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+#+TITLE: Dynamic Tracing on OpenBSD 7.3
+#+DATE: 2023-05-21T23:43:05-04:00
+#+DRAFT: true
+#+DESCRIPTION: Taking a look at OpenBSD's dynamic tracing framework
+#+TAGS[]: openbsd btrace
+#+KEYWORDS[]: openbsd btrace
+#+SLUG:
+#+SUMMARY:
+
+#+ATTR_HTML: :title Puffy with a stopwatch
+#+ATTR_HTML: :alt Puffy with a stopwatch
+[[file:cover.png]]
+
+A couple years ago a new device silently entered the OpenBSD source
+tree.
+
+The =dt(4)= device first appearing in OpenBSD 6.7 with [[https://man.openbsd.org/OpenBSD-6.7/dt][very sparse
+information]]. In the releases since, it's gained quite a bit of
+functionality and now has tools to manipulate it properly. After
+playing around with on my test machine running 7.3-current, it's
+obvious that a lot of work has gone into it.
+
+The documentation has gotten better over the last couple years, but
+there isn't much info about how to leverage it to do anything useful.
+
+The [[https://man.openbsd.org/dt][=dt=]] pseudo device driver is connected to =/dev/dt=, which is
+used to interact with the device through [[https://man.openbsd.org/ioctl.2][=ioctl=]] calls. =dt= stand
+for "dynamic tracer", similar to [[https://en.wikipedia.org/wiki/DTrace][=dtrace=]] and [[https://github.com/iovisor/bpftrace][=bpftrace=]]. It's a
+device that lets you interact with probes on various parts of the
+system.
+
+By default you can't communicate with the device since it can't be
+opened unless the =kern.allowdt= [[https://man.openbsd.org/sysctl.conf][=sysctl=]] flag is set to =1=.
+However you can't set this specific =sysctl= flag while the system's
+=kern.securelevel= is above =0=, which it is under normal circumstances.
+
+The [[https://man.openbsd.org/securelevel][=securelevel=]] of the machine is set during the boot process and
+can't be lowered once the machine is running. That means you need to
+set the flag before the machine's =securelevel= is raised. You can
+do this by adding the following line to [[https://man.openbsd.org/sysctl.conf][=/etc/sysctl.conf=]].
+
+#+begin_src conf
+kern.allowdt=1
+#+end_src
+
+
+You can see in =/etc/rc= that =/etc/sysctl.conf= is loaded very early
+in the boot process, and that the =securelevel= is raised near the end
+of the boot.
+
+#+CAPTION: Taken from =/etc/rc=
+#+begin_src shell
+50 # Apply sysctl.conf(5) settings.
+51 sysctl_conf() {
+52         # do not use a pipe as limits would only be applied to the subshell
+53         set -- $(stripcom /etc/sysctl.conf)
+54         while [[ $# > 0 ]] ; do
+55                 sysctl "$1"
+56
+57                 case "$1" in
+58                 kern.maxproc=*)
+59                         update_limit -p maxproc
+60                         ;;
+61                 kern.maxfiles=*)
+62                         update_limit -n openfiles
+63                         ;;
+64                 esac
+65                 shift
+66         done
+67 }
+
+[...]
+
+589 [[ -f /etc/rc.securelevel ]] && sh /etc/rc.securelevel
+590
+591 # rc.securelevel did not specifically set -1 or 2, so select the default: 1.
+592 (($(sysctl -n kern.securelevel) == 0)) && sysctl kern.securelevel=1
+
+#+end_src
+
+
+You could also run the machine in =Permanently insecure mode= by
+adding ~kern.securelevel=-1~ in =/etc/sysctl.conf=. This would let
+you change the =kern.allowdt= flag at runtime, but it's not
+recommended because it greatly reduces the security of your system.
+
+Once you've added the line to your =sysctl.conf= and rebooted you
+should be able to open the device as root.
+
+The main way to interact with the device is through the [[https://man.openbsd.org/btrace][=btrace=]]
+command. =btrace= is the "bug tracer" tool, used to run scripts
+written in [[https://man.openbsd.org/bt][=bt=]], the bug tracing language. =bt= uses the same syntax
+as Linux's =bpftrace= tool.
