.

       Supported 32 bit classifier return codes from the C program and their
       meanings:
	   0 , denotes a mismatch
	   -1 , denotes the default classid configured from the command line
	   else , everything else will override the default classid to provide
	   a facility for non-linear matching

       Supported 32 bit action return codes from the C program and their
       meanings ( linux/pkt_cls.h ):
	   TC_ACT_OK (0) , will terminate the packet processing pipeline and
	   allows the packet to proceed
	   TC_ACT_SHOT (2) , will terminate the packet processing pipeline and
	   drops the packet
	   TC_ACT_UNSPEC (-1) , will use the default action configured from tc
	   (similarly as returning -1 from a classifier)
	   TC_ACT_PIPE (3) , will iterate to the next action, if available
	   TC_ACT_RECLASSIFY (1) , will terminate the packet processing
	   pipeline and start classification from the beginning
	   else , everything else is an unspecified return code

       Both classifier and action return codes are supported in eBPF and cBPF
       programs.

       To demonstrate restricted C syntax, a minimal toy classifier example is
       provided, which assumes that egress packets, for instance originating
       from a container, have previously been marked in interval [0, 255]. The
       program keeps statistics on different marks for user space and maps the
       classid to the root qdisc with the marking itself as the minor handle:


	   #include <stdint.h>
	   #include <asm/types.h>

	   #include <linux/bpf.h>
	   #include <linux/pkt_sched.h>

	   #include "helpers.h"

	   struct tuple {
		   long packets;
		   long bytes;
	   };

	   #define BPF_MAP_ID_STATS	   1 /* agent's map identifier */
	   #define BPF_MAX_MARK		   256

	   struct bpf_elf_map __section("maps") map_stats = {
		   .type	   =	   BPF_MAP_TYPE_ARRAY,
		   .id		   =	   BPF_MAP_ID_STATS,
		   .size_key	   =	   sizeof(uint32_t),
		   .size_value	   =	   sizeof(struct tuple),
		   .max_elem	   =	   BPF_MAX_MARK,
		   .pinning	   =	   PIN_GLOBAL_NS,
	   };

	   static inline void cls_update_stats(const struct __sk_buff *skb,
					       uint32_t mark)
	   {
		   struct tuple *tu;

		   tu = bpf_map_lookup_elem(&map_stats, &mark);
		   if (likely(tu)) {
			   __sync_fetch_and_add(&tu->packets, 1);
			   __sync_fetch_and_add(&tu->bytes, skb->len);
		   }
	   }

	   __section("cls") int cls_main(struct __sk_buff *skb)
	   {
		   uint32_t mark = skb->mark;

		   if (unlikely(mark >= BPF_MAX_MARK))
			   return 0;

		   cls_update_stats(skb, mark);

		   return TC_H_MAKE(TC_H_ROOT, mark);
	   }

	   char __license[] __section("license") = "GPL";

       Another small example is a port redirector which demuxes destination
       port 80 into the interval [8080, 8087] steered by RSS, that can then be
       attached to ingress qdisc. The exercise of adding the egress
       counterpart and IPv6 support is left to the reader:


	   #include <asm/types.h>
	   #include <asm/byteorder.h>

	   #include <linux/bpf.h>
	   #include <linux/filter.h>
	   #include <linux/in.h>
	   #include <linux/if_ether.h>
	   #include <linux/ip.h>
	   #include <linux/tcp.h>

	   #include "helpers.h"

	   static inline void set_tcp_dport(struct __sk_buff *skb, int nh_off,
					    __u16 old_port, __u16 new_port)
	   {
		   bpf_l4_csum_replace(skb, nh_off + offsetof(struct tcphdr, check),
				       old_port, new_port, sizeof(new_port));
		   bpf_skb_store_bytes(skb, nh_off + offsetof(struct tcphdr, dest),
				       &new_port, sizeof(new_port), 0);
	   }

	   static inline int lb_do_ipv4(struct __sk_buff *skb, int nh_off)
	   {
		   __u16 dport, dport_new = 8080, off;
		   __u8 ip_proto, ip_vl;

		   ip_proto = load_byte(skb, nh_off +
					offsetof(struct iphdr, protocol));
		   if (ip_proto != IPPROTO_TCP)
			   return 0;

		   ip_vl = load_byte(skb, nh_off);
		   if (likely(ip_vl == 0x45))
			   nh_off += sizeof(struct iphdr);
		   else
			   nh_off += (ip_vl & 0xF) << 2;

		   dport = load_half(skb, nh_off + offsetof(struct tcphdr, dest));
		   if (dport != 80)
			   return 0;

		   off = skb->queue_mapping & 7;
		   set_tcp_dport(skb, nh_off - BPF_LL_OFF, __constant_htons(80),
				 __cpu_to_be16(dport_new + off));
		   return -1;
	   }

