Conference Schedule - BSDCan 2020

sysctlinfo: a new interface to visit the FreeBSD sysctl MIB and to pass the objects info to userland

Date: 2020-06-06
Time: 11:30 - 12:15
Room: Stream 2
Level: Intermediate

All times are based on Ottawa time, which is UTC -4. For example, 9AM is 1400 UTC.

The FreeBSD kernel maintains a Management Information Base (MIB) where a component (object) represents a parameter of the system. The sysctl system call explores the MIB to find an object by its Object Identifier (OID) and calls its handler to get or set the value of the parameter. It is often necessary to find an object not to call its handler but to get its info (description, type, name, next object, etc.), so the kernel provides an undocumented interface implemented in kern_sysctl.c.

sysctlinfo is a new interface to explore the sysctl MIB and to pass the info of an object to the userland. The project provides: a README, a manual, helper macros, examples, tests and converted tools.

Primarily sysctlinfo provides a new set of sysctl nodes (corresponding to the kernel interface) to handle an object up to CTL_MAXNAME levels: sysctl.entryfakename, sysctl.entrydesc, sysctl.entrylabel, sysctl.entrykind, sysctl.entryfmt and sysctl.entrynextleaf. Moreover new features have been implemented: the support for the capability mode, sysctl.entryname, sysctl.entryidbyname and sysctl.entrynextnode. To get all the info about an object the kernel needs to find it many times, then the new sysctl.entryallinfo nodes were written, they are 30% more efficient. Finally, byname nodes were added: they search the object by its name avoiding to call sysctl.name2oid (or similar) to explore the MIB just to find the corresponding OID; note, the sysctl.idinputbyname node allows to sysctlbyname(3) to manage a CTLTYPE_NODE with a no-NULL handler, example "kern.proc.pid.<pid>".

sysctlinfo can be installed via sysutils/sysctlinfo-kmod or by applying the sysctlinfo.diff patch (more efficient than the module because uses a shared lock). The interface is used by deskutils/sysctlview, sysutils/nsysctl, devel/libsysctlmibinfo2 and the converted version of sysctl(8), sysctlbyname(3), sysctlnametomib(3), they should be used to get the value of an object with 23/24 levels or if some level-name has only the '\0' character.

Paper

  • http://alfix.gitlab.io/assets/sysctlinfo_bsdcan_2020.pdf

Projects:

  • https://gitlab.com/alfix/sysctlinfo
  • https://gitlab.com/alfix/sysctlbyname-improved

Real World use cases:

  • sysctlview, https://gitlab.com/alfix/sysctlview/
  • nsysctl, https://gitlab.com/alfix/nsysctl
  • sysctlmibinfo2, https://gitlab.com/alfix/sysctlmibinfo2

Slides

The following slides have been made available for this session:

Speaker

Alfonso S. Siciliano