Re: [PATCH v5 00/11] Symbol Namespaces

[Jessica Yu]

+++ Matthias Maennich [06/09/19 11:32 +0100]:
> As of Linux 5.3-rc7, there are 31207 [1] exported symbols in the kernel.
> That is a growth of roughly 1000 symbols since 4.17 (30206 [2]). There
> seems to be some consensus amongst kernel devs that the export surface
> is too large, and hard to reason about.
>
> Generally, these symbols fall in one of these categories:
> 1) Symbols actually meant for drivers
> 2) Symbols that are only exported because functionality is split over
>   multiple modules, yet they really shouldn't be used by modules outside
>   of their own subsystem
> 3) Symbols really only meant for in-tree use
>
> When module developers try to upstream their code, it regularly turns
> out that they are using exported symbols that they really shouldn't be
> using. This problem is even bigger for drivers that are currently
> out-of-tree, which may be using many symbols that they shouldn't be
> using, and that break when those symbols are removed or modified.
>
> This patch allows subsystem maintainers to partition their exported
> symbols into separate namespaces, and module authors to import such
> namespaces only when needed.
>
> This allows subsystem maintainers to more easily limit availability of
> these namespaced symbols to other parts of the kernel. It can also be
> used to partition the set of exported symbols for documentation
> purposes; for example, a set of symbols that is really only used for
> debugging could be in a "SUBSYSTEM_DEBUG" namespace.
>
> I continued the work mainly done by Martijn Coenen.
>
> Changes in v2:
> - Rather than adding and evaluating separate sections __knsimport_NS,
>  use modinfo tags to declare the namespaces a module introduces.
>  Adjust modpost and the module loader accordingly.
> - Also add support for reading multiple modinfo values for the same tag
>  to allow list-like access to modinfo tags.
> - The macros in export.h have been cleaned up to avoid redundancy in the
>  macro parameters (ns, nspost, nspost2).
> - The introduction of relative references in the ksymtab entries caused
>  a rework of the macros to accommodate that configuration as well.
> - Alignment of kernel_symbol in the ksymtab needed to be fixed to allow
>  growing the kernel_symbol struct.
> - Modpost does now also append the namespace suffix to the symbol
>  entries in Module.symvers.
> - The configuration option MODULE_ALLOW_MISSING_NAMESPACE_IMPORTS allows
>  relaxing the enforcement of properly declared namespace imports at
>  module loading time.
> - Symbols can be collectively exported into a namespace by defining
>  DEFAULT_SYMBOL_NAMESPACE in the corresponding Makefile.
> - The requirement for a very recent coccinelle spatch has been lifted by
>  simplifying the script.
> - nsdeps does now ensures MODULE_IMPORT_NS statements are sorted when
>  patching the module source files.
> - Some minor bugs have been addressed in nsdeps to allow it to work with
>  modules that have more than one source file.
> - The RFC for the usb-storage symbols has been simplified by using
>  DEFAULT_SYMBOL_NAMESPACE=USB_STORAGE rather than explicitly exporting
>  each and every symbol into that new namespace.
>
> Changes in v3:
> - Reword the documentation for the
>  MODULE_ALLOW_MISSING_NAMESPACE_IMPORTS option for clarification.
> - Fix printed required version of spatch in coccinelle script.
> - Adopt kbuild changes for modpost: .mod files are no longer generated
>  in .tmp_versions. Similarely, generate the .ns_deps files in the tree
>  along with the .mod files. Also, nsdeps now uses modules.order as
>  source for the list modules to consider.
> - Add an RFC patch to introduce the namespace WATCHDOG_CORE for symbols
>  exported in watchdog_core.c.
>
> Changes in v4:
> - scripts/nsdeps:
>  - exit on first error
>  - support out-of-tree builds O=...
> - scripts/export_report.pl: update for new Module.symvers format
> - scripts/mod/modpost: make the namespace a separate field when
>  exporting to Module.symvers (rather than symbol.NS)
> - include/linux/export.h: fixed style nits
> - kernel/module.c: ensure namespaces are imported before taking a
>  reference to the owner module
> - Documentation: document the Symbol Namespace feature and update
>  references to Module.symvers and EXPORT_SYMBOL*
>
> Changes in v5:
> - Makefile: let 'nsdeps' depend on 'modules' to allow
>  `make clean; make nsdeps` to work
> - scripts/nsdeps: drop 'exit on first error' again as it just makes more
>  problems than it solves
> - drop the watchdog RFC patch for now
>
> This patch series was developed against v5.3-rc7.

Great work Matthias!

I think this patchset is shaping up nicely. As the merge window is
coming up soon, I'd like to queue this up in modules-next by the end
of today to allow for some testing and "soak" time in linux-next. If
there are any more complaints, please speak up.

Thanks!

Jessica