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x-trace

FreeBSD 11+12 kernel hacking example : x-trace, simple tool to trace execution

Description

This is a simple example of FreeBSD 11 kernel hacking. It comes with a driver (sys/), a userland program that communicates with the driver (bin/xtrace) and two tools (bin/xdump, bin/xdot) to dump the content of the binary file created by xtrace.

How to install

# git clone https://github.com/xigh/x-trace
# cd x-trace
# make

How to use it

Load the kernel module:

# kldload sys/xtrace.ko

Trace the execution of /bin/ls:

# bin/xtrace/xtrace /bin/ls
started pid 4467
Makefile	bin		sys		xtrace.out
child returned with 0

Dump the file generated by xtrace:

# bin/xdump/xdump xtrace.out 
 4466: fork pid=4467
  4467: exec '/bin/ls' argc=1 envc=24
	argv:
	   0: /bin/ls
	envv:
	   0: e
	   1: bin/xtSHELL=/bin/csh
	   2: SSH_CLIENT=2a01:....
	   3: LOGNAME=xigh
	   4: PAGER=more
	   5: MAIL=/var/mail/xigh
	   6: PATH=/sbin:/bin:/usr/sbin:/usr/bin:/usr/local/sbin:...
	   7: EDITOR=vi
	   8: ENV=/home/xigh/.shrc
	   9: OLDPWD=/usr/home/xigh/src
	  10: PWD=/usr/home/xigh/src/xtrace
	  11: TERM=xterm-256color
	  12: SSH_TTY=/dev/pts/1
	  13: HOME=/root
	  14: USER=xigh
	  15: SSH_CONNECTION=2a01:....
	  16: BLOCKSIZE=K
	  17: HOSTTYPE=FreeBSD
	  18: VENDOR=amd
	  19: OSTYPE=FreeBSD
	  20: MACHTYPE=x86_64
	  21: SHLVL=1
	  22: GROUP=wheel
	  23: HOST=fbsd11
 4467: exit rval=0

Or use the new xexec tool I created:

# bin/xexec/xexec xtrace.out
?:/bin/sh
	?:/usr/bin/make
	/usr/home/xigh/src/x-trace:/bin/sh "-c" "cc --version || echo 0.0.0"
		/usr/home/xigh/src/x-trace:/usr/bin/cc "--version"
	/usr/home/xigh/src/x-trace:/bin/sh "-c" "echo \"6.0.1 6.0.1)\" | awk -F. '{print $1 * 10000 + $2 * 100 + $3;}'"
		/usr/home/xigh/src/x-trace:/usr/bin/awk "-F." "{print $1 * 10000 + $2 * 100 + $3;}"
	/usr/home/xigh/src/x-trace:/bin/sh "-c" "{ echo \"__FreeBSD_cc_version\" | cc -E - 2>/dev/null || echo __FreeBSD_cc_version; } | sed -n '$p'"
		/usr/home/xigh/src/x-trace:/usr/bin/sed "-n" "$p"
		/usr/home/xigh/src/x-trace:/usr/bin/cc "-E" "-"
			/usr/home/xigh/src/x-trace:/usr/bin/cc "-cc1" "-triple" "x86_64-unknown-freebsd12.0" "-E" "-disable-free" "-disable-llvm-verifier" "-discard-value-names" "-main-file-name" "-" "-mrelocation-model" "static" "-mthread-model" "posix" "-mdisable-fp-elim" "-masm-verbose" "-mconstructor-aliases" "-munwind-tables" "-fuse-init-array" "-target-cpu" "x86-64" "-dwarf-column-info" "-debugger-tuning=gdb" "-resource-dir" "/usr/lib/clang/6.0.1" "-fdebug-compilation-dir" "/usr/home/xigh/src/x-trace" "-ferror-limit" "19" "-fmessage-length" "0" "-fobjc-runtime=gnustep" "-fdiagnostics-show-option" "-o" "-" "-x" "c" "-"
	/usr/home/xigh/src/x-trace:/bin/sh "-c" "(ld --version || echo none) | sed -n 1p"
		/usr/home/xigh/src/x-trace:/usr/bin/ld "--version"
...

Create a graphviz representation of the trace:

