sysctl(9): Ease exporting struct sizes; Discourage doing that
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Authored by olce on May 2 2025, 8:29 PM.

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Reviewers

kib
markj

Commits

rGde6a4418eec9: sysctl(9): Ease exporting struct sizes; Discourage doing that
rGefce9f8a510b: sysctl(9): Ease exporting struct sizes; Discourage doing that
rG713abc9880aa: sysctl(9): Ease exporting struct sizes; Discourage doing that

Summary

Introduce two helpers, the more general SYSCTL_SIZEOF() and a struct-specific
one SYSCTL_SIZEOF_STRUCT() which prepends 'struct' in the description and in the
use of sizeof() but uses the raw structure name as the knob's name. The size of
the object/structure is exported under 'debug.sizeof'.

Existing knobs under 'debug.sizeof' were all converted to use the helpers.

Add a note before the helpers discouraging the introduction of new leaves for
ad-hoc reasons. List alternative means for developers to obtain the size of
arbitrary kernel structures easily (thanks to markj@ for providing these).

No functional change (intended).

Diff Detail

Repository

rG FreeBSD src repository

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Unit

Tests Not Applicable

Event Timeline

olce created this revision.May 2 2025, 8:29 PM

Herald added subscribers: delphij, imp. · View Herald TranscriptMay 2 2025, 8:29 PM

olce requested review of this revision.May 2 2025, 8:29 PM

Harbormaster completed remote builds in B63841: Diff 154718.May 2 2025, 8:29 PM

olce added a child revision: D50122: VFS cache: Export sizeof(struct namecache_ts) in sysctl MIB.May 2 2025, 8:29 PM

guest-patmaddox added a subscriber: guest-patmaddox.May 3 2025, 7:38 PM

After giving up on D50122 and D50123, this one has less value. However, I still think it would be fine to commit it with some comments near the definitions to say people should avoid adding new uses. Objections?

kib accepted this revision.May 5 2025, 6:42 PM

This revision is now accepted and ready to land.May 5 2025, 6:42 PM

Add a note before the new macros that 'debug.sizeof' is considered obsolete and people should not add new leaves to it but instead the listed means to obtain the size of kernel structures.

This revision now requires review to proceed.May 6 2025, 1:06 PM

Harbormaster completed remote builds in B63938: Diff 154937.May 6 2025, 1:06 PM

markj added inline comments.May 6 2025, 2:03 PM

sys/sys/sysctl.h
937	It's not "obsolete" really, it's just that using this tree for ad-hoc purposes is discouraged. The methods below aren't so good for programmatic checking, and something which looks through data structures in kernel memory might want to use a sizeof node as a seatbelt to check for KBI changes. Of course, ideally we'd provide proper interfaces for any such applications, but sometimes we're too slow.