NAME
scanmem - locate and modify a variable in an executing process.
SYNOPSIS
scanmem [--version] [--help] [--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.
--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.
n
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-ids 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.
SEE ALSO
gdb(1) ptrace(2) nethack(6) pidof(8)