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.