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Documentation/networking/filter.txt

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		filter.txt: Linux Socket Filtering
		Written by: Jay Schulist <jschlst@samba.org>
		Linux Socket Filtering aka Berkeley Packet Filter (BPF)
		=======================================================

		Introduction
		============

		Linux Socket Filtering is derived from the Berkeley
		Packet Filter. There are some distinct differences between
		the BSD and Linux Kernel Filtering.

		Linux Socket Filtering (LSF) allows a user-space program to
		attach a filter onto any socket and allow or disallow certain
		types of data to come through the socket. LSF follows exactly
		the same filter code structure as the BSD Berkeley Packet Filter
		(BPF), so referring to the BSD bpf.4 manpage is very helpful in
		creating filters.

		LSF is much simpler than BPF. One does not have to worry about
		devices or anything like that. You simply create your filter
		code, send it to the kernel via the SO_ATTACH_FILTER option and
		if your filter code passes the kernel check on it, you then
		immediately begin filtering data on that socket.

		You can also detach filters from your socket via the
		SO_DETACH_FILTER option. This will probably not be used much
		since when you close a socket that has a filter on it the
		filter is automagically removed. The other less common case
		may be adding a different filter on the same socket where you had another
		filter that is still running: the kernel takes care of removing
		the old one and placing your new one in its place, assuming your
		filter has passed the checks, otherwise if it fails the old filter
		will remain on that socket.

		SO_LOCK_FILTER option allows to lock the filter attached to a
		socket. Once set, a filter cannot be removed or changed. This allows
		one process to setup a socket, attach a filter, lock it then drop
		privileges and be assured that the filter will be kept until the
		socket is closed.

		Examples
		========

		Ioctls-
		setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_FILTER, &Filter, sizeof(Filter));
		setsockopt(sockfd, SOL_SOCKET, SO_DETACH_FILTER, &value, sizeof(value));
		setsockopt(sockfd, SOL_SOCKET, SO_LOCK_FILTER, &value, sizeof(value));

		See the BSD bpf.4 manpage and the BSD Packet Filter paper written by
		Steven McCanne and Van Jacobson of Lawrence Berkeley Laboratory.
		------------

		Linux Socket Filtering (LSF) is derived from the Berkeley Packet Filter.
		Though there are some distinct differences between the BSD and Linux
		Kernel filtering, but when we speak of BPF or LSF in Linux context, we
		mean the very same mechanism of filtering in the Linux kernel.

		BPF allows a user-space program to attach a filter onto any socket and
		allow or disallow certain types of data to come through the socket. LSF
		follows exactly the same filter code structure as BSD's BPF, so referring
		to the BSD bpf.4 manpage is very helpful in creating filters.

		On Linux, BPF is much simpler than on BSD. One does not have to worry
		about devices or anything like that. You simply create your filter code,
		send it to the kernel via the SO_ATTACH_FILTER option and if your filter
		code passes the kernel check on it, you then immediately begin filtering
		data on that socket.

		You can also detach filters from your socket via the SO_DETACH_FILTER
		option. This will probably not be used much since when you close a socket
		that has a filter on it the filter is automagically removed. The other
		less common case may be adding a different filter on the same socket where
		you had another filter that is still running: the kernel takes care of
		removing the old one and placing your new one in its place, assuming your
		filter has passed the checks, otherwise if it fails the old filter will
		remain on that socket.

		SO_LOCK_FILTER option allows to lock the filter attached to a socket. Once
		set, a filter cannot be removed or changed. This allows one process to
		setup a socket, attach a filter, lock it then drop privileges and be
		assured that the filter will be kept until the socket is closed.

		The biggest user of this construct might be libpcap. Issuing a high-level
		filter command like `tcpdump -i em1 port 22` passes through the libpcap
		internal compiler that generates a structure that can eventually be loaded
		via SO_ATTACH_FILTER to the kernel. `tcpdump -i em1 port 22 -ddd`
		displays what is being placed into this structure.

