ld.so(8)
Environment Variables
LD_PRELOAD=<l_so> colon separated list of libso's to be pre loaded
LD_DEBUG=<opts> comma separated list of debug options
=help list available options
=libs show library search path
=files processing of input files
=symbols show search path for symbol lookup
=bindings show against which definition a symbol is bound
LD_PRELOAD: Initialization Order and Link Map
Libraries specified in LD_PRELOAD are loaded from left-to-right but
initialized from right-to-left.
> ldd ./main
>> libc.so.6 => /usr/lib/libc.so.6
> LD_PRELOAD=liba.so:libb.so ./main
-->
preloaded in this order
<--
initialized in this order
The preload order determines:
- the order libraries are inserted into the
link map - the initialization order for libraries
For the example listed above the resulting link map will look like the
following:
+------+ +------+ +------+ +------+
| main | -> | liba | -> | libb | -> | libc |
+------+ +------+ +------+ +------+
This can be seen when running with LD_DEBUG=files:
> LD_DEBUG=files LD_PRELOAD=liba.so:libb.so ./main
# load order (-> determines link map)
>> file=liba.so [0]; generating link map
>> file=libb.so [0]; generating link map
>> file=libc.so.6 [0]; generating link map
# init order
>> calling init: /usr/lib/libc.so.6
>> calling init: <path>/libb.so
>> calling init: <path>/liba.so
>> initialize program: ./main
To verify the link map order we let ld.so resolve the memcpy(3) libc
symbol (used in main) dynamically, while enabling LD_DEBUG=symbols,bindings
to see the resolving in action.
> LD_DEBUG=symbols,bindings LD_PRELOAD=liba.so:libb.so ./main
>> symbol=memcpy; lookup in file=./main [0]
>> symbol=memcpy; lookup in file=<path>/liba.so [0]
>> symbol=memcpy; lookup in file=<path>/libb.so [0]
>> symbol=memcpy; lookup in file=/usr/lib/libc.so.6 [0]
>> binding file ./main [0] to /usr/lib/libc.so.6 [0]: normal symbol `memcpy' [GLIBC_2.14]
Dynamic Linking (x86_64)
Dynamic linking basically works via one indirect jump. It uses a combination of
function trampolines (.plt section) and a function pointer table (.got.plt
section).
On the first call the trampoline sets up some metadata and then jumps to the
ld.so runtime resolve function, which in turn patches the table with the
correct function pointer.
.plt ....... procedure linkage table, contains function trampolines, usually
located in code segment (rx permission)
.got.plt ... global offset table for .plt, holds the function pointer table
Using radare2 we can analyze this in more detail:
[0x00401040]> pd 4 @ section..got.plt
;-- section..got.plt:
;-- .got.plt: ; [22] -rw- section size 32 named .got.plt
;-- _GLOBAL_OFFSET_TABLE_:
[0] 0x00404000 .qword 0x0000000000403e10 ; section..dynamic
[1] 0x00404008 .qword 0x0000000000000000
; CODE XREF from section..plt @ +0x6
[2] 0x00404010 .qword 0x0000000000000000
;-- reloc.puts:
; CODE XREF from sym.imp.puts @ 0x401030
[3] 0x00404018 .qword 0x0000000000401036 ; RELOC 64 puts
[0x00401040]> pd 6 @ section..plt
;-- section..plt:
;-- .plt: ; [12] -r-x section size 32 named .plt
┌─> 0x00401020 ff35e22f0000 push qword [0x00404008]
╎ 0x00401026 ff25e42f0000 jmp qword [0x00404010]
╎ 0x0040102c 0f1f4000 nop dword [rax]
┌ 6: int sym.imp.puts (const char *s);
└ ╎ 0x00401030 ff25e22f0000 jmp qword [reloc.puts]
╎ 0x00401036 6800000000 push 0
└─< 0x0040103b e9e0ffffff jmp sym..plt
- At address
0x00401030in the.pltsection we see the indirect jump forputsusing the function pointer in_GLOBAL_OFFSET_TABLE_[3] (GOT). GOT[3]initially points to instruction after theputstrampoline0x00401036.- This pushes the relocation index
0and then jumps to the first trampoline0x00401020. - The first trampoline jumps to
GOT[2]which will be filled at program startup by theld.sowith its resolve function. - The
ld.soresolve function fixes the relocation referenced by the relocation index pushed by theputstrampoline. - The relocation entry at index
0tells the resolve function which symbol to search for and where to put the function pointer:
As we can see the offset from relocation at index> readelf -r <main> >> Relocation section '.rela.plt' at offset 0x4b8 contains 1 entry: >> Offset Info Type Sym. Value Sym. Name + Addend >> 000000404018 000200000007 R_X86_64_JUMP_SLO 0000000000000000 puts@GLIBC_2.2.5 + 00points toGOT[3].
