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ply

Documentation and language reference is available at wkz.github.io/ply.

A light-weight dynamic tracer for Linux that leverages the kernel's BPF VM in concert with kprobes and tracepoints to attach probes to arbitrary points in the kernel. Most tracers that generate BPF bytecode are based on the LLVM based BCC toolchain. ply on the other hand has no required external dependencies except for libc. In addition to x86_64, ply also runs on aarch64, arm, riscv64 and powerpc. Adding support for more ISAs is easy.

ply follows the Little Language approach of yore, compiling ply scripts into Linux BPF programs that are attached to kprobes and tracepoints in the kernel. The scripts have a C-like syntax, heavily inspired by dtrace(1) and, by extension, awk(1).

The primary goals of ply are:

  • Expose most of the BPF tracing feature-set in such a way that new scripts can be whipped up very quickly to test different hypotheses.

  • Keep dependencies to a minimum. Right now Flex and Bison are required at build-time, leaving libc as the only runtime dependency. Thus, ply is well suited for embedded targets.

If you need more fine-grained control over the kernel/userspace interaction in your tracing, checkout the bcc project which compiles C programs to BPF using LLVM in combination with a python userspace recipient to give you the full six degrees of freedom.

Examples

Here are some one-liner examples to show the kinds of questions that ply can help answer.

What is the distribution of the returned sizes from read(2)s to the VFS?

ply 'kretprobe:vfs_read { @["size"] = quantize(retval); }'

Which processes are receiving errors when reading from the VFS?

ply 'kretprobe:vfs_read if (retval < 0) { @[pid, comm, retval] = count(); }'

Which files are being opened, by who?

ply 'kprobe:do_sys_open { printf("%v(%v): %s\n", comm, uid, str(arg1)); }'

When sending packets, where are we coming from?

ply 'kprobe:dev_queue_xmit { @[stack] = count(); }'

From which hosts and ports are we receiving TCP resets?

ply 'tracepoint:tcp/tcp_receive_reset {
	printf("saddr:%v port:%v->%v\n",
		data->saddr, data->sport, data->dport);
}'

Build and Installation

ply uses GNU's autotools as its build system. When building from a Git clone, use the following steps:

./autogen.sh   # to generate the configure script
./configure
make
make install   # you probably need to be root for this

Contributing

Contributions are welcome! To help you on your way, the test/ directory contains ready-made infrastructure to:

  • Test cross-compilation on all supported architectures.
  • Run a simple test suite on a range of machines using QEMU system emulation.
  • Run interactive sessions on QEMU machines.

A GitHub Action is setup to run these jobs. Please make sure to test your changes locally before opening a PR to avoid unnecessary review cycles.

Maintainers

ply is developed and maintained by Tobias Waldekranz. Please direct all bug reports and pull requests towards the official Github repo.