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![Alexei Starovoitov](/assets/img/avatar_default.png)
Further extend the current BPF documentation to document new BPF engine internals. Joint work with Daniel Borkmann. Signed-off-by: Alexei Starovoitov <ast@plumgrid.com> Signed-off-by: Daniel Borkmann <dborkman@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
688 lines
27 KiB
Text
688 lines
27 KiB
Text
Linux Socket Filtering aka Berkeley Packet Filter (BPF)
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=======================================================
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Introduction
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------------
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Linux Socket Filtering (LSF) is derived from the Berkeley Packet Filter.
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Though there are some distinct differences between the BSD and Linux
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Kernel filtering, but when we speak of BPF or LSF in Linux context, we
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mean the very same mechanism of filtering in the Linux kernel.
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BPF allows a user-space program to attach a filter onto any socket and
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allow or disallow certain types of data to come through the socket. LSF
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follows exactly the same filter code structure as BSD's BPF, so referring
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to the BSD bpf.4 manpage is very helpful in creating filters.
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On Linux, BPF is much simpler than on BSD. One does not have to worry
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about devices or anything like that. You simply create your filter code,
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send it to the kernel via the SO_ATTACH_FILTER option and if your filter
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code passes the kernel check on it, you then immediately begin filtering
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data on that socket.
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You can also detach filters from your socket via the SO_DETACH_FILTER
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option. This will probably not be used much since when you close a socket
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that has a filter on it the filter is automagically removed. The other
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less common case may be adding a different filter on the same socket where
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you had another filter that is still running: the kernel takes care of
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removing the old one and placing your new one in its place, assuming your
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filter has passed the checks, otherwise if it fails the old filter will
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remain on that socket.
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SO_LOCK_FILTER option allows to lock the filter attached to a socket. Once
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set, a filter cannot be removed or changed. This allows one process to
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setup a socket, attach a filter, lock it then drop privileges and be
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assured that the filter will be kept until the socket is closed.
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The biggest user of this construct might be libpcap. Issuing a high-level
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filter command like `tcpdump -i em1 port 22` passes through the libpcap
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internal compiler that generates a structure that can eventually be loaded
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via SO_ATTACH_FILTER to the kernel. `tcpdump -i em1 port 22 -ddd`
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displays what is being placed into this structure.
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Although we were only speaking about sockets here, BPF in Linux is used
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in many more places. There's xt_bpf for netfilter, cls_bpf in the kernel
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qdisc layer, SECCOMP-BPF (SECure COMPuting [1]), and lots of other places
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such as team driver, PTP code, etc where BPF is being used.
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[1] Documentation/prctl/seccomp_filter.txt
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Original BPF paper:
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Steven McCanne and Van Jacobson. 1993. The BSD packet filter: a new
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architecture for user-level packet capture. In Proceedings of the
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USENIX Winter 1993 Conference Proceedings on USENIX Winter 1993
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Conference Proceedings (USENIX'93). USENIX Association, Berkeley,
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CA, USA, 2-2. [http://www.tcpdump.org/papers/bpf-usenix93.pdf]
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Structure
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---------
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User space applications include <linux/filter.h> which contains the
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following relevant structures:
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struct sock_filter { /* Filter block */
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__u16 code; /* Actual filter code */
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__u8 jt; /* Jump true */
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__u8 jf; /* Jump false */
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__u32 k; /* Generic multiuse field */
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};
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Such a structure is assembled as an array of 4-tuples, that contains
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a code, jt, jf and k value. jt and jf are jump offsets and k a generic
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value to be used for a provided code.
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struct sock_fprog { /* Required for SO_ATTACH_FILTER. */
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unsigned short len; /* Number of filter blocks */
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struct sock_filter __user *filter;
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};
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For socket filtering, a pointer to this structure (as shown in
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follow-up example) is being passed to the kernel through setsockopt(2).