git(1)
staging
git add -p [<file>] ............ partial staging (interactive)
Remote
git remote -v .................. list remotes verbose (with URLs)
git remote show [-n] <remote> .. list info for <remote> (like remote HEAD,
remote branches, tracking mapping)
Branching
git branch [-a] ................ list available branches; -a to include
remote branches
git branch -vv ................. list branch & annotate with head sha1 &
remote tracking branch
git branch <bname> ............. create branch with name <bname>
git checkout <bname> ........... switch to branch with name <bname>
git push -u origin <rbname> .... push branch to origin (or other remote), and
setup <rbname> as tracking branch
Resetting
git reset [opt] <ref|commit>
opt:
--mixed .................... resets index, but not working tree
--hard ..................... matches the working tree and index to that
of the tree being switched to any changes to
tracked files in the working tree since
<commit> are lost
git reset HEAD <file> .......... remove file from staging
git reset --soft HEAD~1 ........ delete most recent commit but keep work
git reset --hard HEAD~1 ........ delete most recent commit and delete work
Tags
git tag -a <tname> -m "descr" ........ creates an annotated tag (full object
containing tagger, date, ...)
git tag -l ........................... list available tags
git checkout tag/<tname> ............. checkout specific tag
git checkout tag/<tname> -b <bname> .. checkout specific tag in a new branch
Diff
git diff HEAD:<fname> origin/HEAD:<fname> ... diff files for different refs
git diff -U$(wc -l <fname>) <fname> ......... shows complete file with diffs
instead of usual diff snippets
Log
git log --oneline .... shows log in single line per commit -> alias for
'--pretty=oneline --abbrev-commit'
git log --graph ...... text based graph of commit history
git log --decorate ... decorate log with REFs
File history
git log -p <file> ......... show commit history + diffs for <file>
git log --oneline <file> .. show commit history for <file> in compact format
Patching
git format-patch <opt> <since>/<revision range>
opt:
-N ................... use [PATCH] instead [PATCH n/m] in subject when
generating patch description (for patches spanning
multiple commits)
--start-number <n> ... start output file generation with <n> as start
number instead '1'
since spcifier:
-3 .................. e.g: create a patch from last three commits
<commit hash> ....... create patch with commits starting after <commit hash>
git am <patch> ......... apply patch and create a commit for it
git apply --stat <PATCH> ... see which files the patch would change
git apply --check <PATCH> .. see if the patch can be applied cleanly
git apply <PATCH> .......... apply the patch locally without creating a commit
# eg: generate patches for each commit from initial commit on
git format-patch -N $(git rev-list --max-parents=0 HEAD)
# generate single patch file from a certain commit/ref
git format-patch <COMMIT/REF> --stdout > my-patch.patch
Submodules
git submodule add <url> [<path>] .......... add new submodule to current project
git clone --recursive <url> ............... clone project and recursively all
submodules (same as using
'git submodule update --init
--recursive' after clone)
git submodule update --init --recursive ... checkout submodules recursively
using the commit listed in the
super-project (in detached HEAD)
git submodule update --remote <submod> .... fetch & merge remote changes for
<submod>, this will pull
origin/HEAD or a branch specified
for the submodule
git diff --submodule ...................... show commits that are part of the
submodule diff
Inspection
git ls-tree [-r] <ref> .... show git tree for <ref>, -r to recursively ls sub-trees
git show <obj> ............ show <obj>
git cat-file -p <obj> ..... print content of <obj>
Revision Specifier
HEAD ........ last commit
HEAD~1 ...... last commit-1
HEAD~N ...... last commit-N (linear backwards when in tree structure, check
difference between HEAD^ and HEAD~)
git rev-list --max-parents=0 HEAD ........... first commit
bash(1)
Expansion
Generator
# generate sequence from n to m
{n..m}
# generate sequence from n to m step by s
{n..m..s}
# expand cartesian product
{a,b}{c,d}
Parameter
# default param
# if $foo set, then bar=$foo else bar=some_val
bar=${foo:-some_val}
I/O redirection
Note: The trick with bash I/O redirection is to interpret from left-to-right.
