Opcode(3p) | Perl Programmers Reference Guide | Opcode(3p)

# 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'") 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.

# NOTE

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

# WARNING

The Opcode module does not implement an effective sandbox for evaluating untrusted code with the perl interpreter.

Bugs in the perl interpreter that could be abused to bypass Opcode restrictions are not treated as vulnerabilities. See perlsecpolicy for additional information.

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.

opcodes
  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.
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 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.
opmask
  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
                                            equivalent
        $yes = opset_can($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 padsv_store
          rv2av aassign aelem aelemfast aelemfast_lex aslice kvaslice
          av2arylen aelemfastlex_store
          rv2hv helem hslice kvhslice each values keys exists delete
          aeach akeys avalues multideref argelem argdefelem argcheck
          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 nbit_and
          nbit_xor nbit_or sbit_and sbit_xor sbit_or negate i_negate not
          complement ncomplement scomplement
          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
          isa
          substr vec stringify study pos length index rindex ord chr
          ucfirst lcfirst uc lc fc quotemeta trans transr chop schop
          chomp schomp
          match split qr
          list lslice splice push pop shift unshift reverse
          cond_expr flip flop andassign orassign dorassign and or dor xor
          helemexistsor
          warn die lineseq nextstate scope enter leave
          rv2cv anoncode prototype coreargs avhvswitch anonconst
          emptyavhv
          entersub leavesub leavesublv return method method_named
          method_super method_redir method_redir_super
           -- XXX loops via recursion?
          cmpchain_and cmpchain_dup
          is_bool
          is_weak weaken unweaken
          leaveeval -- needed for Safe to operate, is safe
                       without entereval
          methstart initfield

:base_mem
  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 multiconcat 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.

:base_loop
  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
          goto

:base_io
  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. Usually, to create new filehandles other ops such as open would need to be enabled, if you don't take into account the magical open of ARGV.

          readline rcatline getc read
          formline enterwrite leavewrite
          print say 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 padcv padany padrange introcv clonecv
          once
          rv2gv refgen srefgen ref refassign lvref lvrefslice lvavref
          blessed refaddr reftype
          bless -- could be used to change ownership of objects
                   (reblessing)
           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
          entertrycatch poptry catch leavetrycatch -- similar
          entergiven leavegiven
          enterwhen leavewhen
          break continue
          smartmatch
          pushdefer
          custom -- where should this go
          ceil floor
          is_tainted

:base_math
  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

:base_thread
  These ops are related to multi-threading.

          lock

:default
  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_orig :base_thread

  This list used to contain :base_io prior to Opcode 1.07.

  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) its current definition is unstable while development continues. It will change.

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

: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 warrant having optags defined for them.

  SystemV Interprocess Communications:

          msgctl msgget msgrcv msgsnd
          semctl semget semop
          shmctl shmget shmread shmwrite

:load
  This tag holds opcodes related to loading modules and getting information about calling environment and args.

          require dofile
          caller runcv classname

: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
          hintseval -- constant op holding eval hints
          entereval -- can be used to hide code from initial compile
          reset
          dbstate -- perl -d version of nextstate(ment) opcode

:dangerous
  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

# SEE ALSO

ops -- perl pragma interface to Opcode module.

Safe -- Opcode and namespace limited execution compartments

# AUTHORS

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

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

2025-01-28 | perl v5.40.1