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Example
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-------
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#include <sys/socket.h>
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#include <sys/types.h>
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#include <arpa/inet.h>
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#include <linux/if_ether.h>
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/* ... */
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/* From the example above: tcpdump -i em1 port 22 -dd */
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struct sock_filter code[] = {
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{ 0x28, 0, 0, 0x0000000c },
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{ 0x15, 0, 8, 0x000086dd },
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{ 0x30, 0, 0, 0x00000014 },
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{ 0x15, 2, 0, 0x00000084 },
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{ 0x15, 1, 0, 0x00000006 },
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{ 0x15, 0, 17, 0x00000011 },
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{ 0x28, 0, 0, 0x00000036 },
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{ 0x15, 14, 0, 0x00000016 },
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{ 0x28, 0, 0, 0x00000038 },
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{ 0x15, 12, 13, 0x00000016 },
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{ 0x15, 0, 12, 0x00000800 },
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{ 0x30, 0, 0, 0x00000017 },
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{ 0x15, 2, 0, 0x00000084 },
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{ 0x15, 1, 0, 0x00000006 },
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{ 0x15, 0, 8, 0x00000011 },
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{ 0x28, 0, 0, 0x00000014 },
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{ 0x45, 6, 0, 0x00001fff },
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{ 0xb1, 0, 0, 0x0000000e },
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{ 0x48, 0, 0, 0x0000000e },
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{ 0x15, 2, 0, 0x00000016 },
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{ 0x48, 0, 0, 0x00000010 },
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{ 0x15, 0, 1, 0x00000016 },
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{ 0x06, 0, 0, 0x0000ffff },
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{ 0x06, 0, 0, 0x00000000 },
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};
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struct sock_fprog bpf = {
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.len = ARRAY_SIZE(code),
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.filter = code,
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};
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sock = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
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if (sock < 0)
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/* ... bail out ... */
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ret = setsockopt(sock, SOL_SOCKET, SO_ATTACH_FILTER, &bpf, sizeof(bpf));
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if (ret < 0)
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/* ... bail out ... */
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/* ... */
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close(sock);
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The above example code attaches a socket filter for a PF_PACKET socket
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in order to let all IPv4/IPv6 packets with port 22 pass. The rest will
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be dropped for this socket.
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The setsockopt(2) call to SO_DETACH_FILTER doesn't need any arguments
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and SO_LOCK_FILTER for preventing the filter to be detached, takes an
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integer value with 0 or 1.
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Note that socket filters are not restricted to PF_PACKET sockets only,
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but can also be used on other socket families.
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Summary of system calls:
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* setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_FILTER, &val, sizeof(val));
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* setsockopt(sockfd, SOL_SOCKET, SO_DETACH_FILTER, &val, sizeof(val));
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* setsockopt(sockfd, SOL_SOCKET, SO_LOCK_FILTER, &val, sizeof(val));
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Normally, most use cases for socket filtering on packet sockets will be
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covered by libpcap in high-level syntax, so as an application developer
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you should stick to that. libpcap wraps its own layer around all that.
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Unless i) using/linking to libpcap is not an option, ii) the required BPF
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filters use Linux extensions that are not supported by libpcap's compiler,
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iii) a filter might be more complex and not cleanly implementable with
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libpcap's compiler, or iv) particular filter codes should be optimized
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differently than libpcap's internal compiler does; then in such cases
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writing such a filter "by hand" can be of an alternative. For example,
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xt_bpf and cls_bpf users might have requirements that could result in
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more complex filter code, or one that cannot be expressed with libpcap
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(e.g. different return codes for various code paths). Moreover, BPF JIT
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implementors may wish to manually write test cases and thus need low-level
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access to BPF code as well.
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BPF engine and instruction set
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------------------------------
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Under tools/net/ there's a small helper tool called bpf_asm which can
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be used to write low-level filters for example scenarios mentioned in the
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previous section. Asm-like syntax mentioned here has been implemented in
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bpf_asm and will be used for further explanations (instead of dealing with
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less readable opcodes directly, principles are the same). The syntax is
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closely modelled after Steven McCanne's and Van Jacobson's BPF paper.
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The BPF architecture consists of the following basic elements:
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Element Description
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A 32 bit wide accumulator
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X 32 bit wide X register
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M[] 16 x 32 bit wide misc registers aka "scratch memory
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store", addressable from 0 to 15
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A program, that is translated by bpf_asm into "opcodes" is an array that
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consists of the following elements (as already mentioned):
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op:16, jt:8, jf:8, k:32
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The element op is a 16 bit wide opcode that has a particular instruction
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encoded. jt and jf are two 8 bit wide jump targets, one for condition
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"jump if true", the other one "jump if false". Eventually, element k
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contains a miscellaneous argument that can be interpreted in different
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ways depending on the given instruction in op.