# stdout & stderr to file
command >file 2>&1
# equivalent
command &>file
# stderr to stdout & stdout to file
command 2>&1 >file
Explanation
j>&i
Duplicate fd i to fd j, making j a copy of i. See dup2(2).
Example:
command 2>&1 >file
- duplicate
fd 1tofd 2, effectively redirectingstderrtostdout - redirect
stdouttofile
tmux(1)
Terminology:
sessionis a collection of pseudo terminals which can have multiplewindowswindowuses the entire screen and can be split into rectangularpanespaneis a single pseudo terminal instance
Tmux cli
# Session
tmux creates new session
tmux ls list running sessions
tmux kill-session -t <s> kill running session <s>
tmux attach -t <s> [-d] attach to session <s>, detach other clients [-d]
tmux detach -s <s> detach all clients from session <s>
# Environment
tmux showenv -g show global tmux environment variables
tmux setenv -g <var> <val> set variable in global tmux env
# Misc
tmux source-file <file> source config <file>
tmux lscm list available tmux commnds
tmux show -g show global tmux options
tmux display <msg> display message in tmux status line
Scripting
# Session
tmux list-sessions -F '#S' list running sessions, only IDs
# Window
tmux list-windows -F '#I' -t <s> list window IDs for session <s>
tmux selectw -t <s>:<w> select window <w> in session <s>
# Pane
tmux list-panes -F '#P' -t <s>:<w> list pane IDs for window <w> in session <s>
tmux selectp -t <s>:<w>.<p> select pane <p> in window <w> in session <s>
# Run commands
tmux send -t <s>:<w>.<p> "ls" C-m send cmds/keys to pane
tmux run -t <p> <sh-cmd> run shell command <sh-cmd> in background and report output on pane -t <p>
For example cycle through all panes in all windows in all sessions:
# bash
for s in $(tmux list-sessions -F '#S'); do
for w in $(tmux list-windows -F '#I' -t $s); do
for p in $(tmux list-panes -F '#P' -t $s:$w); do
echo $s:$w.$p
done
done
done
Bindings
prefix d detach from current session
prefix c create new window
prefix w open window list
prefix $ rename session
prefix , rename window
prefix . move current window
Following bindings are specific to my tmux.conf:
C-s prefix
# Panes
prefix s horizontal split
prefix v vertical split
prefix f toggle maximize/minimize current pane
# Movement
prefix Tab toggle between window
prefix h move to pane left
prefix j move to pane down
prefix k move to pane up
prefix l move to pane right
# Resize
prefix C-h resize pane left
prefix C-j resize pane down
prefix C-k resize pane up
prefix C-l resize pane right
# Copy/Paste
prefix C-v enter copy mode
prefix C-p paste yanked text
prefix C-b open copy-buffer list
# In Copy Mode
v enable visual mode
y yank selected text
Command mode
To enter command mode prefix :.
Some useful commands are:
setw synchronize-panes on/off enables/disables synchronized input to all panes
list-keys -t vi-copy list keymaps for vi-copy mode
awk(1)
awk [opt] program [input]
-F <sepstr> field separator string (can be regex)
program awk program
input file or stdin if not file given
Input processing
Input is processed in two stages:
- Splitting input into a sequence of
records. By default split atnewlinecharacter, but can be changed via the builtinRSvariable. - Splitting a
recordintofields. By default strings withoutwhitespace, but can be changed via the builtin variableFSor command line option-F.
Fields are accessed as follows:
$0wholerecord$1field one$2field two- ...
Program
An awk program is composed of pairs of the form:
pattern { action }
The program is run against each record in the input stream. If a pattern
matches a record the corresponding action is executed and can access the
fields.
INPUT
|
v
record ----> ∀ pattern matched
| |
v v
fields ----> run associated action
Any valid awk expr can be a pattern.
Special pattern
awk provides two special patterns, BEGIN and END, which can be used
multiple times. Actions with those patterns are executed exactly once.
