Design Decisions - Choice of helper library to impelement the capability feature

Problem

Technically, capabilities can be configured via the Linux kernel’s syscall interface. But when it comes to provide a user interface (e.g. crinit task configuration file), we require a reliable API to transform human readable capability identifiers into their numerical representation.

Influencing factors

  • Besides aforementioned necessity to transform capability identifiers, libraries also offer convenience APIs to set capabilities in the kernel. This is less error prone than using the low level syscall interface.

  • When it comes to choose between different libraries, license agreements play an important role

Assumptions

None

Considered Alternatives

1) Use ro library at all

All capability related features are implemented via the syscall interface.

pros

  • No additional license to handle

cons

  • No human readable configuration interface (capability identifiers from /linux/capabilities.h cannot be used): capabilities have to be provided by their numerical values. This is error prone.

  • Conversion between numerical and textual representation of capabilities needs to be implemented and kept in sync with the Linux kernel by ourselves. This is error prone.

2) libcap

Library for capability handling. License BSD-3-Clause OR GPL-2.0-only

pros

  • This library provides all necessary APIs and is especially capable of converting between numerical and textual representation of capabilities.

  • There is an easy to use and well documented API available.

  • (Compared to libcap-ng) The license is better to handle.

cons

  • None that actually matter.

3) libcap-ng

Alternative library for capability handling. License GPL-2.0-or-later, LGPL-2.1-or-later

pros

  • The library provides an equally good feature/API set as libcap.

cons

  • Its license is not really clear, libcap seems to be easier to handle

Decision

Alternative 2 is taken. Feature set of both libraries is roughly the same. The license decided for libcap.

Rationale

Open Points

References

  • https://man7.org/linux/man-pages/man3/libcap.3.html

  • https://github.com/stevegrubb/libcap-ng

  • https://git.kernel.org/pub/scm/libs/libcap/libcap.git/