		Although we were only speaking about sockets here, BPF in Linux is used
		in many more places. There's xt_bpf for netfilter, cls_bpf in the kernel
		qdisc layer, SECCOMP-BPF (SECure COMPuting [1]), and lots of other places
		such as team driver, PTP code, etc where BPF is being used.

		[1] Documentation/prctl/seccomp_filter.txt

		Original BPF paper:

		Steven McCanne and Van Jacobson. 1993. The BSD packet filter: a new
		architecture for user-level packet capture. In Proceedings of the
		USENIX Winter 1993 Conference Proceedings on USENIX Winter 1993
		Conference Proceedings (USENIX'93). USENIX Association, Berkeley,
		CA, USA, 2-2. [http://www.tcpdump.org/papers/bpf-usenix93.pdf]

		Structure
		---------

		User space applications include <linux/filter.h> which contains the
		following relevant structures:

		struct sock_filter { /* Filter block */
		__u16 code; /* Actual filter code */
		__u8 jt; /* Jump true */
		__u8 jf; /* Jump false */
		__u32 k; /* Generic multiuse field */
		};

		Such a structure is assembled as an array of 4-tuples, that contains
		a code, jt, jf and k value. jt and jf are jump offsets and k a generic
		value to be used for a provided code.

		struct sock_fprog { /* Required for SO_ATTACH_FILTER. */
		unsigned short len; /* Number of filter blocks */
		struct sock_filter __user *filter;
		};

		For socket filtering, a pointer to this structure (as shown in
		follow-up example) is being passed to the kernel through setsockopt(2).

		Example
		-------

		#include <sys/socket.h>
		#include <sys/types.h>
		#include <arpa/inet.h>
		#include <linux/if_ether.h>
		/* ... */

		/* From the example above: tcpdump -i em1 port 22 -dd */
		struct sock_filter code[] = {
		{ 0x28, 0, 0, 0x0000000c },
		{ 0x15, 0, 8, 0x000086dd },
		{ 0x30, 0, 0, 0x00000014 },
		{ 0x15, 2, 0, 0x00000084 },
		{ 0x15, 1, 0, 0x00000006 },
		{ 0x15, 0, 17, 0x00000011 },
		{ 0x28, 0, 0, 0x00000036 },
		{ 0x15, 14, 0, 0x00000016 },
		{ 0x28, 0, 0, 0x00000038 },
		{ 0x15, 12, 13, 0x00000016 },
		{ 0x15, 0, 12, 0x00000800 },
		{ 0x30, 0, 0, 0x00000017 },
		{ 0x15, 2, 0, 0x00000084 },
		{ 0x15, 1, 0, 0x00000006 },
		{ 0x15, 0, 8, 0x00000011 },
		{ 0x28, 0, 0, 0x00000014 },
		{ 0x45, 6, 0, 0x00001fff },
		{ 0xb1, 0, 0, 0x0000000e },
		{ 0x48, 0, 0, 0x0000000e },
		{ 0x15, 2, 0, 0x00000016 },
		{ 0x48, 0, 0, 0x00000010 },
		{ 0x15, 0, 1, 0x00000016 },
		{ 0x06, 0, 0, 0x0000ffff },
		{ 0x06, 0, 0, 0x00000000 },
		};

		struct sock_fprog bpf = {
		.len = ARRAY_SIZE(code),
		.filter = code,
		};

		sock = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
		if (sock < 0)
		/* ... bail out ... */

		ret = setsockopt(sock, SOL_SOCKET, SO_ATTACH_FILTER, &bpf, sizeof(bpf));
		if (ret < 0)
		/* ... bail out ... */

		/* ... */
		close(sock);

		The above example code attaches a socket filter for a PF_PACKET socket
		in order to let all IPv4/IPv6 packets with port 22 pass. The rest will
		be dropped for this socket.

		The setsockopt(2) call to SO_DETACH_FILTER doesn't need any arguments
		and SO_LOCK_FILTER for preventing the filter to be detached, takes an
		integer value with 0 or 1.