BEGINactions are run before processing the first recordENDactions are run after processing the last record
Special variables
RSrecord separator: first char is the record separator, by defaultFSfield separator: regex to split records into fields, by defaultNRnumber record: number of current record
Special statements & functions
-
printf "fmt", args...Print format string, args are comma separated.
%sstring%ddecimal%xhex%ffloat
Width can be specified as
%Ns, this reservesNchars for a string. For floats one can use%N.Mf,Nis the total number including.andM. -
strftime("fmt")Print time stamp formatted by
fmt.%Yfull year (eg 2020)%mmonth (01-12)%dday (01-31)%Falias for%Y-%m-%d%Hhour (00-23)%Mminute (00-59)%Ssecond (00-59)%Talias for%H:%M:%S
Examples
Filter records
awk 'NR%2 == 0 { print $0 }' <file>
The pattern NR%2 == 0 matches every second record and the action { print $0 }
prints the whole record.
Capture in variables
# /proc/<pid>/status
# Name: cat
# ...
# VmRSS: 516 kB
# ...
for f in /proc/*/status; do
cat $f | awk '
/^VmRSS/ { rss = $2/1024 }
/^Name/ { name = $2 }
END { printf "%16s %6d MB\n", name, rss }';
done | sort -k2 -n
We capture values from VmRSS and Name into variables and print them at the
END once processing all records is done.
Run shell command and capture output
cat /proc/1/status | awk '
/^Pid/ {
"ps --no-header -o user " $2 | getline user;
print user
}'
We build a ps command line and capture the first line of the processes output
in the user variable and then print it.
gdb(1)
CLI
gdb [opts] [prg [-c coredump | -p pid]]
gdb [opts] --args prg <prg-args>
opts:
-p <pid> attach to pid
-c <coredump> use <coredump>
-x <file> execute script <file> before prompt
-ex <cmd> execute command <cmd> before prompt
--tty <tty> set I/O tty for debugee
Interactive usage
tty <tty>
Set <tty> as tty for debugee.
Make sure nobody reads from target tty, easiest is to spawn a shell
and run following in target tty:
> while true; do sleep 1024; done
set follow-fork-mode <child | parent>
Specify which process to follow when debuggee makes a fork(2)
syscall.
sharedlibrary [<regex>]
Load symbols of shared libs loaded by debugee. Optionally use <regex>
to filter libs for symbol loading.
break [-qualified] <sym> thread <tnum>
Set a breakpoint only for a specific thread.
-qualified: Tred <sym> as fully qualified symbol (quiet handy to set
breakpoints on C symbols in C++ contexts)
rbreak <regex>
Set breakpoints matching <regex>, where matching internally is done
on: .*<regex>.*
command [<bp_list>]
Define commands to run after breakpoint hit. If <bp_list> is not
specified attach command to last created breakpoint. Command block
terminated with 'end' token.
<bp_list>: Space separates list, eg 'command 2 5-8' to run command
for breakpoints: 2,5,6,7,8.
info functions [<regex>]
List functions matching <regex>. List all functions if no <regex>
provided.
info variables [<regex>]
List variables matching <regex>. List all variables if no <regex>
provided.
info handle [<signal>]
Print how to handle <signal>. If no <signal> specified print for all
signals.
handle <signal> <action>
Configure how gdb handles <signal> sent to debugee.
<action>:
stop/nostop Catch signal in gdb and break.
print/noprint Print message when gdb catches signal.
pass/nopass Pass signal down to debugee.
catch signal <signal>
Create a catchpoint for <signal>.
User commands (macros)
Gdb allows to create & document user commands as follows:
define <cmd>
# cmds
end
document <cmd>
# docu
end
To get all user commands or documentations one can use:
help user-defined
help <cmd>
Hooks
Gdb allows to create two types of command hooks
hook-will be run before<cmd>hookpost-will be run after<cmd>
define hook-<cmd>
# cmds
end
define hookpost-<cmd>
# cmds
end
Examples
Catch SIGSEGV and execute commands
This creates a catchpoint for the SIGSEGV signal and attached the command
to it.
catch signal SIGSEGV
command
bt
c
end
Run backtrace on thread 1 (batch mode)
gdb --batch -ex 'thread 1' -ex 'bt' -p <pid>
Script gdb for automating debugging sessions
To script gdb add commands into a file and pass it to gdb via -x.