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The instruction set consists of load, store, branch, alu, miscellaneous
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and return instructions that are also represented in bpf_asm syntax. This
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table lists all bpf_asm instructions available resp. what their underlying
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opcodes as defined in linux/filter.h stand for:
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Instruction Addressing mode Description
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ld 1, 2, 3, 4, 10 Load word into A
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ldi 4 Load word into A
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ldh 1, 2 Load half-word into A
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ldb 1, 2 Load byte into A
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ldx 3, 4, 5, 10 Load word into X
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ldxi 4 Load word into X
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ldxb 5 Load byte into X
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st 3 Store A into M[]
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stx 3 Store X into M[]
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jmp 6 Jump to label
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ja 6 Jump to label
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jeq 7, 8 Jump on k == A
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jneq 8 Jump on k != A
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jne 8 Jump on k != A
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jlt 8 Jump on k < A
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jle 8 Jump on k <= A
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jgt 7, 8 Jump on k > A
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jge 7, 8 Jump on k >= A
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jset 7, 8 Jump on k & A
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add 0, 4 A + <x>
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sub 0, 4 A - <x>
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mul 0, 4 A * <x>
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div 0, 4 A / <x>
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mod 0, 4 A % <x>
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neg 0, 4 !A
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and 0, 4 A & <x>
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or 0, 4 A | <x>
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xor 0, 4 A ^ <x>
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lsh 0, 4 A << <x>
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rsh 0, 4 A >> <x>
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tax Copy A into X
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txa Copy X into A
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ret 4, 9 Return
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The next table shows addressing formats from the 2nd column:
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Addressing mode Syntax Description
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0 x/%x Register X
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1 [k] BHW at byte offset k in the packet
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2 [x + k] BHW at the offset X + k in the packet
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3 M[k] Word at offset k in M[]
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4 #k Literal value stored in k
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5 4*([k]&0xf) Lower nibble * 4 at byte offset k in the packet
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6 L Jump label L
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7 #k,Lt,Lf Jump to Lt if true, otherwise jump to Lf
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8 #k,Lt Jump to Lt if predicate is true
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9 a/%a Accumulator A
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10 extension BPF extension
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The Linux kernel also has a couple of BPF extensions that are used along
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with the class of load instructions by "overloading" the k argument with
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a negative offset + a particular extension offset. The result of such BPF
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extensions are loaded into A.
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Possible BPF extensions are shown in the following table:
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Extension Description
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len skb->len
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proto skb->protocol
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type skb->pkt_type
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poff Payload start offset
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ifidx skb->dev->ifindex
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nla Netlink attribute of type X with offset A
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nlan Nested Netlink attribute of type X with offset A
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mark skb->mark
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queue skb->queue_mapping
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hatype skb->dev->type
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rxhash skb->rxhash
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cpu raw_smp_processor_id()
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vlan_tci vlan_tx_tag_get(skb)
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vlan_pr vlan_tx_tag_present(skb)
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These extensions can also be prefixed with '#'.
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Examples for low-level BPF:
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** ARP packets:
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ldh [12]
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jne #0x806, drop
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ret #-1
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drop: ret #0
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** IPv4 TCP packets:
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ldh [12]
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jne #0x800, drop
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ldb [23]
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jneq #6, drop
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ret #-1
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drop: ret #0
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** (Accelerated) VLAN w/ id 10:
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ld vlan_tci
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jneq #10, drop
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ret #-1
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drop: ret #0
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** SECCOMP filter example:
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ld [4] /* offsetof(struct seccomp_data, arch) */
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jne #0xc000003e, bad /* AUDIT_ARCH_X86_64 */
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ld [0] /* offsetof(struct seccomp_data, nr) */
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jeq #15, good /* __NR_rt_sigreturn */
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jeq #231, good /* __NR_exit_group */
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jeq #60, good /* __NR_exit */
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jeq #0, good /* __NR_read */
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jeq #1, good /* __NR_write */
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jeq #5, good /* __NR_fstat */
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jeq #9, good /* __NR_mmap */
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jeq #14, good /* __NR_rt_sigprocmask */
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jeq #13, good /* __NR_rt_sigaction */
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jeq #35, good /* __NR_nanosleep */
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bad: ret #0 /* SECCOMP_RET_KILL */
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good: ret #0x7fff0000 /* SECCOMP_RET_ALLOW */
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The above example code can be placed into a file (here called "foo"), and
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then be passed to the bpf_asm tool for generating opcodes, output that xt_bpf
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and cls_bpf understands and can directly be loaded with. Example with above
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ARP code:
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$ ./bpf_asm foo
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4,40 0 0 12,21 0 1 2054,6 0 0 4294967295,6 0 0 0,
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In copy and paste C-like output:
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$ ./bpf_asm -c foo
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{ 0x28, 0, 0, 0x0000000c },
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{ 0x15, 0, 1, 0x00000806 },
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{ 0x06, 0, 0, 0xffffffff },
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{ 0x06, 0, 0, 0000000000 },
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In particular, as usage with xt_bpf or cls_bpf can result in more complex BPF
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filters that might not be obvious at first, it's good to test filters before
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attaching to a live system. For that purpose, there's a small tool called
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bpf_dbg under tools/net/ in the kernel source directory. This debugger allows
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for testing BPF filters against given pcap files, single stepping through the
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BPF code on the pcap's packets and to do BPF machine register dumps.