		Note that socket filters are not restricted to PF_PACKET sockets only,
		but can also be used on other socket families.

		Summary of system calls:

		* setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_FILTER, &val, sizeof(val));
		* setsockopt(sockfd, SOL_SOCKET, SO_DETACH_FILTER, &val, sizeof(val));
		* setsockopt(sockfd, SOL_SOCKET, SO_LOCK_FILTER, &val, sizeof(val));

		Normally, most use cases for socket filtering on packet sockets will be
		covered by libpcap in high-level syntax, so as an application developer
		you should stick to that. libpcap wraps its own layer around all that.

		Unless i) using/linking to libpcap is not an option, ii) the required BPF
		filters use Linux extensions that are not supported by libpcap's compiler,
		iii) a filter might be more complex and not cleanly implementable with
		libpcap's compiler, or iv) particular filter codes should be optimized
		differently than libpcap's internal compiler does; then in such cases
		writing such a filter "by hand" can be of an alternative. For example,
		xt_bpf and cls_bpf users might have requirements that could result in
		more complex filter code, or one that cannot be expressed with libpcap
		(e.g. different return codes for various code paths). Moreover, BPF JIT
		implementors may wish to manually write test cases and thus need low-level
		access to BPF code as well.

		BPF engine and instruction set
		------------------------------

		Under tools/net/ there's a small helper tool called bpf_asm which can
		be used to write low-level filters for example scenarios mentioned in the
		previous section. Asm-like syntax mentioned here has been implemented in
		bpf_asm and will be used for further explanations (instead of dealing with
		less readable opcodes directly, principles are the same). The syntax is
		closely modelled after Steven McCanne's and Van Jacobson's BPF paper.

		The BPF architecture consists of the following basic elements:

		Element Description

		A 32 bit wide accumulator
		X 32 bit wide X register
		M[] 16 x 32 bit wide misc registers aka "scratch memory
		store", addressable from 0 to 15

		A program, that is translated by bpf_asm into "opcodes" is an array that
		consists of the following elements (as already mentioned):

		op:16, jt:8, jf:8, k:32

		The element op is a 16 bit wide opcode that has a particular instruction
		encoded. jt and jf are two 8 bit wide jump targets, one for condition
		"jump if true", the other one "jump if false". Eventually, element k
		contains a miscellaneous argument that can be interpreted in different
		ways depending on the given instruction in op.

		The instruction set consists of load, store, branch, alu, miscellaneous
		and return instructions that are also represented in bpf_asm syntax. This
		table lists all bpf_asm instructions available resp. what their underlying
		opcodes as defined in linux/filter.h stand for:

		Instruction Addressing mode Description

		ld 1, 2, 3, 4, 10 Load word into A
		ldi 4 Load word into A
		ldh 1, 2 Load half-word into A
		ldb 1, 2 Load byte into A
		ldx 3, 4, 5, 10 Load word into X
		ldxi 4 Load word into X
		ldxb 5 Load byte into X

		st 3 Store A into M[]
		stx 3 Store X into M[]

		jmp 6 Jump to label
		ja 6 Jump to label
		jeq 7, 8 Jump on k == A
		jneq 8 Jump on k != A
		jne 8 Jump on k != A
		jlt 8 Jump on k < A
		jle 8 Jump on k <= A
		jgt 7, 8 Jump on k > A
		jge 7, 8 Jump on k >= A
		jset 7, 8 Jump on k & A

		add 0, 4 A + <x>
		sub 0, 4 A - <x>
		mul 0, 4 A * <x>
		div 0, 4 A / <x>
		mod 0, 4 A % <x>
		neg 0, 4 !A
		and 0, 4 A & <x>
		or 0, 4 A \| <x>
		xor 0, 4 A ^ <x>
		lsh 0, 4 A << <x>
		rsh 0, 4 A >> <x>

		tax Copy A into X
		txa Copy X into A

		ret 4, 9 Return

		The next table shows addressing formats from the 2nd column:

