asm - ASSEMBLE

ceccomp asm [-c WHEN] [-a ARCH] [-f FMT] [TEXT]

Assemble TEXT to RAW. Use it to embed hand written filter rules into C code or to see the original code of some TEXT.

WHEN

ARCH

FMT

TEXT

Please check out TEXT GRAMMAR REFERENCE section to see how to write a rule by hand. Some examples will be displayed in EXAMPLES section.

disasm - DISASSEMBLE

ceccomp disasm [-c WHEN] [-a ARCH] [RAW]

Disassemble RAW to TEXT. Use it to see what does a filter do if you could not access filter via trace and have to manually extract the filter out.

WHEN

ARCH

RAW

emu - EMULATE

ceccomp emu [-c WHEN] [-a ARCH] [-q] TEXT SYSCALL_NAME/SYSCALL_NR [ARGS[0] ARGS[1] ... ARGS[5] PC]

Emulate what will happen if syscall(SYSCALL_NR, ARGS[0], ARGS[1], ..., ARGS[5]) from PC is called following rules described in TEXT. Use it to see the result without actually running it in program or you don’t want to examine the filter rule manually. This subcommand can be used to automatically examining a filter.

WHEN

SYSCALL_NAME/SYSCALL_NR

ARGS[0-5] and PC

ARCH

TEXT

-q, --quiet

trace - TRACE FILTER IN RUNTIME

ceccomp trace [-c WHEN] [-q] [-o FILE] PROGRAM [program-args]
              [-c WHEN] [-q] -p PID [-s]

The first line captures filters PROGRAM loads in runtime by tracing it; the second line extract seccomp filters from PID, or trace PID to capture subsequent seccomp filters; once fetched filters, print them in TEXT. You can only choose one of the two formats above. Use this if running the program is the simplest way to fetch bpf filters or a program with seccomp filters installed is waiting for input.

WHEN

FILE

PROGRAM

PID

-s, --seize

-q, --quiet

probe - TEST COMMON SYSCALLS INSTANTLY

ceccomp probe [-c WHEN] [-o FILE] [-q] PROGRAM [program-args]

Run PROGRAM with program-args to captures FIRST seccomp filter, and then kill all children. Use it when a quick check against a program is needed, and detect potential seccomp rule issues.

All argument descriptions can be found in trace - TRACE FILTER IN RUNTIME section.

The output for this subcommand is the emulating result of common syscalls like execve, open and so on. If the filter itself is not capable of blocking syscalls, you could know that with a glance.

Typical output for this subcommand is described below, more detailed example could be found in EXAMPLES section.

open      -> ALLOW
read      -> ALLOW
write     -> ALLOW
execve    -> KILL
execveat  -> KILL
mmap      -> ALLOW
mprotect  -> ALLOW
openat    -> ALLOW
sendfile  -> ALLOW
ptrace    -> ERRNO(1)
fork      -> ALLOW

Comment and Label

ceccomp disasm displays a lot of things, but some of them are optional for asm.

#Label  CODE  JT   JF      K
#---------------------------------
 L0001: 0x06 0x00 0x00 0x7fff0000 return ALLOW
#---------------------------------

Any text after # will be discarded by asm like some script languages.

Empty lines are accepted.

Label declaration is an identifier at the beginning of line and suffixed by : like L0001. An identifier is a string starts with alpha and contains with only alphanumeric characters and underscore _. Label is only necessary if it’s the destination of goto, these redundant labels added by disasm are for readability. E.g. in if ($A == 0) goto somewhere, somewhere is a label and must be declared after the statement. Label declaration can take a line separately, or be put in front of statement.

The CODE, JT, JF and K value generated by disasm will be discarded by asm, asm only parse the effective statement after K.

line  CODE  JT   JF      K
=================================
0000: 0x06 0x00 0x00 0x7fff0000  return ALLOW

Assignment

A can be set to seccomp attributes directly. But X can not be assigned with seccomp attributes directly due to kernel limit.

$A = $arch
$A = $syscall_nr

To assign A with those 64-bit long fields, low_ or high_ prefix is needed.

$A = $low_pc
$A = $high_pc
$A = $low_args[0]
$A = $high_args[0]
...
$A = $low_args[5]
$A = $high_args[5]

A special attribute is sizeof(struct seccomp_data), that can be assigned to A or X directly.

$A = $scmp_data_len
$X = $scmp_data_len

Temporary memory is 32-bit, to access them, you could use hex or dec as index. Both A and X is assignable. Assigning immediate values to A or X accepts any format of number if you imply the correct base by "0x" or "0b".

$X = $mem[0]
$A = $mem[0xf]
$A = $mem[15] # both hex and dec index are OK
$A = 0
$X = 0x3b
$A = 0b1111
$A = 0333

You could also assign X to A or in the reverse order. Assign X or A to temporary memory is definitely okay.

$A = $X
$X = $A
$mem[3] = $X
$mem[0x4] = $A

Arithmetic Operations

Various operations can be applied to A.

$A += 30
$A -= 4
$A *= 9
$A /= 1
$A &= 7
$A >>= 6

The right value can be X.

$A &= $X
$A |= $X
$A ^= $X
$A <<= $X

And there is a way to negativate A.

$A = -$A

Jump Downwards If ...

Unconditional jump:

goto L3

Jump if:

if ($A == execve) goto L3
if ($A != 1234) goto L4
if ($A & $X) goto L5
if !($A & 7) goto L6
if ($A <= $X) goto L7

If true jump to ... if false jump to...:

if ($A > $X) goto L3, else goto L4
if ($A >= 4567) goto L5, else goto L6

ONLY in conditions, you CAN replace number with syscall name or arch name. In example above, 0x3b is replaced by execve. All the syscall name will be resolved to syscall number under your selected arch. If you want to resolve a syscall name in foreign arch (not equal to your selected arch), please prepend a arch and dot. For example, your arch is x86_64, and you are writing aarch64 rules, then please write like:

if ($A == aarch64.read) goto 5

Note that if you manually set arch to aarch64 with -a aarch64, you can omit aarch64. in statement.

Return Code

Return value of register A:

return $A

Or return a immediate value, with extra field in (). Actions including TRACE, TRAP and ERRNO accept an extra field, without (), they are treated as action(0).

return 0x13371337
return KILL
return KILL_PROCESS
return TRAP(123)
return ERRNO(0)
return TRACE
return TRACE(3)
return LOG
return NOTIFY

Short Example

The following TEXT is valid for asm, which blocks execve and execveat for amd64 syscalls:

$A = $syscall_nr
if ($A == execve) goto forbid
if ($A == execveat) goto forbid
return ALLOW
forbid: return KILL