# bin/xdot/xdot xtrace.out
digraph G {
	"0:/usr/bin/make\n0:/usr/bin/make\n" -> "1:/bin/sh\n0:sh\n1:-c\n2:cc --version || echo 0...\n"
	"1:/bin/sh\n0:sh\n1:-c\n2:cc --version || echo 0...\n" -> "2:/usr/bin/cc\n0:cc\n1:--version\n"
	"0:/usr/bin/make\n0:/usr/bin/make\n" -> "3:/bin/sh\n0:sh\n1:-c\n2:echo '3.8.0 3.8.0)' | ...\n"
	"3:/bin/sh\n0:sh\n1:-c\n2:echo '3.8.0 3.8.0)' | ...\n" -> "5:/usr/bin/awk\n0:awk\n1:-F.\n2:{print $1 * 10000 + $2...\n"
	"0:/usr/bin/make\n0:/usr/bin/make\n" -> "6:/bin/sh\n0:sh\n1:-c\n2:{ echo '__FreeBSD_cc_v...\n"
	"6:/bin/sh\n0:sh\n1:-c\n2:{ echo '__FreeBSD_cc_v...\n" -> "8:/usr/bin/sed\n0:sed\n1:-n\n2:$p\n"
	"10:/usr/bin/cc\n0:cc\n1:-E\n2:-\n" -> "11:/usr/bin/cc\n0:/usr/bin/cc\n1:-cc1\n2:-triple\n3:x86_64-unknown-freebsd...\n4:-E\n5:-disable-free\n6:-disable-llvm-verifier\n7:-main-file-name\n8:-\n9:-mrelocation-model\n10:static\n11:-mthread-model\n12:posix\n13:-mdisable-fp-elim\n14:-masm-verbose\n15:-mconstructor-aliases\n16:-munwind-tables\n17:-target-cpu\n18:x86-64\n19:-dwarf-column-info\n20:-debugger-tuning=gdb\n21:-resource-dir\n22:/usr/bin/../lib/clang/...\n23:-fdebug-compilation-dir\n24:/usr/home/xigh/src/xtrace\n25:-ferror-limit\n26:19\n27:-fmessage-length\n28:0\n29:-fobjc-runtime=gnustep\n30:-fdiagnostics-show-option\n31:-o\n32:-\n33:-x\n34:c\n35:-\n"
	"0:/usr/bin/make\n0:/usr/bin/make\n" -> "12:/bin/sh\n0:/bin/sh\n1:-e\n2:-c\n3:target=all;  for dir i...\n"
	"12:/bin/sh\n0:/bin/sh\n1:-e\n2:-c\n3:target=all;  for dir i...\n" -> "13:/usr/bin/make\n0:/usr/bin/make\n1:all\n2:DIRPRFX=sys/\n"
	"13:/usr/bin/make\n0:/usr/bin/make\n1:all\n2:DIRPRFX=sys/\n" -> "14:/bin/sh\n0:sh\n1:-c\n2:find /usr/src/sys/ -na...\n"
	"13:/usr/bin/make\n0:/usr/bin/make\n1:all\n2:DIRPRFX=sys/\n" -> "14:/usr/bin/find\n0:find\n1:/usr/src/sys/\n2:-name\n3:*_if.m\n"
	"13:/usr/bin/make\n0:/usr/bin/make\n1:all\n2:DIRPRFX=sys/\n" -> "15:/bin/sh\n0:/bin/sh\n1:-e\n2:-c\n3:echo 'Warning: Object ...\n"
	"12:/bin/sh\n0:/bin/sh\n1:-e\n2:-c\n3:target=all;  for dir i...\n" -> "16:/usr/bin/make\n0:/usr/bin/make\n1:all\n2:DIRPRFX=bin/\n"
	"16:/usr/bin/make\n0:/usr/bin/make\n1:all\n2:DIRPRFX=bin/\n" -> "17:/bin/sh\n0:/bin/sh\n1:-e\n2:-c\n3:target=all;  for dir i...\n"
	"17:/bin/sh\n0:/bin/sh\n1:-e\n2:-c\n3:target=all;  for dir i...\n" -> "18:/usr/bin/make\n0:/usr/bin/make\n1:all\n2:DIRPRFX=bin/xtrace/\n"
	"18:/usr/bin/make\n0:/usr/bin/make\n1:all\n2:DIRPRFX=bin/xtrace/\n" -> "19:/bin/sh\n0:/bin/sh\n1:-e\n2:-c\n3:echo 'Warning: Object ...\n"
	"17:/bin/sh\n0:/bin/sh\n1:-e\n2:-c\n3:target=all;  for dir i...\n" -> "20:/usr/bin/make\n0:/usr/bin/make\n1:all\n2:DIRPRFX=bin/xdump/\n"
	"20:/usr/bin/make\n0:/usr/bin/make\n1:all\n2:DIRPRFX=bin/xdump/\n" -> "21:/bin/sh\n0:/bin/sh\n1:-e\n2:-c\n3:echo 'Warning: Object ...\n"
	"17:/bin/sh\n0:/bin/sh\n1:-e\n2:-c\n3:target=all;  for dir i...\n" -> "22:/usr/bin/make\n0:/usr/bin/make\n1:all\n2:DIRPRFX=bin/xdot/\n"
	"22:/usr/bin/make\n0:/usr/bin/make\n1:all\n2:DIRPRFX=bin/xdot/\n" -> "23:/bin/sh\n0:/bin/sh\n1:-e\n2:-c\n3:echo 'Warning: Object ...\n"
}

My image

How it works

When the kernel module loads, it install hooks on a few syscalls:

cb_fork = sysent[SYS_fork].sy_call;
sysent[SYS_fork].sy_call =  &xtrace_fork;
cb_vfork = sysent[SYS_vfork].sy_call;
sysent[SYS_vfork].sy_call =  &xtrace_vfork;
cb_exit = sysent[SYS_exit].sy_call;
sysent[SYS_exit].sy_call =  &xtrace_exit;
cb_exec = sysent[SYS_execve].sy_call;
sysent[SYS_execve].sy_call =  &xtrace_exec;
cb_chdir = sysent[SYS_chdir].sy_call;
sysent[SYS_chdir].sy_call = &xtrace_chdir;

Then call the original syscall:

static sy_call_t *cb_exit = 0;
static int xtrace_exit(struct thread *td, void *uap) {
       return cb_exit(td, uap);
}

Licence

Copyright (c) 2013-2019 Philippe Anel. All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

  1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
  2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
  3. Neither the name of the University nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.