		Addressing mode Syntax Description

		0 x/%x Register X
		1 [k] BHW at byte offset k in the packet
		2 [x + k] BHW at the offset X + k in the packet
		3 M[k] Word at offset k in M[]
		4 #k Literal value stored in k
		5 4([k]&0xf) Lower nibble 4 at byte offset k in the packet
		6 L Jump label L
		7 #k,Lt,Lf Jump to Lt if true, otherwise jump to Lf
		8 #k,Lt Jump to Lt if predicate is true
		9 a/%a Accumulator A
		10 extension BPF extension

		The Linux kernel also has a couple of BPF extensions that are used along
		with the class of load instructions by "overloading" the k argument with
		a negative offset + a particular extension offset. The result of such BPF
		extensions are loaded into A.

		Possible BPF extensions are shown in the following table:

		Extension Description

		len skb->len
		proto skb->protocol
		type skb->pkt_type
		poff Payload start offset
		ifidx skb->dev->ifindex
		nla Netlink attribute of type X with offset A
		nlan Nested Netlink attribute of type X with offset A
		mark skb->mark
		queue skb->queue_mapping
		hatype skb->dev->type
		rxhash skb->rxhash
		cpu raw_smp_processor_id()
		vlan_tci vlan_tx_tag_get(skb)
		vlan_pr vlan_tx_tag_present(skb)

		These extensions can also be prefixed with '#'.
		Examples for low-level BPF:

		** ARP packets:

		ldh [12]
		jne #0x806, drop
		ret #-1
		drop: ret #0

		** IPv4 TCP packets:

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

		** (Accelerated) VLAN w/ id 10:

		ld vlan_tci
		jneq #10, drop
		ret #-1
		drop: ret #0

		** SECCOMP filter example:

		ld [4] /* offsetof(struct seccomp_data, arch) */
		jne #0xc000003e, bad /* AUDIT_ARCH_X86_64 */
		ld [0] /* offsetof(struct seccomp_data, nr) */
		jeq #15, good /* __NR_rt_sigreturn */
		jeq #231, good /* __NR_exit_group */
		jeq #60, good /* __NR_exit */
		jeq #0, good /* __NR_read */
		jeq #1, good /* __NR_write */
		jeq #5, good /* __NR_fstat */
		jeq #9, good /* __NR_mmap */
		jeq #14, good /* __NR_rt_sigprocmask */
		jeq #13, good /* __NR_rt_sigaction */
		jeq #35, good /* __NR_nanosleep */
		bad: ret #0 /* SECCOMP_RET_KILL */
		good: ret #0x7fff0000 /* SECCOMP_RET_ALLOW */

		The above example code can be placed into a file (here called "foo"), and
		then be passed to the bpf_asm tool for generating opcodes, output that xt_bpf
		and cls_bpf understands and can directly be loaded with. Example with above
		ARP code:

		$ ./bpf_asm foo
		4,40 0 0 12,21 0 1 2054,6 0 0 4294967295,6 0 0 0,

		In copy and paste C-like output:

		$ ./bpf_asm -c foo
		{ 0x28, 0, 0, 0x0000000c },
		{ 0x15, 0, 1, 0x00000806 },
		{ 0x06, 0, 0, 0xffffffff },
		{ 0x06, 0, 0, 0000000000 },

		In particular, as usage with xt_bpf or cls_bpf can result in more complex BPF
		filters that might not be obvious at first, it's good to test filters before
		attaching to a live system. For that purpose, there's a small tool called
		bpf_dbg under tools/net/ in the kernel source directory. This debugger allows
		for testing BPF filters against given pcap files, single stepping through the
		BPF code on the pcap's packets and to do BPF machine register dumps.

		Starting bpf_dbg is trivial and just requires issuing:

		# ./bpf_dbg

		In case input and output do not equal stdin/stdout, bpf_dbg takes an
		alternative stdin source as a first argument, and an alternative stdout
		sink as a second one, e.g. `./bpf_dbg test_in.txt test_out.txt`.

