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diff --git a/content/posts/openbsd-looking-at-btrace/index.org b/content/posts/openbsd-looking-at-btrace/index.org new file mode 100644 index 0000000..bfd7674 --- /dev/null +++ b/content/posts/openbsd-looking-at-btrace/index.org @@ -0,0 +1,312 @@ +#+TITLE: Looking at btrace 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: + +* What it is + + 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. + +* How to enable it + + 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. + +* What it can do + + 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. + +* Examples + + - 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. |