For example create run.gdb:
set pagination off
break mmap
command
info reg rdi rsi rdx
bt
c
end
#initial drop
c
This script can be used as:
gdb --batch -x ./run.gdb -p <pid>
Know Bugs
Workaround command + finish bug
When using finish inside a command block, commands after finish are not
executed. To workaround that bug one can create a wrapper function which calls
finish.
define handler
bt
finish
info reg rax
end
command
handler
end
radare2(1)
pd <n> [@ <addr>] # print disassembly for <n> instructions
# with optional temporary seek to <addr>
flags
fs # list flag-spaces
fs <fs> # select flag-space <fs>
f # print flags of selected flag-space
help
?*~<kw> # '?*' list all commands and '~' grep for <kw>
?*~... # '..' less mode /'...' interactive search
relocation
> r2 -B <baddr> <exe> # open <exe> mapped to addr <baddr>
oob <addr> # reopen current file at <baddr>
emacs(1)
help
C-h ? list available help modes
C-h f describe function
C-h v describe variable
C-h c <KEY> print command bound to <KEY>
C-h k <KEY> describe command bound to <KEY>
C-h b list buffer local key-bindings
<kseq> C-h list possible key-bindings with <kseq>
eg C-x C-h -> list key-bindings beginning with C-x
package manager
package-refresh-contents refresh package list
package-list-packages list available/installed packages
window
C-x 0 kill focused window
C-x 1 kill all other windows
C-x 2 split horizontal
C-x 3 split vertical
yank/paste
C-<SPACE> set start mark to select text
M-w copy selected text
C-w kill selected text
C-y paste selected text
M-y cycle through kill-ring
block/rect
C-x <SPC> activate rectangle-mark-mode
M-x string-rectangle <RET> insert text in marked rect
mass edit
C-x h mark whole buffer (mark-whole-buffer)
M-x delete-matching-line <RET> delete lines matching regex
M-x % search & replace region (query-replace)
C-M-x % search & replace regex (query-replace-regexp)
grep
M-x find-grep <RET> run find-grep result in *grep* buffer
n/p navigate next/previous match in *grep* buffer
lisp mode
M-x lisp-interaction-mode activate lisp mode
C-M-x evaluate top expr under cursor
C-x C-e eval-last-sexp
C-u C-x C-e eval-last-sexp and prints result in current buffer
narrow
C-x n n show only focused region (narrow)
C-x n w show whole buffer (wide)
org
M-up/M-down re-arrange items in same hierarchy
M-left/M-right change item hierarchy
C-RET create new item below current
C-S-RET create new TODO item below current
S-left/S-right cycle TODO states
org source
<s TAB generate a source block
C-c ' edit source block (in lang specific buffer)
C-c C-c eval source block
fish(1)
keymaps
Shift-Tab ........... tab-completion with search
Alt-Up / Alt-Down ... search history with token under the cursor
Alt-l ............... list content of dir under cursor
Alt-p ............... append '2>&1 | less;' to current cmdline
debug
status print-stack-trace .. prints function stacktrace (can be used in scripts)
breakpoint ................ halt script execution and gives shell (C-d | exit
to continue)
strace(1)
strace [opts] [prg]
-f .......... follow child processes on fork(2)
-p <pid> .... attach to running process
-s <size> ... max string size, truncate of longer (default: 32)
-e <expr> ... expression for trace filtering
-o <file> ... log output into <file>
-c .......... dump syscall statitics at the end
<expr>:
trace=syscall[,syscall] .... trace only syscall listed
trace=file ................. trace all syscall that take a filename as arg
trace=process .............. trace process management related syscalls
trace=signal ............... trace signal related syscalls
signal ..................... trace signals delivered to the process
Examples
Trace open(2) & socket(2) syscalls for a running process + child processes:
strace -f -e trace=open,socket -p <pid>
Trace signals delivered to a running process:
strace -f -e signal -p <pid>
lsof(8)
lsof
-a ......... AND slection filters instead ORing (OR: default)
-p <pid> ... filter by <pid>
+fg ........ show file flags for file descripros
-n ......... don't convert network addr to hostnames
-P ......... don't convert network port to service names
-i <@h[:p]>. show connections to h (hostname|ip addr) with optional port p
file flags:
R/W/RW ..... read/write/read-write
CR ......... create
AP ......... append
TR ......... truncate
Examples
File flags
Show open files with file flags for process:
lsof +fg -p <pid>
Open TCP connections
Show open tcp connections for $USER:
lsof -a -u $USER -i tcp
Note: -a ands the results. If -a is not given all open files matching
$USER and all tcp connections are listed (ored).