	   __section("lb") int lb_main(struct __sk_buff *skb)
	   {
		   int ret = 0, nh_off = BPF_LL_OFF + ETH_HLEN;

		   if (likely(skb->protocol == __constant_htons(ETH_P_IP)))
			   ret = lb_do_ipv4(skb, nh_off);

		   return ret;
	   }

	   char __license[] __section("license") = "GPL";

       The related helper header file helpers.h in both examples was:


	   /* Misc helper macros. */
	   #define __section(x) __attribute__((section(x), used))
	   #define offsetof(x, y) __builtin_offsetof(x, y)
	   #define likely(x) __builtin_expect(!!(x), 1)
	   #define unlikely(x) __builtin_expect(!!(x), 0)

	   /* Object pinning settings */
	   #define PIN_NONE	  0
	   #define PIN_OBJECT_NS  1
	   #define PIN_GLOBAL_NS  2

	   /* ELF map definition */
	   struct bpf_elf_map {
	       __u32 type;
	       __u32 size_key;
	       __u32 size_value;
	       __u32 max_elem;
	       __u32 flags;
	       __u32 id;
	       __u32 pinning;
	       __u32 inner_id;
	       __u32 inner_idx;
	   };

	   /* Some used BPF function calls. */
	   static int (*bpf_skb_store_bytes)(void *ctx, int off, void *from,
					     int len, int flags) =
		 (void *) BPF_FUNC_skb_store_bytes;
	   static int (*bpf_l4_csum_replace)(void *ctx, int off, int from,
					     int to, int flags) =
		 (void *) BPF_FUNC_l4_csum_replace;
	   static void *(*bpf_map_lookup_elem)(void *map, void *key) =
		 (void *) BPF_FUNC_map_lookup_elem;

	   /* Some used BPF intrinsics. */
	   unsigned long long load_byte(void *skb, unsigned long long off)
	       asm ("llvm.bpf.load.byte");
	   unsigned long long load_half(void *skb, unsigned long long off)
	       asm ("llvm.bpf.load.half");

       Best practice, we recommend to only have a single eBPF classifier
       loaded in tc and perform all necessary matching and mangling from there
       instead of a list of individual classifier and separate actions. Just a
       single classifier tailored for a given use-case will be most efficient
       to run.


   eBPF DEBUGGING
       Both tc filter and action commands for bpf support an optional verbose
       parameter that can be used to inspect the eBPF verifier log. It is
       dumped by default in case of an error.

       In case the eBPF/cBPF JIT compiler has been enabled, it can also be
       instructed to emit a debug output of the resulting opcode image into
       the kernel log, which can be read via dmesg(1) :

	   echo 2 > /proc/sys/net/core/bpf_jit_enable

       The Linux kernel source tree ships additionally under tools/net/ a
       small helper called bpf_jit_disasm that reads out the opcode image dump
       from the kernel log and dumps the resulting disassembly:

	   bpf_jit_disasm -o

       Other than that, the Linux kernel also contains an extensive eBPF/cBPF
       test suite module called test_bpf . Upon ...

	   modprobe test_bpf

       ... it performs a diversity of test cases and dumps the results into
       the kernel log that can be inspected with dmesg(1) . The results can
       differ depending on whether the JIT compiler is enabled or not. In case
       of failed test cases, the module will fail to load. In such cases, we
       urge you to file a bug report to the related JIT authors, Linux kernel
       and networking mailing lists.


   cBPF
       Although we generally recommend switching to implementing eBPF
       classifier and actions, for the sake of completeness, a few words on
       how to program in cBPF will be lost here.

       Likewise, the bpf_jit_enable switch can be enabled as mentioned
       already. Tooling such as bpf_jit_disasm is also independent whether
       eBPF or cBPF code is being loaded.

       Unlike in eBPF, classifier and action are not implemented in restricted
       C, but rather in a minimal assembler-like language or with the help of
       other tooling.

