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522397d05e
Currently resolve_btfids updates .BTF_ids section of an ELF file
in-place, based on the contents of provided BTF, usually within the
same input file, and optionally a BTF base.
Change resolve_btfids behavior to enable BTF transformations as part
of its main operation. To achieve this, in-place ELF write in
resolve_btfids is replaced with generation of the following binaries:
* ${1}.BTF with .BTF section data
* ${1}.BTF_ids with .BTF_ids section data if it existed in ${1}
* ${1}.BTF.base with .BTF.base section data for out-of-tree modules
The execution of resolve_btfids and consumption of its output is
orchestrated by scripts/gen-btf.sh introduced in this patch.
The motivation for emitting binary data is that it allows simplifying
resolve_btfids implementation by delegating ELF update to the $OBJCOPY
tool [1], which is already widely used across the codebase.
There are two distinct paths for BTF generation and resolve_btfids
application in the kernel build: for vmlinux and for kernel modules.
For the vmlinux binary a .BTF section is added in a roundabout way to
ensure correct linking. The patch doesn't change this approach, only
the implementation is a little different.
Before this patch it worked as follows:
* pahole consumed .tmp_vmlinux1 [2] and added .BTF section with
llvm-objcopy [3] to it
* then everything except the .BTF section was stripped from .tmp_vmlinux1
into a .tmp_vmlinux1.bpf.o object [2], later linked into vmlinux
* resolve_btfids was executed later on vmlinux.unstripped [4],
updating it in-place
After this patch gen-btf.sh implements the following:
* pahole consumes .tmp_vmlinux1 and produces a *detached* file with
raw BTF data
* resolve_btfids consumes .tmp_vmlinux1 and detached BTF to produce
(potentially modified) .BTF, and .BTF_ids sections data
* a .tmp_vmlinux1.bpf.o object is then produced with objcopy copying
BTF output of resolve_btfids
* .BTF_ids data gets embedded into vmlinux.unstripped in
link-vmlinux.sh by objcopy --update-section
For kernel modules, creating a special .bpf.o file is not necessary,
and so embedding of sections data produced by resolve_btfids is
straightforward with objcopy.
With this patch an ELF file becomes effectively read-only within
resolve_btfids, which allows deleting elf_update() call and satellite
code (like compressed_section_fix [5]).
Endianness handling of .BTF_ids data is also changed. Previously the
"flags" part of the section was bswapped in sets_patch() [6], and then
Elf_Type was modified before elf_update() to signal to libelf that
bswap may be necessary. With this patch we explicitly bswap entire
data buffer on load and on dump.
[1] https://lore.kernel.org/bpf/131b4190-9c49-4f79-a99d-c00fac97fa44@linux.dev/
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/scripts/link-vmlinux.sh?h=v6.18#n110
[3] https://git.kernel.org/pub/scm/devel/pahole/pahole.git/tree/btf_encoder.c?h=v1.31#n1803
[4] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/scripts/link-vmlinux.sh?h=v6.18#n284
[5] https://lore.kernel.org/bpf/20200819092342.259004-1-jolsa@kernel.org/
[6] https://lore.kernel.org/bpf/cover.1707223196.git.vmalik@redhat.com/
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20251219181825.1289460-3-ihor.solodrai@linux.dev
33 lines
1.1 KiB
Makefile
33 lines
1.1 KiB
Makefile
# SPDX-License-Identifier: GPL-2.0
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pahole-ver := $(CONFIG_PAHOLE_VERSION)
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pahole-flags-y :=
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JOBS := $(patsubst -j%,%,$(filter -j%,$(MAKEFLAGS)))
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ifeq ($(call test-le, $(pahole-ver), 125),y)
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pahole-flags-y += --btf_gen_floats -j$(JOBS)
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pahole-flags-$(call test-ge, $(pahole-ver), 125) += --skip_encoding_btf_inconsistent_proto --btf_gen_optimized
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else
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# Switch to using --btf_features for v1.26 and later.
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pahole-flags-$(call test-ge, $(pahole-ver), 126) = -j$(JOBS) --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func,decl_tag_kfuncs
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pahole-flags-$(call test-ge, $(pahole-ver), 130) += --btf_features=attributes
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endif
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pahole-flags-$(CONFIG_PAHOLE_HAS_LANG_EXCLUDE) += --lang_exclude=rust
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export PAHOLE_FLAGS := $(pahole-flags-y)
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resolve-btfids-flags-y :=
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resolve-btfids-flags-$(CONFIG_WERROR) += --fatal_warnings
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resolve-btfids-flags-$(if $(KBUILD_EXTMOD),y) += --distill_base
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resolve-btfids-flags-$(if $(KBUILD_VERBOSE),y) += --verbose
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export RESOLVE_BTFIDS_FLAGS := $(resolve-btfids-flags-y)
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