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Starting bpf_dbg is trivial and just requires issuing:
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# ./bpf_dbg
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In case input and output do not equal stdin/stdout, bpf_dbg takes an
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alternative stdin source as a first argument, and an alternative stdout
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sink as a second one, e.g. `./bpf_dbg test_in.txt test_out.txt`.
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Other than that, a particular libreadline configuration can be set via
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file "~/.bpf_dbg_init" and the command history is stored in the file
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"~/.bpf_dbg_history".
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Interaction in bpf_dbg happens through a shell that also has auto-completion
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support (follow-up example commands starting with '>' denote bpf_dbg shell).
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The usual workflow would be to ...
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> 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
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Loads a BPF filter from standard output of bpf_asm, or transformed via
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e.g. `tcpdump -iem1 -ddd port 22 | tr '\n' ','`. Note that for JIT
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debugging (next section), this command creates a temporary socket and
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loads the BPF code into the kernel. Thus, this will also be useful for
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JIT developers.
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> load pcap foo.pcap
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Loads standard tcpdump pcap file.
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> run [<n>]
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bpf passes:1 fails:9
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Runs through all packets from a pcap to account how many passes and fails
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the filter will generate. A limit of packets to traverse can be given.
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> disassemble
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l0: ldh [12]
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l1: jeq #0x800, l2, l5
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l2: ldb [23]
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l3: jeq #0x1, l4, l5
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l4: ret #0xffff
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l5: ret #0
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Prints out BPF code disassembly.
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> dump
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/* { op, jt, jf, k }, */
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{ 0x28, 0, 0, 0x0000000c },
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{ 0x15, 0, 3, 0x00000800 },
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{ 0x30, 0, 0, 0x00000017 },
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{ 0x15, 0, 1, 0x00000001 },
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{ 0x06, 0, 0, 0x0000ffff },
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{ 0x06, 0, 0, 0000000000 },
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Prints out C-style BPF code dump.
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> breakpoint 0
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breakpoint at: l0: ldh [12]
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> breakpoint 1
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breakpoint at: l1: jeq #0x800, l2, l5
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...
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Sets breakpoints at particular BPF instructions. Issuing a `run` command
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will walk through the pcap file continuing from the current packet and
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break when a breakpoint is being hit (another `run` will continue from
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the currently active breakpoint executing next instructions):
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> run
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-- register dump --
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pc: [0] <-- program counter
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code: [40] jt[0] jf[0] k[12] <-- plain BPF code of current instruction
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curr: l0: ldh [12] <-- disassembly of current instruction
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A: [00000000][0] <-- content of A (hex, decimal)
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X: [00000000][0] <-- content of X (hex, decimal)
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M[0,15]: [00000000][0] <-- folded content of M (hex, decimal)
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-- packet dump -- <-- Current packet from pcap (hex)
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len: 42
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0: 00 19 cb 55 55 a4 00 14 a4 43 78 69 08 06 00 01
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16: 08 00 06 04 00 01 00 14 a4 43 78 69 0a 3b 01 26
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32: 00 00 00 00 00 00 0a 3b 01 01
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(breakpoint)
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>
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> breakpoint
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breakpoints: 0 1
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Prints currently set breakpoints.
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> step [-<n>, +<n>]
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Performs single stepping through the BPF program from the current pc
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offset. Thus, on each step invocation, above register dump is issued.
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This can go forwards and backwards in time, a plain `step` will break
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on the next BPF instruction, thus +1. (No `run` needs to be issued here.)
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> select <n>
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Selects a given packet from the pcap file to continue from. Thus, on
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the next `run` or `step`, the BPF program is being evaluated against
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the user pre-selected packet. Numbering starts just as in Wireshark
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with index 1.