		Other than that, a particular libreadline configuration can be set via
		file "~/.bpf_dbg_init" and the command history is stored in the file
		"~/.bpf_dbg_history".

		Interaction in bpf_dbg happens through a shell that also has auto-completion
		support (follow-up example commands starting with '>' denote bpf_dbg shell).
		The usual workflow would be to ...

		> load bpf 6,40 0 0 12,21 0 3 2048,48 0 0 23,21 0 1 1,6 0 0 65535,6 0 0 0
		Loads a BPF filter from standard output of bpf_asm, or transformed via
		e.g. `tcpdump -iem1 -ddd port 22 \| tr '\n' ','`. Note that for JIT
		debugging (next section), this command creates a temporary socket and
		loads the BPF code into the kernel. Thus, this will also be useful for
		JIT developers.

		> load pcap foo.pcap
		Loads standard tcpdump pcap file.

		> run [<n>]
		bpf passes:1 fails:9
		Runs through all packets from a pcap to account how many passes and fails
		the filter will generate. A limit of packets to traverse can be given.

		> disassemble
		l0: ldh [12]
		l1: jeq #0x800, l2, l5
		l2: ldb [23]
		l3: jeq #0x1, l4, l5
		l4: ret #0xffff
		l5: ret #0
		Prints out BPF code disassembly.

		> dump
		/* { op, jt, jf, k }, */
		{ 0x28, 0, 0, 0x0000000c },
		{ 0x15, 0, 3, 0x00000800 },
		{ 0x30, 0, 0, 0x00000017 },
		{ 0x15, 0, 1, 0x00000001 },
		{ 0x06, 0, 0, 0x0000ffff },
		{ 0x06, 0, 0, 0000000000 },
		Prints out C-style BPF code dump.

		> breakpoint 0
		breakpoint at: l0: ldh [12]
		> breakpoint 1
		breakpoint at: l1: jeq #0x800, l2, l5
		...
		Sets breakpoints at particular BPF instructions. Issuing a `run` command
		will walk through the pcap file continuing from the current packet and
		break when a breakpoint is being hit (another `run` will continue from
		the currently active breakpoint executing next instructions):

		> run
		-- register dump --
		pc: [0] <-- program counter
		code: [40] jt[0] jf[0] k[12] <-- plain BPF code of current instruction
		curr: l0: ldh [12] <-- disassembly of current instruction
		A: [00000000][0] <-- content of A (hex, decimal)
		X: [00000000][0] <-- content of X (hex, decimal)
		M[0,15]: [00000000][0] <-- folded content of M (hex, decimal)
		-- packet dump -- <-- Current packet from pcap (hex)
		len: 42
		0: 00 19 cb 55 55 a4 00 14 a4 43 78 69 08 06 00 01
		16: 08 00 06 04 00 01 00 14 a4 43 78 69 0a 3b 01 26
		32: 00 00 00 00 00 00 0a 3b 01 01
		(breakpoint)
		>

		> breakpoint
		breakpoints: 0 1
		Prints currently set breakpoints.

		> step [-<n>, +<n>]
		Performs single stepping through the BPF program from the current pc
		offset. Thus, on each step invocation, above register dump is issued.
		This can go forwards and backwards in time, a plain `step` will break
		on the next BPF instruction, thus +1. (No `run` needs to be issued here.)

		> select <n>
		Selects a given packet from the pcap file to continue from. Thus, on
		the next `run` or `step`, the BPF program is being evaluated against
		the user pre-selected packet. Numbering starts just as in Wireshark
		with index 1.

		> quit
		#
		Exits bpf_dbg.

