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[bpf-next,v3,1/4] bpf: Add __bpf_kfunc tag for marking kernel functions as kfuncs

Message ID 20230201173016.342758-2-void@manifault.com (mailing list archive)
State Accepted
Commit 57e7c169cd6afa093d858b8edfb9bceaf2e1c93b
Delegated to: BPF
Headers show
Series bpf: Add __bpf_kfunc tag for kfunc definitions | expand

Checks

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netdev/fixes_present success Fixes tag not required for -next series
netdev/subject_prefix success Link
netdev/cover_letter success Series has a cover letter
netdev/patch_count success Link
netdev/header_inline success No static functions without inline keyword in header files
netdev/build_32bit success Errors and warnings before: 1419 this patch: 1419
netdev/cc_maintainers warning 1 maintainers not CCed: yhs@fb.com
netdev/build_clang success Errors and warnings before: 149 this patch: 149
netdev/module_param success Was 0 now: 0
netdev/verify_signedoff success Signed-off-by tag matches author and committer
netdev/check_selftest success No net selftest shell script
netdev/verify_fixes success No Fixes tag
netdev/build_allmodconfig_warn success Errors and warnings before: 1414 this patch: 1414
netdev/checkpatch fail ERROR: Macros with complex values should be enclosed in parentheses
netdev/kdoc success Errors and warnings before: 0 this patch: 0
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Commit Message

David Vernet Feb. 1, 2023, 5:30 p.m. UTC 
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.

Acked-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: David Vernet <void@manifault.com>
---
 include/linux/btf.h | 8 ++++++++
 1 file changed, 8 insertions(+)
diff mbox series

Patch

diff --git a/include/linux/btf.h b/include/linux/btf.h
index e9b90d9c3569..49e0fe6d8274 100644
--- a/include/linux/btf.h
+++ b/include/linux/btf.h
@@ -72,6 +72,14 @@ 
 #define KF_DESTRUCTIVE  (1 << 6) /* kfunc performs destructive actions */
 #define KF_RCU          (1 << 7) /* kfunc only takes rcu pointer arguments */
 
+/*
+ * Tag marking a kernel function as a kfunc. This is meant to minimize the
+ * amount of copy-paste that kfunc authors have to include for correctness so
+ * as to avoid issues such as the compiler inlining or eliding either a static
+ * kfunc, or a global kfunc in an LTO build.
+ */
+#define __bpf_kfunc __used noinline
+
 /*
  * Return the name of the passed struct, if exists, or halt the build if for
  * example the structure gets renamed. In this way, developers have to revisit