Opcode - Disable named opcodes when compiling perl code


  use Opcode;


Perl code is always compiled into an internal format before execution.

Evaluating perl code (e.g. via ``eval'' or ``do 'file''') causes the code to be compiled into an internal format and then, provided there was no error in the compilation, executed. The internal format is based on many distinct opcodes.

By default no opmask is in effect and any code can be compiled.

The Opcode module allow you to define an operator mask to be in effect when perl next compiles any code. Attempting to compile code which contains a masked opcode will cause the compilation to fail with an error. The code will not be executed.


The Opcode module is not usually used directly. See the ops pragma and Safe modules for more typical uses.


The authors make no warranty, implied or otherwise, about the suitability of this software for safety or security purposes.

The authors shall not in any case be liable for special, incidental, consequential, indirect or other similar damages arising from the use of this software.

Your mileage will vary. If in any doubt do not use it.

Operator Names and Operator Lists

The canonical list of operator names is the contents of the array PL_op_name defined and initialised in file opcode.h of the Perl source distribution (and installed into the perl library).

Each operator has both a terse name (its opname) and a more verbose or recognisable descriptive name. The opdesc function can be used to return a list of descriptions for a list of operators.

Many of the functions and methods listed below take a list of operators as parameters. Most operator lists can be made up of several types of element. Each element can be one of

an operator name (opname)

Operator names are typically small lowercase words like enterloop, leaveloop, last, next, redo etc. Sometimes they are rather cryptic like gv2cv, i_ncmp and ftsvtx.

an operator tag name (optag)

Operator tags can be used to refer to groups (or sets) of operators. Tag names always begin with a colon. The Opcode module defines several optags and the user can define others using the define_optag function.

a negated opname or optag

An opname or optag can be prefixed with an exclamation mark, e.g., !mkdir. Negating an opname or optag means remove the corresponding ops from the accumulated set of ops at that point.

an operator set (opset)

An opset as a binary string of approximately 44 bytes which holds a set or zero or more operators.

The opset and opset_to_ops functions can be used to convert from a list of operators to an opset and vice versa.

Wherever a list of operators can be given you can use one or more opsets. See also Manipulating Opsets below.

Opcode Functions

The Opcode package contains functions for manipulating operator names tags and sets. All are available for export by the package.


In a scalar context opcodes returns the number of opcodes in this version of perl (around 350 for perl-5.7.0).

In a list context it returns a list of all the operator names. (Not yet implemented, use @names = opset_to_ops(full_opset).)

opset (OP, ...)

Returns an opset containing the listed operators.

opset_to_ops (OPSET)

Returns a list of operator names corresponding to those operators in the set.

opset_to_hex (OPSET)

Returns a string representation of an opset. Can be handy for debugging.


Returns an opset which includes all operators.


Returns an opset which contains no operators.

invert_opset (OPSET)

Returns an opset which is the inverse set of the one supplied.

verify_opset (OPSET, ...)

Returns true if the supplied opset looks like a valid opset (is the right length etc) otherwise it returns false. If an optional second parameter is true then verify_opset will croak on an invalid opset instead of returning false.

Most of the other Opcode functions call verify_opset automatically and will croak if given an invalid opset.

define_optag (OPTAG, OPSET)

Define OPTAG as a symbolic name for OPSET. Optag names always start with a colon :.

The optag name used must not be defined already (define_optag will croak if it is already defined). Optag names are global to the perl process and optag definitions cannot be altered or deleted once defined.

It is strongly recommended that applications using Opcode should use a leading capital letter on their tag names since lowercase names are reserved for use by the Opcode module. If using Opcode within a module you should prefix your tags names with the name of your module to ensure uniqueness and thus avoid clashes with other modules.

opmask_add (OPSET)

Adds the supplied opset to the current opmask. Note that there is currently no mechanism for unmasking ops once they have been masked. This is intentional.


Returns an opset corresponding to the current opmask.

opdesc (OP, ...)

This takes a list of operator names and returns the corresponding list of operator descriptions.

opdump (PAT)

Dumps to STDOUT a two column list of op names and op descriptions. If an optional pattern is given then only lines which match the (case insensitive) pattern will be output.

It's designed to be used as a handy command line utility:

        perl -MOpcode=opdump -e opdump
        perl -MOpcode=opdump -e 'opdump Eval'

Manipulating Opsets

Opsets may be manipulated using the perl bit vector operators & (and), | (or), ^ (xor) and ~ (negate/invert).

However you should never rely on the numerical position of any opcode within the opset. In other words both sides of a bit vector operator should be opsets returned from Opcode functions.

Also, since the number of opcodes in your current version of perl might not be an exact multiple of eight, there may be unused bits in the last byte of an upset. This should not cause any problems (Opcode functions ignore those extra bits) but it does mean that using the ~ operator will typically not produce the same 'physical' opset 'string' as the invert_opset function.

TO DO (maybe)

    $bool = opset_eq($opset1, $opset2)  true if opsets are logically eqiv
    $yes = opset_can($opset, @ops)      true if $opset has all @ops set
    @diff = opset_diff($opset1, $opset2) => ('foo', '!bar', ...)

