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kfuncs are functions defined in the kernel, which may be invoked by BPF programs. They may or may not also be used as regular kernel functions, implying that they may be static (in which case the compiler could e.g. inline it away, or elide one or more arguments), or it could have external linkage, but potentially be elided in an LTO build if a function is observed to never be used, and is stripped from the final kernel binary. This has already resulted in some issues, such as those discussed in [0] wherein changes in DWARF that identify when a parameter has been optimized out can break BTF encodings (and in general break the kfunc). [0]: https://lore.kernel.org/all/1675088985-20300-2-git-send-email-alan.maguire@oracle.com/ We therefore require some convenience macro that kfunc developers can use just add to their kfuncs, and which will prevent all of the above issues from happening. This is in contrast with what we have today, where some kfunc definitions have "noinline", some have "__used", and others are static and have neither. Note that longer term, this mechanism may be replaced by a macro that more closely resembles EXPORT_SYMBOL_GPL(), as described in [1]. For now, we're going with this shorter-term approach to fix existing issues in kfuncs. [1]: https://lore.kernel.org/lkml/Y9AFT4pTydKh+PD3@maniforge.lan/ Note as well that checkpatch complains about this patch with the following: ERROR: Macros with complex values should be enclosed in parentheses +#define __bpf_kfunc __used noinline There seems to be a precedent for using this pattern in other places such as compiler_types.h (see e.g. __randomize_layout and noinstr), so it seems appropriate. Signed-off-by: David Vernet <void@manifault.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Acked-by: Stanislav Fomichev <sdf@google.com> Link: https://lore.kernel.org/bpf/20230201173016.342758-2-void@manifault.com |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.