Ubuntu Manpage: scanmem - locate and modify a variable in an executing process.

NAME

       scanmem - locate and modify a variable in an executing process.

SYNOPSIS

       scanmem [--version] [--help] [--debug] [--backend] target-program-pid

DESCRIPTION

       scanmem  is  an interactive debugging utility that can be used to isolate the address of a
       variable in an executing process by  successively  scanning  the  process'  address  space
       looking  for  matching  values. By informing scanmem how the value of the variable changes
       over time, it can determine  the  actual  location  (or  locations)  of  the  variable  by
       successively  eliminating  non-matches.  scanmem determines where to look by searching for
       mappings with read / write permission,  these  are  referred  to  as  regions.  Users  can
       eliminate  regions  they believe are likely unrelated to the target variable (for example,
       located in a shared library unrelated to the variable in question), this will improve  the
       speed of the scan, which can initially be quite slow in large programs.

       Once  a  variable has been found, scanmem can monitor the variable, or change it to a user
       specified value, either once, or continually over a period of time.

       scanmem works similarly to the " pokefinders " once commonly used to cheat at video games,
       this  function  is  a  good  demonstration  of  how  to  use  scanmem , and is used in the
       documentation.

USAGE

       scanmem should be invoked with the process id of the program  you  wish  to  debug  as  an
       argument.  Once  started, scanmem accepts interactive commands. These are described below,
       however entering help  at  the  >  prompt  will  allow  you  to  access  scanmem's  online
       documentation.

       The  target-program-pid  can  be  specified  in  decimal,  hexadecimal, or octal using the
       standard C language notation (leading 0x for hexadecimal, leading 0  for  octal,  anything
       else is assumed to be decimal).

       --version

       Print version and exit.

       --help

       Print a short description of command line options then exit.

       --debug

       Run in debug mode, more information will be outputted.

       --backend

       Work as backend, normal users should not use this paratmeter.

COMMANDS

While in interactive mode, scanmem prints a decimal number followed by > , the number is
the current number of possible candidates for the target variable that are known. 0
indicates that no possible variables have been eliminated yet.

Where n represents any number in decimal, octal or hexadecimal, this command tells scanmem
that the current value of the target variable is exactly n. scanmem will begin a search
of the entire address space, or the existsing known matches (if any), eliminating any
variable that does not have this value.

set [match-id][,match-id,...]=]value[/delay] [...]

Set the value value into the match numbers match-id , or if just value is specified, all
known matches. value can be specified in standard C language notation. All known matches,
along with their match-id's can be displayed using the list command. Multiple match-id's
can be specified, separated with commas and terminated with an = sign. To set a value
continually, suffix the command with / followed by the number of seconds to wait between
sets. You can interrupt the set command with ^C to return to the scanmem prompt. This can
be used to sustain the value of a variable which decreases overtime, for example a timer
that is decremented every second can be set to 100 every 10 seconds to prevent some
property from ever changing.

This command is used to change the value of the variable(s) once found by elimination.
Please note, some applications will store values in multiple locations.

dump <address> <length> [<filename>]

Dump the memory region starting from <address> of length <length> into a human-readable
format.

If <filename> is given, data will be saved into the file, otherwise data will be displayed
into stdout in a human readable format

write <value_type> <address> <value>

Manually set the value of the variable at the speicified address.

Names of value_type are subject to change in different versions of scanmem, see more info
using the `help write' command.

Tells scanmem that it should eliminate all matched variables that have not increased since
the last search. For example, if the value of a variable is known to be zero when a
program is executed, but increases over time, this command can be used several times to
eliminate variables that have decreased or not changed.

As for > but indicates that the target variable has decreased since the last scan.

As for > but indicates that the target variable has not changed since last scan.

snapshot

Save a snapshot of existing program state, for use with > , < , and = , although other
commands can still be used.

list

List all the possible candidates currently known, including their address, last known
value and possible types. The value in the first column is the match id, and can be used
in conjunction with the delete command to eliminate matches.

delete [match-id]

Delete match match-id , which can be found from the output of the list command. To delete
all matches, see the reset command, or to delete all matches associated with a particular
library, see the dregion command, which also removes any associated matches. Pleae note
that match-ids may be recalculated after matches are removed or added.

watch [match-id]

Monitor the value of match-id , and print its value as it changes. Every change is printed
along with a timestamp, you can interrupt this command with ^C to stop monitoring.

pid [new-pid]

Print out the process id of the current target program, or change the target to new-pid ,
which will reset existing regions and matches.

lregions

List all the known regions, this can be used in combination with the dregion command to
eliminate regions that the user believes are not related to the variable in question, thus
reducing the address space required to search. The value in the first column is the
region-id which must be passed to the dregion command. The size and path (if applicable)
is also printed. This can be used to eliminate regions located in shared libraries that
are unlikely to be relevant to the variable required.

dregion [!][region-id][,region-id][,...]