+
+Using the =bt= language and =btrace= tool, you can profile system
+internals and probe the inner workings of many programs. You can see
+which processes are forking, opening files, reading and writing to
+file descriptors, using pipes, using chown, pledging, listening on a
+socket, etc. You can see the full kernel and userland stack traces,
+process ids, function arguments, command names, thread id, cpu id,
+and return values.
+
+You can see the full list of supported probes with =btrace -l=.
+
+#+CAPTION: A sample of the available probes
+#+begin_src
+# doas btrace -l
+profile:hz:97
+interval:hz:1
+syscall:exit:entry
+syscall:exit:return
+syscall:fork:entry
+syscall:fork:return
+syscall:read:entry
+syscall:read:return
+syscall:write:entry
+syscall:write:return
+syscall:open:entry
+syscall:open:return
+syscall:close:entry
+syscall:close:return
+syscall:getentropy:entry
+syscall:getentropy:return
+syscall:__tfork:entry
+syscall:__tfork:return
+syscall:unlink:entry
+syscall:unlink:return
+syscall:wait4:entry
+syscall:wait4:return
+syscall:chdir:entry
+syscall:chdir:return
+syscall:fchdir:entry
+syscall:fchdir:return
+syscall:mknod:entry
+syscall:mknod:return
+syscall:chmod:entry
+syscall:chmod:return
+syscall:chown:entry
+syscall:chown:return
+[...]
+tracepoint:raw_syscalls:sys_enter(register_t)
+tracepoint:raw_syscalls:sys_exit(register_t)
+tracepoint:uvm:fault(vaddr_t, vm_fault_t, vm_prot_t)
+tracepoint:uvm:map_insert(vaddr_t, vaddr_t, vm_prot_t)
+tracepoint:uvm:map_remove(vaddr_t, vaddr_t, vm_prot_t)
+tracepoint:uvm:malloc(int, void *, size_t, int)
+tracepoint:uvm:free(int, void *, size_t)
+tracepoint:uvm:pool_get(void *, void *, int)
+tracepoint:uvm:pool_put(void *, void *)
+tracepoint:vfs:bufcache_rel(long, int, int64_t)
+tracepoint:vfs:bufcache_take(long, int, int64_t)
+tracepoint:vfs:cleaner(long, int, long, long)
+tracepoint:vmm:guest_enter(void *, void *)
+tracepoint:vmm:guest_exit(void *, void *, uint64_t)
+tracepoint:vmm:inout(void *, uint16_t, uint8_t, uint32_t)
+[...]
+#+end_src
+
+
+Currently it seems most probes are for system calls =syscall:*= but
+there are also a couple trace points for other function calls, like
+the scheduler =tracepoint:sched:enqueue=, virtual memory system
+=tracepoint:uvm:malloc=, virtual machine manager (vmm)
+=tracepoint:vmm:guest_enter=, etc.
+
+Trace points are often functions that have arguments that you can
+inspect as part of your probe.
+
+You can print values every time a probe is called using =printf=.
+You can also collect aggregates values, build simple histograms,
+calculate sums, or get min/max values. Check [[https://man.openbsd.org/bt.5][=bt=]] man page for full
+syntax. Since it's based on the =bpftrace= syntax you should also be
+able to look at those resources for ideas of how to use it.
+
+The =bt= language is somewhat similar to awk.
+
+=bt= programs have probes, filters, and actions.
+
+They're put together like this.
+
+#+begin_src
+PROBE /FILTER/ { ACTION(S) }
+#+end_src
+
+The action is called every time the probe gets activated, and you
+can optionally filter the probe to only activate on certain
+conditions like when it's a certain PID or thread ID using the
+filter format ~/pid == 1234/~
+
+There are special =BEGIN= and =END= probes that are called once at
+the beginning and end of program execution, which can be used to set
+and clear variables.
+
+Variables are global and take the form of =@var= for a scalars or =@var[key]= for
+a maps. There are functions like =clear(@map)= and =delete(@map[key])= that
+operate on map values.
+
+You can also do basic math operations on values.
