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Linux Socket Filtering aka Berkeley Packet Filter (BPF)
=======================================================
Introduction
------------
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->hash
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.
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>