Delete the region region-id , along with any matches from the match list. The region-id
can be found in the output of the lregions command. A leading ! indicates the list should
be inverted.

reset

Forget all known matches and start again.

shell [shell-command]

Execute shell-command using /bin/sh, then return.

option <name> <value>

Change options in runtime. See `help option` for all possible names/values.

version

Print the version of scanmem in use.

help

Print a short summary of available commands.

exit

Detach from the target program and exit immediately.

EXAMPLES

       Cheat at nethack, on systems where nethack is not installed sgid.

       $ scanmem `pidof nethack`
       info: attaching to pid 13070.
       info: maps file located at /proc/13070/maps opened.
       info: 17 suitable regions found.
       Please enter current value, or "help" for other commands.
       0>

       The 0 in the scanmem prompt indicates we currently have no candidates, so I enter how much
       gold I currently have (91 pieces) and let scanmem find the potential candidates.

       0> 91
       info: searching 0xbfffa000 - 0xc0000000...........ok
       info: searching 0x401c2000 - 0x401e3000...........ok
       info: searching 0x401c1000 - 0x401c2000...........ok
       info: searching 0x401b6000 - 0x401b8000...........ok
       info: searching 0x401b5000 - 0x401b6000...........ok
       info: searching 0x40189000 - 0x4018a000...........ok
       info: searching 0x40188000 - 0x40189000...........ok
       info: searching 0x40181000 - 0x40183000...........ok
       info: searching 0x4017f000 - 0x40181000...........ok
       info: searching 0x40070000 - 0x40071000...........ok
       info: searching 0x40068000 - 0x40070000...........ok
       info: searching 0x40030000 - 0x40031000...........ok
       info: searching 0x40029000 - 0x4002a000...........ok
       info: searching 0x4001f000 - 0x40020000...........ok
       info: searching 0x40016000 - 0x40017000...........ok
       info: searching 0x081d4000 - 0x0820a000...........ok
       info: searching 0x081b7000 - 0x081d4000...........ok
       info: we currently have 16 matches.
       16> list
       [ 0] 0x081c1f34 {        91} (/usr/share/games/nethack/nethack)
       [ 1] 0x081c1780 {        91} (/usr/share/games/nethack/nethack)
       [ 2] 0x081be436 {        91} (/usr/share/games/nethack/nethack)
       [ 3] 0x081eeffc {        91} (unassociated, typically .bss)
       [ 4] 0x081ee0c0 {        91} (unassociated, typically .bss)
       [ 5] 0x081eddb8 {        91} (unassociated, typically .bss)
       [ 6] 0x081d6d88 {        91} (unassociated, typically .bss)
       [ 7] 0x4001fcd3 {        91} (/lib/libnss_compat-2.3.5.so)
       [ 8] 0x40029fe3 {        91} (/lib/libnss_nis-2.3.5.so)
       [ 9] 0x40029f8b {        91} (/lib/libnss_nis-2.3.5.so)
       [10] 0x40029efb {        91} (/lib/libnss_nis-2.3.5.so)
       [11] 0x40029bff {        91} (/lib/libnss_nis-2.3.5.so)
       [12] 0x401d18d3 {        91} (unassociated, typically .bss)
       [13] 0x401d156f {        91} (unassociated, typically .bss)
       [14] 0x401d120b {        91} (unassociated, typically .bss)
       [15] 0xbfffd76c {        91} (unassociated, typically .bss)
       16>

       16  potential  matches  were found, many of them are clearly unrelated, as they're part of
       unrelated libraries (libnss_nis.so). We could make scanmem eliminate these manually  using
       the  delete  command,  however  just  waiting until the amount of gold changes and telling
       scanmem the new value should be enough. I find some more gold, and tell  scanmem  the  new
       value, 112.

       16> 112
       info: we currently have 1 matches.
       info: match identified, use "set" to modify value.
       info: enter "help" for other commands.
       1> list
       [ 0] 0x081d6d88 {       112} (unassociated, typically .bss)

       Only  one  of the 16 original candidates now have the value 112, so this must be where the
       amount of gold is stored. I'll try setting it to 10,000 pieces.

       1> set 10000
       info: setting *0x081d6d88 to 10000...
       1>

       The resulting nethack status:

       Dlvl:1  $:10000 HP:15(15) Pw:2(2) AC:7  Exp:1

NOTES

       scanmem has been tested on multiple large programs, including the  3d  shoot-em-up  quake3
       linux.

       Obviously, scanmem can crash your program if used incorrectly.

       Some  programs  store  values in multiple locations, this is why set will change all known
       matches.

BUGS

       The first scan can be very slow on large programs, this is not a  problem  for  subsequent
       scans as huge portions of the address space are usually eliminated. This could be improved
       in future, perhaps by assuming all integers are aligned by default. Suggestions welcome.

       The snapshot command uses memory inefficiently, and should probably not be used  on  large
       programs.  In future this will use a more intelligent format.

HOMEPAGE

       http://code.google.com/p/scanmem/

AUTHORS

       Tavis Ormandy <taviso(a)sdf.lonestar.org> http://taviso.decsystem.org/
       Eli   Dupree  <elidupree(a)charter.net>
       WANG  Lu      <coolwanglu(a)gmail.com>

       All bug reports, suggestions or feedback welcome.