Currently, the kernel uses pahole version checking as the way to
determine which BTF encoding features to request from pahole. This
means that such features have to be tied to a specific version and
as new features are added, additional clauses in scripts/pahole-flags.sh
have to be added; for example
if [ "${pahole_ver}" -ge "125" ]; then
extra_paholeopt="${extra_paholeopt} --skip_encoding_btf_inconsistent_proto --btf_gen_optimized"
fi
To better future-proof this process, this series introduces a
single "btf_features" parameter that uses a comma-separated list
of encoding options. This is helpful because
- the semantics are simpler for the user; the list comprises the set of
BTF features asked for, rather than having to specify a combination of
--skip_encoding_btf_feature and --btf_gen_feature options; and
- any version of pahole that supports --btf_features can accept the
option list; unknown options are silently ignored. As a result, there
would be no need to add additional version clauses beyond
if [ "${pahole_ver}" -ge "126" ]; then
extra_pahole_opt="-j --lang_exclude=rust
--btf_features=encode_force,var,float,decl_tag,type_tag,enum64,optimized_func,consistent_func"
fi
Newly-supported features would simply be appended to the btf_features
list, and these would have impact on BTF encoding only if the features
were supported by pahole. This means pahole will not require a version
bump when new BTF features are added, and should ease the burden of
coordinating such changes between bpf-next and dwarves.
Patches 1 and 2 are preparatory work, while patch 3 adds the
--btf_features support. Patch 4 provides a means of querying
the supported feature set since --btf_features will not error
out when it encounters unrecognized features (this ensures
an older pahole without a requested feature will not dump warnings
in the build log for kernel/module BTF generation). Patch 5
adds --btf_features_strict, which is identical to --btf_features
aside from the fact it will fail if an unrecognized feature is used.
See [1] for more background on this topic.
Changes since v2 [2]:
- added acks from Andrii and Jiri (patches 1-5)
- merged suggestions from Eduard which simplify and clean up code
considerably; these changes fix issues with --btf_features_strict
while providing better diagnostic output when an unknown feature
is encountered (specifying the unknown feature if in verbose
or strict modes). Added Suggested-bys from Eduard for the
relevant patches (Eduard, patches 3,5)
Changes since RFC [3]:
- ensure features are disabled unless requested; use "default" field in
"struct btf_features" to specify the conf_load default value which
corresponds to the feature being disabled. For
conf_load->btf_gen_floats for example, the default value is false,
while for conf_load->skip_encoding_btf_type_tags the default is
true; in both cases the intent is to _not_ encode the associated
feature by default. However if the user specifies "float" or
"type_tag" in --btf_features, the default conf_load value is negated,
resulting in a BTF encoding that contains floats and type tags
(Eduard, patch 3)
- clarify feature default/setting behaviour and how it only applies
when --btf_features is used (Eduard, patch 3)
- ensure we do not run off the end of the feature_list[] array
(Eduard, patch 3)
- rather than having each struct btf_feature record the offset in the
conf_load structure of the boolean (requiring us to later do pointer
math to update it), record the pointers to the boolean conf_load
values associated with each feature (Jiri, patch 3)
- allow for multiple specifications of --btf_features, enabling the
union of all features specified (Andrii, patch 3)
- rename function-related optimized/consistent to optimized_func and
consistent_func in recognition of the fact they are function-specific
(Andrii, patch 3)
- add a strict version of --btf_features, --btf_features_strict that
will error out if an unrecognized feature is used (Andrii, patch 5)
[1] https://lore.kernel.org/bpf/CAEf4Bzaz1UqqxuZ7Q+KQee-HLyY1nwhAurBE2n9YTWchqoYLbg@xxxxxxxxxxxxxx/
[2] https://lore.kernel.org/bpf/20231013153359.88274-1-alan.maguire@xxxxxxxxxx/
[3] https://lore.kernel.org/bpf/20231011091732.93254-1-alan.maguire@xxxxxxxxxx/
Alan Maguire (5):
btf_encoder, pahole: move btf encoding options into conf_load
dwarves: move ARRAY_SIZE() to dwarves.h
pahole: add --btf_features support
pahole: add --supported_btf_features
pahole: add --btf_features_strict to reject unknown BTF features
btf_encoder.c | 8 +-
btf_encoder.h | 2 +-
dwarves.c | 16 ----
dwarves.h | 19 +++++
man-pages/pahole.1 | 32 ++++++++
pahole.c | 191 ++++++++++++++++++++++++++++++++++++++++++---
6 files changed, 234 insertions(+), 34 deletions(-)
--
2.31.1
