This series is intended to help pave the way to allow pahole to
handle compiler-generated BTF as input to its encoding process.
The BTF loader currently supports loading BTF for data structure
and function query but not for BTF generation. [1] describes how
gcc can generate (and the linker deduplicate) BTF but even there
we will need pahole to post-process the results so it is valuable
to support reading BTF in pahole as input to start exploring how
to handle cases like [1].
The intention of this series is not yet to fully support
linker-deduplicated BTF, but to start down that road by supporting
reading BTF from (multiple) object files and support its deduplication
and BTF output. To that end a few changes are needed in the
intermediate representation generated by the BTF loader; these are
done in patch 1. Patch 2 then fixes up a few cases where we needed
to special-case the intermediate representation when it came from
BTF. Patches 3/4 ensure that we retrieve ELF info when loading BTF
for encoding. Patch 5 is intended to support cases where we read BTF from
multiple objects and - if the "encode_force" BTF feature is specified -
we continue as long as BTF is found in at least one of those sources.
Finally patch 6 tests pahole when BTF is used as input.
This allows us to experiment with "gcc -gbtf". For example adding
-gbtf to the KCFLAGS environment variable for a kernel build, we can
then run pahole across the set of vmlinux objects via
pahole --format_path=btf -J --btf_features=encode_force \
--btf_encode_detached=/tmp/vmlinux.btf $(ar t vmlinux.a)
This is clearly not the right way to do this for kernel builds -
we want to make use of linker deduplication as in [1] - but the fact
that even a hack like this works does demonstrate the viability of
handling BTF as input to vmlinux BTF encoding. It also allows us
to provide a comparison between compiler-generated BTF with pahole
deduplication and compiler-generated BTF with linker deduplication
when the latter becomes available.
[1] https://lore.kernel.org/dwarves/87ldqf1i19.fsf@xxxxxxxxxxxxx/
Alan Maguire (6):
btf_loader: Make BTF representation match DWARF
pfunct: Fix up function display with updated prototype representation
pahole: Add btf_encode to conf_load
btf_loader: read ELF for BTF encoding
btf_encoder: Do not error out if BTF is not found in some input files
tests: Add test of pahole using BTF as input to BTF generation
btf_loader.c | 36 +++++++++++--
dwarves.c | 14 +++++-
dwarves.h | 1 +
dwarves_fprintf.c | 2 +-
pahole.c | 15 +++---
pfunct.c | 2 +-
tests/btf_2_btf.sh | 122 +++++++++++++++++++++++++++++++++++++++++++++
7 files changed, 176 insertions(+), 16 deletions(-)
create mode 100755 tests/btf_2_btf.sh
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2.43.5
