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libsystrap and trace-syscalls

Interposing on system calls is useful for instrumentation or "virtualisation"-like tools. The code in this repository lets you build programs that can trap, reflect on and modify system calls as they happen within a running program. It consists of a library (libsystrap) and an example tool (, a preloadable tracing library).


Traditionally there are a few ways of interposing on system calls

  • overriding the C library's wrappers
  • ptrace(TRACE_ME)
  • breakpointing on any syscall instruction

The code in this repository takes the latter approach. The former two approaches both have drawbacks: not all syscalls go via libc wrappers, and ptrace requires a separate thread to trace from.

The main drawback of the breakpointing approach is that you take a double trap, increasing the overhead of syscalls (one trap to the interposition code, the other to actually do the syscall). Also, you must parse the instructions on all executable pages before you allow them to run... handling self-modifying or dynamically generated code (JIT) becomes tricky. We solve the latter by a bootstrapping approach: the JIT must create executable mappings by making system calls, which we can intercept as they happen, and perform our breakpointing on the dynamically generated instructions.

What about newer ways like SECCOMP-BPF on Linux? The short answer is: I don't know. It may be possible to use these within the library, to the user's benefit. My first reaction is that newer kernel-side mechanisms are more restrictive, more complex, more change-prone, and interact badly with programs that themselves want to use those features. But they may be worth using despite that.


This is a simple library to do the breakpointing and install a SIGILL handler (which, unlike handling SIGTRAP, doesn't break debugging).

This is a strace-like tool.

This is an extension of trace-syscalls that also collects further semantic information about syscalls, including (thanks to dwarfidl) for system call names and type signatures scraped from the kernel DWARF. It also uses libfootprints to walk the memory foorprints of each call. This will soon be split into its own repository, since it has many more dependencies and is very fragile/experimental at present.


Unless you really know what you're doing, do `make' contrib/ first, and cross your fingers.

Then do `make' in src/, and then in example/. You can use trace-syscalls as a starting point for whatever you want to build.

The contrib/ directory's Makefile builds a big pile of dependencies. You must use this! You can't just use stock libraries on your system, because to pull the code into a preloadable libsystrap-enabled library, it needs to be built as PIC archives. Usually, a Debian-style stock packaging (of glibc, say) will build PIC shared objects and non-PIC archives -- i.e. not the PIC achive we need. Anyway, the contrib/Makefile should take care of all this.

More detail

Some notes on the contents of libsystrap:

trap.c contains the logic of the trapping mechanism, its code is standalone (does not rely on external libraries) and must run before any other code. It replaces system calls in the original code with traps and installs handling mechanisms.

`instr.c' actually does the instrumentation.

do-syscall.c is the interface between the previously mentioned trapping mechanism and the user-provided handling tools.

raw-syscalls.c contains raw assembly implementations of the various sytem calls required in the library itself, since it cannot rely on the C library.

restorer.c is a copy of glibc's signal restorer needed to return into user code.


Unless stated otherwise, the source code in this distribution is made available under the terms of the GNU Lesser General Public License version 3. See the files LICENSE.lgpl3 and LICENSE.gpl3.

would like to discuss alternative licensing arrangements, please contact the principal author, Stephen Kell

Note that a few specific files are under different licensing terms. This is clearly stated at the top of the file. Currently the paths of these files are as follows.


Note also that building this software involves downloading and compiling source code from other projects (collected under the contrib/ directory). Consequently, the output binaries (libsystrap.a, and others) are, in some cases, subject to licensing terms imposed by those projects. To determine the terms applying to a binary you have built, one reliable method is to use the debugging information in that binary to establish which source files it embodies. In particular, x86_decode.c from libx86decode originates in the Xen project and is licensed as "GPLv2 or later".


Monitor, rewrite and/or otherwise trap system calls... on Linux/x86{,-64} only, for now.



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