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diff --git a/content/posts/openbsd-looking-at-btrace/index.org b/content/posts/openbsd-looking-at-btrace/index.org deleted file mode 100644 index 24b6171..0000000 --- a/content/posts/openbsd-looking-at-btrace/index.org +++ /dev/null @@ -1,310 +0,0 @@ -#+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. |