This patch adds a minimal BPF debugger that "emulates" the kernel's
BPF engine (w/o extensions) and allows for single stepping (forwards
and backwards through BPF code) or running with >=1 breakpoints through
selected or all packets from a pcap file with a provided user filter
in order to facilitate verification of a BPF program. When a breakpoint
is being hit, it dumps all register contents, decoded instructions and
in case of branches both decoded branch targets as well as other useful
information.
Having this facility is in particular useful to verify BPF programs
against given test traffic *before* attaching to a live system.
With the general availability of cls_bpf, xt_bpf, socket filters,
team driver and e.g. PTP code, all BPF users, quite often a single
more complex BPF program is being used. Reasons for a more complex
BPF program are primarily to optimize execution time for making a
verdict when multiple simple BPF programs are combined into one in
order to prevent parsing same headers multiple times. In particular,
for cls_bpf that can have various return paths for encoding flowids,
and xt_bpf to come to a fw verdict this can be the case.
Therefore, as this can result in more complex and harder to debug
code, it would be very useful to have this minimal tool for testing
purposes. It can also be of help for BPF JIT developers as filters
are "test attached" to the kernel on a temporary socket thus
triggering a JIT image dump when enabled. The tool uses an interactive
libreadline shell with auto-completion and history support.
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This is a minimal stand-alone user space helper, that allows for debugging or
verification of emitted BPF JIT images. This is in particular useful for
emitted opcode debugging, since minor bugs in the JIT compiler can be fatal.
The disassembler is architecture generic and uses libopcodes and libbfd.
How to get to the disassembly, example:
1) `echo 2 > /proc/sys/net/core/bpf_jit_enable`
2) Load a BPF filter (e.g. `tcpdump -p -n -s 0 -i eth1 host 192.168.20.0/24`)
3) Run e.g. `bpf_jit_disasm -o` to disassemble the most recent JIT code output
`bpf_jit_disasm -o` will display the related opcodes to a particular instruction
as well. Example for x86_64:
$ ./bpf_jit_disasm
94 bytes emitted from JIT compiler (pass:3, flen:9)
ffffffffa0356000 + <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 0xe0(%rdi),%r8
1b: mov $0xc,%esi
20: callq 0xffffffffe0d01b71
25: cmp $0x86dd,%eax
2a: jne 0x000000000000003d
2c: mov $0x14,%esi
31: callq 0xffffffffe0d01b8d
36: cmp $0x6,%eax
[...]
5c: leaveq
5d: retq
$ ./bpf_jit_disasm -o
94 bytes emitted from JIT compiler (pass:3, flen:9)
ffffffffa0356000 + <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
[...]
5c: leaveq
c9
5d: retq
c3
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