		JIT compiler
		------------

		The Linux kernel has a built-in BPF JIT compiler for x86_64, SPARC, PowerPC,
		ARM and s390 and can be enabled through CONFIG_BPF_JIT. The JIT compiler is
		transparently invoked for each attached filter from user space or for internal
		kernel users if it has been previously enabled by root:

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

		For JIT developers, doing audits etc, each compile run can output the generated
		opcode image into the kernel log via:

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

		Example output from dmesg:

		[ 3389.935842] flen=6 proglen=70 pass=3 image=ffffffffa0069c8f
		[ 3389.935847] JIT code: 00000000: 55 48 89 e5 48 83 ec 60 48 89 5d f8 44 8b 4f 68
		[ 3389.935849] JIT code: 00000010: 44 2b 4f 6c 4c 8b 87 d8 00 00 00 be 0c 00 00 00
		[ 3389.935850] JIT code: 00000020: e8 1d 94 ff e0 3d 00 08 00 00 75 16 be 17 00 00
		[ 3389.935851] JIT code: 00000030: 00 e8 28 94 ff e0 83 f8 01 75 07 b8 ff ff 00 00
		[ 3389.935852] JIT code: 00000040: eb 02 31 c0 c9 c3

		In the kernel source tree under tools/net/, there's bpf_jit_disasm for
		generating disassembly out of the kernel log's hexdump:

		# ./bpf_jit_disasm
		70 bytes emitted from JIT compiler (pass:3, flen:6)
		ffffffffa0069c8f + <x>:
		0: push %rbp
		1: mov %rsp,%rbp
		4: sub $0x60,%rsp
		8: mov %rbx,-0x8(%rbp)
		c: mov 0x68(%rdi),%r9d
		10: sub 0x6c(%rdi),%r9d
		14: mov 0xd8(%rdi),%r8
		1b: mov $0xc,%esi
		20: callq 0xffffffffe0ff9442
		25: cmp $0x800,%eax
		2a: jne 0x0000000000000042
		2c: mov $0x17,%esi
		31: callq 0xffffffffe0ff945e
		36: cmp $0x1,%eax
		39: jne 0x0000000000000042
		3b: mov $0xffff,%eax
		40: jmp 0x0000000000000044
		42: xor %eax,%eax
		44: leaveq
		45: retq

		Issuing option `-o` will "annotate" opcodes to resulting assembler
		instructions, which can be very useful for JIT developers:

		# ./bpf_jit_disasm -o
		70 bytes emitted from JIT compiler (pass:3, flen:6)
		ffffffffa0069c8f + <x>:
		0: push %rbp
		55
		1: mov %rsp,%rbp
		48 89 e5
		4: sub $0x60,%rsp
		48 83 ec 60
		8: mov %rbx,-0x8(%rbp)
		48 89 5d f8
		c: mov 0x68(%rdi),%r9d
		44 8b 4f 68
		10: sub 0x6c(%rdi),%r9d
		44 2b 4f 6c
		14: mov 0xd8(%rdi),%r8
		4c 8b 87 d8 00 00 00
		1b: mov $0xc,%esi
		be 0c 00 00 00
		20: callq 0xffffffffe0ff9442
		e8 1d 94 ff e0
		25: cmp $0x800,%eax
		3d 00 08 00 00
		2a: jne 0x0000000000000042
		75 16
		2c: mov $0x17,%esi
		be 17 00 00 00
		31: callq 0xffffffffe0ff945e
		e8 28 94 ff e0
		36: cmp $0x1,%eax
		83 f8 01
		39: jne 0x0000000000000042
		75 07
		3b: mov $0xffff,%eax
		b8 ff ff 00 00
		40: jmp 0x0000000000000044
		eb 02
		42: xor %eax,%eax
		31 c0
		44: leaveq
		c9
		45: retq
		c3

		For BPF JIT developers, bpf_jit_disasm, bpf_asm and bpf_dbg provides a useful
		toolchain for developing and testing the kernel's JIT compiler.

		Misc
		----

		Also trinity, the Linux syscall fuzzer, has built-in support for BPF and
		SECCOMP-BPF kernel fuzzing.

		Written by
		----------

		The document was written in the hope that it is found useful and in order
		to give potential BPF hackers or security auditors a better overview of
		the underlying architecture.

		Jay Schulist <jschlst@samba.org>
		Daniel Borkmann <dborkman@redhat.com>

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