Thursday, 22 November 2018

High-level tracing with bpftrace

Bpftrace is a new high-level tracing language for Linux using the extended Berkeley packet filter (eBPF).  It is a very powerful and flexible tracing front-end that enables systems to be analyzed much like DTrace.

The bpftrace tool is now installable as a snap. From the command line one can install it and enable it to use system tracing as follows:

 sudo snap install bpftrace  
 sudo snap connect bpftrace:system-trace  

To illustrate the power of bpftrace, here are some simple one-liners:

 # trace openat() system calls
 sudo bpftrace -e 'tracepoint:syscalls:sys_enter_openat { printf("%d %s %s\n", pid, comm, str(args->filename)); }'   
 Attaching 1 probe...  
 1080 irqbalance /proc/interrupts  
 1080 irqbalance /proc/stat  
 2255 dmesg /etc/ld.so.cache  
 2255 dmesg /lib/x86_64-linux-gnu/libtinfo.so.5  
 2255 dmesg /lib/x86_64-linux-gnu/librt.so.1  
 2255 dmesg /lib/x86_64-linux-gnu/libc.so.6  
 2255 dmesg /lib/x86_64-linux-gnu/libpthread.so.0  
 2255 dmesg /usr/lib/locale/locale-archive  
 2255 dmesg /lib/terminfo/l/linux  
 2255 dmesg /home/king/.config/terminal-colors.d  
 2255 dmesg /etc/terminal-colors.d  
 2255 dmesg /dev/kmsg  
 2255 dmesg /usr/lib/x86_64-linux-gnu/gconv/gconv-modules.cache  

 # count system calls using tracepoints:  
 sudo bpftrace -e 'tracepoint:syscalls:sys_enter_* { @[probe] = count(); }'  
 @[tracepoint:syscalls:sys_enter_getsockname]: 1  
 @[tracepoint:syscalls:sys_enter_kill]: 1  
 @[tracepoint:syscalls:sys_enter_prctl]: 1  
 @[tracepoint:syscalls:sys_enter_epoll_wait]: 1  
 @[tracepoint:syscalls:sys_enter_signalfd4]: 2  
 @[tracepoint:syscalls:sys_enter_utimensat]: 2  
 @[tracepoint:syscalls:sys_enter_set_robust_list]: 2  
 @[tracepoint:syscalls:sys_enter_poll]: 2  
 @[tracepoint:syscalls:sys_enter_socket]: 3  
 @[tracepoint:syscalls:sys_enter_getrandom]: 3  
 @[tracepoint:syscalls:sys_enter_setsockopt]: 3  
 ...  

Note that it is recommended to use bpftrace with Linux 4.9 or higher.

The bpftrace github project page has an excellent README guide with some worked examples and is a very good place to start.  There is also a very useful reference guide and one-liner tutorial too.

If you have any useful btftrace one-liners, it would be great to share them. This is an amazingly powerful tool, and it would be interesting to see how it will be used.

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