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> quit
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#
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Exits bpf_dbg.
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JIT compiler
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------------
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The Linux kernel has a built-in BPF JIT compiler for x86_64, SPARC, PowerPC,
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ARM and s390 and can be enabled through CONFIG_BPF_JIT. The JIT compiler is
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transparently invoked for each attached filter from user space or for internal
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kernel users if it has been previously enabled by root:
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echo 1 > /proc/sys/net/core/bpf_jit_enable
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For JIT developers, doing audits etc, each compile run can output the generated
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opcode image into the kernel log via:
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echo 2 > /proc/sys/net/core/bpf_jit_enable
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Example output from dmesg:
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[ 3389.935842] flen=6 proglen=70 pass=3 image=ffffffffa0069c8f
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[ 3389.935847] JIT code: 00000000: 55 48 89 e5 48 83 ec 60 48 89 5d f8 44 8b 4f 68
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[ 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
|
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[ 3389.935852] JIT code: 00000040: eb 02 31 c0 c9 c3
|
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|
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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
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|
8: mov %rbx,-0x8(%rbp)
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|
c: mov 0x68(%rdi),%r9d
|
|
10: sub 0x6c(%rdi),%r9d
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|
14: mov 0xd8(%rdi),%r8
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|
1b: mov $0xc,%esi
|
|
20: callq 0xffffffffe0ff9442
|
|
25: cmp $0x800,%eax
|
|
2a: jne 0x0000000000000042
|
|
2c: mov $0x17,%esi
|
|
31: callq 0xffffffffe0ff945e
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|
36: cmp $0x1,%eax
|
|
39: jne 0x0000000000000042
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|
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.
|
|
|
|
BPF kernel internals
|
|
--------------------
|
|
Internally, for the kernel interpreter, a different BPF instruction set
|
|
format with similar underlying principles from BPF described in previous
|
|
paragraphs is being used. However, the instruction set format is modelled
|
|
closer to the underlying architecture to mimic native instruction sets, so
|
|
that a better performance can be achieved (more details later).
|
|
|
|
It is designed to be JITed with one to one mapping, which can also open up
|
|
the possibility for GCC/LLVM compilers to generate optimized BPF code through
|
|
a BPF backend that performs almost as fast as natively compiled code.
|
|
|
|
The new instruction set was originally designed with the possible goal in
|
|
mind to write programs in "restricted C" and compile into BPF with a optional
|
|
GCC/LLVM backend, so that it can just-in-time map to modern 64-bit CPUs with
|
|
minimal performance overhead over two steps, that is, C -> BPF -> native code.
|
|
|
|
Currently, the new format is being used for running user BPF programs, which
|
|
includes seccomp BPF, classic socket filters, cls_bpf traffic classifier,
|
|
team driver's classifier for its load-balancing mode, netfilter's xt_bpf
|
|
extension, PTP dissector/classifier, and much more. They are all internally
|
|
converted by the kernel into the new instruction set representation and run
|
|
in the extended interpreter. For in-kernel handlers, this all works
|
|
transparently by using sk_unattached_filter_create() for setting up the
|
|
filter, resp. sk_unattached_filter_destroy() for destroying it. The macro
|
|
SK_RUN_FILTER(filter, ctx) transparently invokes the right BPF function to
|
|
run the filter. 'filter' is a pointer to struct sk_filter that we got from
|
|
sk_unattached_filter_create(), and 'ctx' the given context (e.g. skb pointer).
|
|
All constraints and restrictions from sk_chk_filter() apply before a
|
|
conversion to the new layout is being done behind the scenes!
|
|
|
|
Currently, for JITing, the user BPF format is being used and current BPF JIT
|
|
compilers reused whenever possible. In other words, we do not (yet!) perform
|
|
a JIT compilation in the new layout, however, future work will successively
|
|
migrate traditional JIT compilers into the new instruction format as well, so
|
|
that they will profit from the very same benefits. Thus, when speaking about
|
|
JIT in the following, a JIT compiler (TBD) for the new instruction format is
|
|
meant in this context.
