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Opcode

Section: Perl Programmers Reference Guide (3pm)
Updated: 2014-10-30
Index

NAME

Opcode - Disable named opcodes when compiling perl code

SYNOPSIS

  use Opcode;

DESCRIPTION

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

Evaluating perl code (e.g. via ``eval'' or ``do 'file''') causesthe 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 ineffect when perl next compiles any code. Attempting to compile codewhich contains a masked opcode will cause the compilation to failwith an error. The code will not be executed.

NOTE

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

WARNING

The authors make no warranty, implied or otherwise, about thesuitability 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 useof 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 arrayPL_op_name defined and initialised in file opcode.h of the Perlsource distribution (and installed into the perl library).

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

Many of the functions and methods listed below take a list ofoperators as parameters. Most operator lists can be made up of severaltypes 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 crypticlike 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 severaloptags 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 theaccumulated set of ops at that point.

an operator set (opset)

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

The opset and opset_to_ops functions can be used to convert froma 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 namestags and sets. All are available for export by the package.

opcodes

In a scalar context opcodes returns the number of opcodes in thisversion 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 inthe set.

opset_to_hex (OPSET)

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

full_opset

Returns an opset which includes all operators.

empty_opset

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 theright length etc) otherwise it returns false. If an optional secondparameter is true then verify_opset will croak on an invalid opsetinstead of returning false.

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

define_optag (OPTAG, OPSET)

Define OPTAG as a symbolic name for OPSET. Optag names always startwith a colon ":".

The optag name used must not be defined already (define_optag willcroak if it is already defined). Optag names are global to the perlprocess and optag definitions cannot be altered or deleted oncedefined.

It is strongly recommended that applications using Opcode should use aleading capital letter on their tag names since lowercase names arereserved for use by the Opcode module. If using Opcode within a moduleyou should prefix your tags names with the name of your module toensure uniqueness and thus avoid clashes with other modules.

opmask_add (OPSET)

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

opmask

Returns an opset corresponding to the current opmask.

opdesc (OP, ...)

This takes a list of operator names and returns the corresponding listof 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 opcodewithin the opset. In other words both sides of a bit vector operatorshould be opsets returned from Opcode functions.

Also, since the number of opcodes in your current version of perl mightnot be an exact multiple of eight, there may be unused bits in the lastbyte of an upset. This should not cause any problems (Opcode functionsignore those extra bits) but it does mean that using the ~ operatorwill typically not produce the same 'physical' opset 'string' as theinvert_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

:base_core

    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

:base_mem

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

    concat repeat join range

    anonlist anonhash

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

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

:base_loop

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

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

:base_io

These ops enable filehandle (rather than filename) based input andoutput. These are safe on the assumption that only pre-existingfilehandles are available for use. To create new filehandles other opssuch 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

:base_orig

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

    crypt

    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

:base_math

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

    atan2 sin cos exp log sqrt

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

    rand srand

:base_thread

These ops are related to multi-threading.

    lock threadsv

:default

A handy tag name for a reasonable default set of ops. (The current opsallowed 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!

:filesys_read

    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

    fileno

:sys_db

    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

:browse

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

The :browse tag represents the next step beyond :default. It it asuperset 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

:filesys_open

    sysopen open close    umask binmode

    open_dir closedir -- other dir ops are in :base_io

:filesys_write

    link unlink rename symlink truncate

    mkdir rmdir

    utime chmod chown

    fcntl -- not strictly filesys related, but possibly as dangerous?

:subprocess

    backtick system

    fork

    wait waitpid

    glob -- access to Cshell via <`rm *`>

:ownprocess

    exec exit kill

    time tms -- could be used for timing attacks (paranoid?)

:others

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

SystemV Interprocess Communications:

    msgctl msgget msgrcv msgsnd

    semctl semget semop

    shmctl shmget shmread shmwrite

:still_to_be_decided

    chdir    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

    reset

    dbstate -- perl -d version of nextstate(ment) opcode

:dangerous

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

    syscall dump chroot

AUTHORS

Originally designed and implemented by Malcolm Beattie,mbeattieAATTsable.ox.ac.uk as part of Safe version 1.

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

This document was created byman2html,using the manual pages.