Open connection to specific host
Show open connections to localhost for $USER:
lsof -a -u $USER -i @localhost
pidstat(1)
pidstat [opt] [interval] [cont]
-U [user] show username instead UID, optionally only show for user
-r memory statistics
-d I/O statistics
-h single line per process and no lines with average
Page fault and memory utilization
pidstat -r -p <pid> [interval] [count]
minor_pagefault: Happens when the page needed is already in memory but not
allocated to the faulting process, in that case the kernel
only has to create a new page-table entry pointing to the
shared physical page (not required to load a memory page from
disk).
major_pagefault: Happens when the page needed is NOT in memory, the kernel
has to create a new page-table entry and populate the
physical page (required to load a memory page from disk).
I/O statistics
pidstat -d -p <pid> [interval] [count]
/usr/bin/time(1)
# statistics of process run
/usr/bin/time -v <cmd>
pgrep(1)
pgrep [opts] <pattern>
-n only list newest matching process
-u <usr> only show matching for user <usr>
-l additionally list command
-a additionally list command + arguments
Debug newest process
For example attach gdb to newest zsh process from $USER.
gdb -p $(pgrep -n -u $USER zsh)
pstack(1)
pstack <pid>
Dump stack for all threads of process.
perf(1)
perf list show supported hw/sw events
perf stat
-p <pid> .. show stats for running process
-I <ms> ... show stats periodically over interval <ms>
-e <ev> ... filter for events
perf top
-p <pid> .. show stats for running process
-F <hz> ... sampling frequency
-K ........ hide kernel threads
perf record
-p <pid> ............... record stats for running process
-F <hz> ................ sampling frequency
--call-graph <method> .. [fp, dwarf, lbr] method how to caputre backtrace
fp : use frame-pointer, need to compile with
-fno-omit-frame-pointer
dwarf: use .cfi debug information
lbr : use hardware last branch record facility
-g ..................... short-hand for --call-graph fp
-e <ev> ................ filter for events
perf report
-n .................... annotate symbols with nr of samples
--stdio ............... report to stdio, if not presen tui mode
-g graph,0.5,caller ... show caller based call chains with value >0.5
Useful <ev>:
page-faults
minor-faults
major-faults
cpu-cycles`
task-clock
Flamegraph
Flamegraph with single event trace
perf record -g -e cpu-cycles -p <pid>
perf script | FlameGraph/stackcollapse-perf.pl | FlameGraph/flamegraph.pl > cycles-flamegraph.svg
Flamegraph with multiple event traces
perf record -g -e cpu-cycles,page-faults -p <pid>
perf script --per-event-dump
# fold & generate as above
OProfile
operf -g -p <pid>
-g ...... caputre call-graph information
opreport [opt] FILE
show time spent per binary image
-l ...... show time spent per symbol
-c ...... show callgraph information (see below)
-a ...... add column with time spent accumulated over child nodes
ophelp show supported hw/sw events
od(1)
od [opts] <file>
-An don't print addr info
-tx4 print hex in 4 byte chunks
-ta print as named character
-tc printable chars or backslash escape
-w4 print 4 bytes per line
-j <n> skip <n> bytes from <file> (hex if start with 0x)
-N <n> dump <n> bytes (hex of start with 0x)
ASCII to hex string
echo -n AAAABBBB | od -An -w4 -tx4
>> 41414141
>> 42424242
echo -n '\x7fELF\n' | od -tx1 -ta -tc
>> 0000000 7f 45 4c 46 0a # tx1
>> del E L F nl # ta
>> 177 E L F \n # tc
Extract parts of file
For example .rodata section from an elf file. We can use readelf to get the
offset into the file where the .rodata section starts.
readelf -W -S foo
>> Section Headers:
>> [Nr] Name Type Address Off Size ES Flg Lk Inf Al
>> ...