Predefined Opcode Tags

    null stub scalar pushmark wantarray const defined undef
    rv2sv sassign
    rv2av aassign aelem aelemfast aslice av2arylen
    rv2hv helem hslice each values keys exists delete
    preinc i_preinc predec i_predec postinc i_postinc postdec i_postdec
    int hex oct abs pow multiply i_multiply divide i_divide
    modulo i_modulo add i_add subtract i_subtract
    left_shift right_shift bit_and bit_xor bit_or negate i_negate
    not complement
    lt i_lt gt i_gt le i_le ge i_ge eq i_eq ne i_ne ncmp i_ncmp
    slt sgt sle sge seq sne scmp
    substr vec stringify study pos length index rindex ord chr
    ucfirst lcfirst uc lc quotemeta trans chop schop chomp schomp
    match split qr
    list lslice splice push pop shift unshift reverse
    cond_expr flip flop andassign orassign and or xor
    warn die lineseq nextstate scope enter leave setstate
    rv2cv anoncode prototype
    entersub leavesub leavesublv return method method_named -- XXX loops via recursion?
    leaveeval -- needed for Safe to operate, is safe without entereval

These memory related ops are not included in :base_core because they can easily be used to implement a resource attack (e.g., consume all available memory).

    concat repeat join range
    anonlist anonhash

Note that despite the existence of this optag a memory resource attack may still be possible using only :base_core ops.

Disabling these ops is a very heavy handed way to attempt to prevent a memory resource attack. It's probable that a specific memory limit mechanism will be added to perl in the near future.


These loop ops are not included in :base_core because they can easily be used to implement a resource attack (e.g., consume all available CPU time).

    grepstart grepwhile
    mapstart mapwhile
    enteriter iter
    enterloop leaveloop unstack
    last next redo

These ops enable filehandle (rather than filename) based input and output. These are safe on the assumption that only pre-existing filehandles are available for use. To create new filehandles other ops such as open would need to be enabled.

    readline rcatline getc read
    formline enterwrite leavewrite
    print sysread syswrite send recv
    eof tell seek sysseek
    readdir telldir seekdir rewinddir

These are a hotchpotch of opcodes still waiting to be considered

    gvsv gv gelem
    padsv padav padhv padany
    rv2gv refgen srefgen ref
    bless -- could be used to change ownership of objects (reblessing)
    pushre regcmaybe regcreset regcomp subst substcont
    sprintf prtf -- can core dump
    tie untie
    dbmopen dbmclose
    sselect select
    pipe_op sockpair
    getppid getpgrp setpgrp getpriority setpriority localtime gmtime
    entertry leavetry -- can be used to 'hide' fatal errors
    custom -- where should this go

These ops are not included in :base_core because of the risk of them being used to generate floating point exceptions (which would have to be caught using a $SIG{FPE} handler).

    atan2 sin cos exp log sqrt

These ops are not included in :base_core because they have an effect beyond the scope of the compartment.

    rand srand

These ops are related to multi-threading.

    lock threadsv

A handy tag name for a reasonable default set of ops. (The current ops allowed are unstable while development continues. It will change.)

    :base_core :base_mem :base_loop :base_io :base_orig :base_thread

If safety matters to you (and why else would you be using the Opcode module?) then you should not rely on the definition of this, or indeed any other, optag!

    stat lstat readlink
    ftatime ftblk ftchr ftctime ftdir fteexec fteowned fteread
    ftewrite ftfile ftis ftlink ftmtime ftpipe ftrexec ftrowned
    ftrread ftsgid ftsize ftsock ftsuid fttty ftzero ftrwrite ftsvtx
    fttext ftbinary
    ghbyname ghbyaddr ghostent shostent ehostent      -- hosts
    gnbyname gnbyaddr gnetent snetent enetent         -- networks
    gpbyname gpbynumber gprotoent sprotoent eprotoent -- protocols
    gsbyname gsbyport gservent sservent eservent      -- services
    gpwnam gpwuid gpwent spwent epwent getlogin       -- users
    ggrnam ggrgid ggrent sgrent egrent                -- groups

A handy tag name for a reasonable default set of ops beyond the :default optag. Like :default (and indeed all the other optags) its current definition is unstable while development continues. It will change.

The :browse tag represents the next step beyond :default. It it a superset of the :default ops and adds :filesys_read the :sys_db. The intent being that scripts can access more (possibly sensitive) information about your system but not be able to change it.

    :default :filesys_read :sys_db
    sysopen open close
    umask binmode
    open_dir closedir -- other dir ops are in :base_io
    link unlink rename symlink truncate
    mkdir rmdir
    utime chmod chown
    fcntl -- not strictly filesys related, but possibly as dangerous?
    backtick system
    wait waitpid
    glob -- access to Cshell via <`rm *`>
    exec exit kill
    time tms -- could be used for timing attacks (paranoid?)

This tag holds groups of assorted specialist opcodes that don't warrant having optags defined for them.

SystemV Interprocess Communications:

    msgctl msgget msgrcv msgsnd
    semctl semget semop
    shmctl shmget shmread shmwrite
    flock ioctl
    socket getpeername ssockopt
    bind connect listen accept shutdown gsockopt getsockname
    sleep alarm -- changes global timer state and signal handling
    sort -- assorted problems including core dumps
    tied -- can be used to access object implementing a tie
    pack unpack -- can be used to create/use memory pointers
    entereval -- can be used to hide code from initial compile
    require dofile
    caller -- get info about calling environment and args
    dbstate -- perl -d version of nextstate(ment) opcode

This tag is simply a bucket for opcodes that are unlikely to be used via a tag name but need to be tagged for completeness and documentation.

    syscall dump chroot


ops(3) -- perl pragma interface to Opcode module.

Safe(3) -- Opcode and namespace limited execution compartments


Originally designed and implemented by Malcolm Beattie, as part of Safe version 1.

Split out from Safe module version 1, named opcode tags and other changes added by Tim Bunce.