|
|
|
|
Some core changes of the new internal format:
|
|
|
|
- Number of registers increase from 2 to 10:
|
|
|
|
The old format had two registers A and X, and a hidden frame pointer. The
|
|
new layout extends this to be 10 internal registers and a read-only frame
|
|
pointer. Since 64-bit CPUs are passing arguments to functions via registers
|
|
the number of args from BPF program to in-kernel function is restricted
|
|
to 5 and one register is used to accept return value from an in-kernel
|
|
function. Natively, x86_64 passes first 6 arguments in registers, aarch64/
|
|
sparcv9/mips64 have 7 - 8 registers for arguments; x86_64 has 6 callee saved
|
|
registers, and aarch64/sparcv9/mips64 have 11 or more callee saved registers.
|
|
|
|
Therefore, BPF calling convention is defined as:
|
|
|
|
* R0 - return value from in-kernel function
|
|
* R1 - R5 - arguments from BPF program to in-kernel function
|
|
* R6 - R9 - callee saved registers that in-kernel function will preserve
|
|
* R10 - read-only frame pointer to access stack
|
|
|
|
Thus, all BPF registers map one to one to HW registers on x86_64, aarch64,
|
|
etc, and BPF calling convention maps directly to ABIs used by the kernel on
|
|
64-bit architectures.
|
|
|
|
On 32-bit architectures JIT may map programs that use only 32-bit arithmetic
|
|
and may let more complex programs to be interpreted.
|
|
|
|
R0 - R5 are scratch registers and BPF program needs spill/fill them if
|
|
necessary across calls. Note that there is only one BPF program (== one BPF
|
|
main routine) and it cannot call other BPF functions, it can only call
|
|
predefined in-kernel functions, though.
|
|
|
|
- Register width increases from 32-bit to 64-bit:
|
|
|
|
Still, the semantics of the original 32-bit ALU operations are preserved
|
|
via 32-bit subregisters. All BPF registers are 64-bit with 32-bit lower
|
|
subregisters that zero-extend into 64-bit if they are being written to.
|
|
That behavior maps directly to x86_64 and arm64 subregister definition, but
|
|
makes other JITs more difficult.
|
|
|
|
32-bit architectures run 64-bit internal BPF programs via interpreter.
|
|
Their JITs may convert BPF programs that only use 32-bit subregisters into
|
|
native instruction set and let the rest being interpreted.
|
|
|
|
Operation is 64-bit, because on 64-bit architectures, pointers are also
|
|
64-bit wide, and we want to pass 64-bit values in/out of kernel functions,
|
|
so 32-bit BPF registers would otherwise require to define register-pair
|
|
ABI, thus, there won't be able to use a direct BPF register to HW register
|
|
mapping and JIT would need to do combine/split/move operations for every
|
|
register in and out of the function, which is complex, bug prone and slow.
|
|
Another reason is the use of atomic 64-bit counters.
|
|
|
|
- Conditional jt/jf targets replaced with jt/fall-through:
|
|
|
|
While the original design has constructs such as "if (cond) jump_true;
|
|
else jump_false;", they are being replaced into alternative constructs like
|
|
"if (cond) jump_true; /* else fall-through */".
|
|
|
|
- Introduces bpf_call insn and register passing convention for zero overhead
|
|
calls from/to other kernel functions:
|
|
|
|
After a kernel function call, R1 - R5 are reset to unreadable and R0 has a
|
|
return type of the function. Since R6 - R9 are callee saved, their state is
|
|
preserved across the call.
|
|
|
|
Also in the new design, BPF is limited to 4096 insns, which means that any
|
|
program will terminate quickly and will only call a fixed number of kernel
|
|
functions. Original BPF and the new format are two operand instructions,
|
|
which helps to do one-to-one mapping between BPF insn and x86 insn during JIT.
|
|
|
|
The input context pointer for invoking the interpreter function is generic,
|
|
its content is defined by a specific use case. For seccomp register R1 points
|
|
to seccomp_data, for converted BPF filters R1 points to a skb.
|
|
|
|
A program, that is translated internally consists of the following elements:
|
|
|
|
op:16, jt:8, jf:8, k:32 ==> op:8, a_reg:4, x_reg:4, off:16, imm:32
|
|
|
|
Just like the original BPF, the new format runs within a controlled environment,
|
|
is deterministic and the kernel can easily prove that. The safety of the program
|
|
can be determined in two steps: first step does depth-first-search to disallow
|
|
loops and other CFG validation; second step starts from the first insn and
|
|
descends all possible paths. It simulates execution of every insn and observes
|
|
the state change of registers and stack.
|
|
|
|
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>
|
|
Alexei Starovoitov <ast@plumgrid.com>
|