>> [15] .rodata PROGBITS 00000000004009c0 0009c0 000030 00 A 0 0 16
With the offset of -j 0x0009c0 we can dump -N 0x30 bytes from the beginning of
the .rodata section as follows:
od -j 0x0009c0 -N 0x30 -tx4 -w4 foo
>> 0004700 00020001
>> 0004704 00000000
>> *
>> 0004740 00000001
>> 0004744 00000002
>> 0004750 00000003
>> 0004754 00000004
Note: Numbers starting with 0x will be interpreted as hex by od.
xxd(1)
xxd [opts]
-p dump continuous hexdump
-r convert hexdump into binary ('revert')
-e dump as little endian mode
-i output as C array
ASCII to hex stream
echo -n 'aabb' | xxd -p
>> 61616262
Hex to binary stream
echo -n '61616262' | xxd -p -r
>> aabb
ASCII to binary
echo -n '\x7fELF' | xxd -p | xxd -p -r | file -p -
>> ELF
ASCII to C array (hex encoded)
xxd -i <(echo -n '\x7fELF')
>> unsigned char _proc_self_fd_11[] = {
>> 0x7f, 0x45, 0x4c, 0x46
>> };
>> unsigned int _proc_self_fd_11_len = 4;
readelf(1)
readelf [opts] <elf>
-W|--wide wide output, dont break output at 80 chars
-h print ELF header
-S print section headers
-l print program headers + segment mapping
-d print .dynamic section (dynamic link information)
--syms print symbol tables (.symtab .dynsym)
--dyn-syms print dynamic symbol table (exported symbols for dynamic linker)
-r print relocation sections (.rel.*, .rela.*)
objdump(1)
objdump [opts] <elf>
-M intel use intil syntax
-d disassemble text section
-D disassemble all sections
-S mix disassembly with source code
-C demangle
-j <section> display info for section
--[no-]show-raw-insn [dont] show object code next to disassembly
Disassemble section
For example .plt section:
objdump -j .plt -d <elf>
nm(1)
nm [opts] <elf>
-C demangle
-u undefined only
c++filt(1)
Demangle symbol
c++-filt <symbol_str>
Demangle stream
For example dynamic symbol table:
readelf -W --dyn-syms <elf> | c++filt
c++
Type deduction
Force compile error to see what auto is deduced to.
auto foo = bar();
// force compile error
typename decltype(foo)::_;
glibc
malloc tracer mtrace(3)
Trace memory allocation and de-allocation to detect memory leaks.
Need to call mtrace(3) to install the tracing hooks.
If we can't modify the binary to call mtrace we can create a small shared
library and pre-load it.
// libmtrace.c
#include <mcheck.h>
__attribute__((constructor)) static void init_mtrace() { mtrace(); }
Compile as:
gcc -shared -fPIC -o libmtrace.so libmtrace.c
To generate the trace file run:
export MALLOC_TRACE=<file>
LD_PRELOAD=./libmtrace.so <binary>
Note: If MALLOC_TRACE is not set mtrace won't install tracing hooks.
To get the results of the trace file:
mtrace <binary> $MALLOC_TRACE
malloc check mallopt(3)
Configure action when glibc detects memory error.
export MALLOC_CHECK_=<N>
Useful values:
1 print detailed error & continue
3 print detailed error + stack trace + memory mappings & abort
7 print simple error message + stack trace + memory mappings & abort
gcc(1)
CLI
Preprocessing
While debugging can be helpful to just pre-process files.
gcc -E [-dM] ...
-Erun only preprocessor-dMlist only#definestatements
Builtins
__builtin_expect(expr, cond)
Give the compiler a hint which branch is hot, so it can lay out the code accordingly to reduce number of jump instructions. See on compiler explorer.
echo "
extern void foo();
extern void bar();
void run0(int x) {
if (__builtin_expect(x,0)) { foo(); }
else { bar(); }
}
void run1(int x) {
if (__builtin_expect(x,1)) { foo(); }
else { bar(); }
}
" | gcc -O2 -S -masm=intel -o /dev/stdout -xc -
Will generate something similar to the following.
run0:baris on the path without branchrun1:foois on the path without branch
run0:
test edi, edi
jne .L4
xor eax, eax
jmp bar
.L4:
xor eax, eax
jmp foo
run1:
test edi, edi
je .L6
xor eax, eax
jmp foo
.L6:
xor eax, eax
jmp bar