Plash: tools for practical least privilege

FAQs: frequently asked questions

If Plash relies on replacing libc, doesn't this mean that processes can get around the access restrictions by making system calls directly?

No. Plash takes away a process's authority by putting it in a chroot() jail and running it under a freshly-allocated user ID. This stops all of the filename-related system calls from doing anything much.

The modified glibc is not used for taking away authority. It is only used for giving authority back via a different channel. glibc will communicate with a server process via a socket; this is how the filename-related Unix calls such as open() are implemented. The server can send the process file descriptors via the socket.

If a sandboxed program bypasses glibc, it will only be able to see the contents of the chroot jail. If you link a sandboxed program with the regular glibc, it probably won't work.

Why don't you intercept libc calls using an LD_PRELOADed library rather than using a replacement

Plash needs to be able to intercept all calls to functions such as open(). Using an LD_PRELOADed library can only replace open() as seen from outside of It is not able to replace's internal uses of open(). These include:

  • fopen() calls open()
  • calls to open() to read configuration files such as /etc/hosts, /etc/hosts, /etc/resolv.conf
  • calls to open() to read locale data

Other tools, such as fakeroot, fakechroot and cowdancer, use LD_PRELOADed libraries which replace fopen() as well as open(), but they are not able to handle the other cases.

It used to be that you could intercept's internal calls by defining __open and __libc_open in an LD_PRELOADed library. But newer versions of glibc resolve these symbols at static linking time now, so you can't. This was changed for efficiency, so that there are fewer relocations to do at dynamic link time, and so that the calls don't have to go through a jump table. There are also some cases in where the "open" syscall is inlined, such as when using open_not_cancel (a macro).

More importantly, Plash needs to replace the dynamic linker ( so that it doesn't use the "open" syscall, and you can't do that with LD_PRELOAD.

Why don't you use Linux's ptrace() syscall to intercept system calls instead?

Firstly, performance: ptrace() is slow, because it intercepts all system calls, and the monitor process can only read the traced process's address space one word at a time. In contrast, Plash does not need to intercept frequently-used calls such as read() and write() at all.

Secondly, ptrace() can only be used to allow or block a traced process's system calls. This leads to TOCTTOU (time-of-check to time-of-use) race conditions when checking whether to allow operations using filenames, particularly when symlinks are involved. ptrace() on its own does not let us virtualize the file namespace, as Plash does.

Systrace addresses some of the problems of ptrace(), but it is not included in mainline versions of the Linux kernel.

It used to be that there was a race condition in which a newly forked process would not initially be traced, which mean that ptrace() was not secure for sandboxing programs that need to use fork(). I believe this has now been fixed.