       The raw interface with tc takes opcodes directly. For example, the most
       minimal classifier matching on every packet resulting in the default
       classid of 1:1 looks like:

	   tc filter add dev em1 parent 1: bpf bytecode '1,6 0 0 4294967295,'
	   flowid 1:1

       The first decimal of the bytecode sequence denotes the number of
       subsequent 4-tuples of cBPF opcodes. As mentioned, such a 4-tuple
       consists of c t f k decimals, where c represents the cBPF opcode, t the
       jump true offset target, f the jump false offset target and k the
       immediate constant/literal. Here, this denotes an unconditional return
       from the program with immediate value of -1.

       Thus, for egress classification, Willem de Bruijn implemented a minimal
       stand-alone helper tool under the GNU General Public License version 2
       for iptables(8) BPF extension, which abuses the libpcap internal
       classic BPF compiler, his code derived here for usage with tc(8) :


	   #include <pcap.h>
	   #include <stdio.h>

	   int main(int argc, char **argv)
	   {
		   struct bpf_program prog;
		   struct bpf_insn *ins;
		   int i, ret, dlt = DLT_RAW;

		   if (argc < 2 || argc > 3)
			   return 1;
		   if (argc == 3) {
			   dlt = pcap_datalink_name_to_val(argv[1]);
			   if (dlt == -1)
				   return 1;
		   }

		   ret = pcap_compile_nopcap(-1, dlt, &prog, argv[argc - 1],
					     1, PCAP_NETMASK_UNKNOWN);
		   if (ret)
			   return 1;

		   printf("%d,", prog.bf_len);
		   ins = prog.bf_insns;

		   for (i = 0; i < prog.bf_len - 1; ++ins, ++i)
			   printf("%u %u %u %u,", ins->code,
				  ins->jt, ins->jf, ins->k);
		   printf("%u %u %u %u",
			  ins->code, ins->jt, ins->jf, ins->k);

		   pcap_freecode(&prog);
		   return 0;
	   }

       Given this small helper, any tcpdump(8) filter expression can be abused
       as a classifier where a match will result in the default classid:

	   bpftool EN10MB 'tcp[tcpflags] & tcp-syn != 0' > /var/bpf/tcp-syn
	   tc filter add dev em1 parent 1: bpf bytecode-file /var/bpf/tcp-syn
	   flowid 1:1

       Basically, such a minimal generator is equivalent to:

	   tcpdump -iem1 -ddd 'tcp[tcpflags] & tcp-syn != 0' | tr '  ',' >
	   /var/bpf/tcp-syn

       Since libpcap does not support all Linux' specific cBPF extensions in
       its compiler, the Linux kernel also ships under tools/net/ a minimal
       BPF assembler called bpf_asm for providing full control. For detailed
       syntax and semantics on implementing such programs by hand, see
       references under FURTHER READING .

       Trivial toy example in bpf_asm for classifying IPv4/TCP packets, saved
       in a text file called foobar :


	   ldh [12]
	   jne #0x800, drop
	   ldb [23]
	   jneq #6, drop
	   ret #-1
	   drop: ret #0

       Similarly, such a classifier can be loaded as:

	   bpf_asm foobar > /var/bpf/tcp-syn
	   tc filter add dev em1 parent 1: bpf bytecode-file /var/bpf/tcp-syn
	   flowid 1:1

       For BPF classifiers, the Linux kernel provides additionally under
       tools/net/ a small BPF debugger called bpf_dbg , which can be used to
       test a classifier against pcap files, single-step or add various
       breakpoints into the classifier program and dump register contents
       during runtime.

       Implementing an action in classic BPF is rather limited in the sense
       that packet mangling is not supported. Therefore, it's generally
       recommended to make the switch to eBPF, whenever possible.


FURTHER READING
       Further and more technical details about the BPF architecture can be
       found in the Linux kernel source tree under
       Documentation/networking/filter.txt .

       Further details on eBPF tc(8) examples can be found in the iproute2
       source tree under examples/bpf/ .


SEE ALSO
       tc(8), tc-ematch(8) bpf(2) bpf(4)


AUTHORS
       Manpage written by Daniel Borkmann.

       Please report corrections or improvements to the Linux kernel
       networking mailing list: <netdev@vger.kernel.org>

iproute2			  18 May 2015
					   BPF classifier and actions in tc(8)