+
+Here are a couple examples of how it can be used:
+
+- Print the process name and paid every time =fork= is called
+  #+begin_src
+  doas btrace -e 'syscall:fork:entry { printf("%s[%d]\n", comm, pid) }'
+  ksh[68490]
+  ksh[68490]
+  ksh[68490]
+  ksh[68490]
+  sh[83762]
+  ksh[68490]
+  ksh[87548]
+  ksh[87548]
+  ksh[87548]
+  sh[33064]
+  smtpd[32246]
+  smtpd[33076]
+  smtpd[33076]
+  ksh[88916]
+  smtpd[33076]
+  smtpd[33076]
+  smtpd[33076]
+  ksh[52579]
+  smtpd[33076]
+  smtpd[33076]
+
+  #+end_src
+
+- Count how many time processes call =read= (values printed after Ctrl-C)
+  #+begin_src
+  doas btrace -e 'syscall:read:entry { @[comm] = count() }'
+  ^C
+  @[sshd]: 243
+  @[ksh]: 48
+  @[less]: 31
+  @[cat]: 2
+  #+end_src
+
+- See which processes are calling which syscalls, and how many times
+  #+begin_src
+  doas btrace -e 'tracepoint:raw_syscalls:sys_enter { @[comm, arg0] = count() }'
+  ^C
+  @[sshd, 48]: 354
+  @[top, 49]: 292
+  @[sshd, 109]: 176
+  @[top, 86]: 155
+  @[top, 202]: 124
+  @[sshd, 3]: 92
+  @[sshd, 4]: 92
+  @[less, 86]: 82
+  @[ksh, 4]: 55
+  @[less, 49]: 42
+  @[top, 46]: 36
+  @[sshd, 54]: 34
+  @[top, 169]: 31
+  @[ksh, 3]: 28
+  @[top, 3]: 27
+  @[ksh, 202]: 26
+  @[top, 252]: 22
+  @[ksh, 46]: 20
+  @[top, 253]: 19
+  @[ksh, 40]: 18
+  @[top, 5]: 18
+  @[top, 6]: 18
+  @[top, 74]: 18
+  @[top, 53]: 16
+  @[top, 54]: 15
+  @[ksh, 54]: 14
+  @[ksh, 99]: 14
+  @[less, 159]: 14
+  @[less, 74]: 14
+  @[top, 159]: 14
+  @[top, 4]: 11
+  @[ksh, 48]: 10
+  @[ksh, 6]: 9
+  @[sshd, 73]: 9
+  @[ksh, 38]: 8
+  #+end_src
+
+  You can check the syscall numbers here:
+  https://github.com/openbsd/src/blob/master/sys/kern/syscalls.master
+
+- See a histogram of the size of =read()= calls for a certain process
+  #+begin_src
+  doas btrace -e 'syscall:read:return /pid == 5353/ { @readsize = hist(retval) }'
+  ^C
+  @readsize:
+  [0, 1)             3 |@@@@@@@@@@@@@@@@@@@                                 |
+  [2, 4)             3 |@@@@@@@@@@@@@@@@@@@                                 |
+  [32, 64)           8 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
+  #+end_src
+
+- See a histogram of how long it takes for a =read()= to return
+  #+begin_src
+  doas btrace -e 'syscall:read:entry { @start[tid] = nsecs } syscall:read:return /@start[tid]/ { @times = hist(nsecs - @start[tid]); delete(@start[tid]) } END { clear(@start) }'
+  ^C@times:
+  [0]                3 |@@@                                                 |
+  [1K, 2K)           3 |@@@                                                 |
+  [2K, 4K)          50 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
+  [4K, 8K)          24 |@@@@@@@@@@@@@@@@@@@@@@@@                            |
+  [8K, 16K)          8 |@@@@@@@@                                            |
+  [16M, 32M)         2 |@@                                                  |
+  [32M, 64M)         1 |@                                                   |
+  [64M, 128M)        5 |@@@@@                                               |
+  [128M, 256M)       4 |@@@@                                                |
+  #+end_src
+
+There are also a couple of examples in =/usr/share/btrace= that use
+sampling and command line arguments.
+
+You can also check out the Linux [[https://github.com/iovisor/bpftrace/blob/master/docs/reference_guide.md][=bpftrace= guide]] for ideas.