resolve_btfids: Support for BTF modifications #6430
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Rename the member of `struct object` holding the path to BTF data if provided via --btf arg. `btf_path` is less ambiguous. Signed-off-by: Ihor Solodrai <[email protected]>
Increase the lifetime of parsed BTF in resolve_btfids by factoring load_btf() routine out of symbols_resolve() and storing the base_btf and btf pointers in the struct object. Signed-off-by: Ihor Solodrai <[email protected]>
Instead of using multiple flags, make struct btf_id tagged with an enum value indicating its kind in the context of resolve_btfids. Signed-off-by: Ihor Solodrai <[email protected]>
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.
This patch changes 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}.distilled_base.btf with .BTF.base section data (for
out-of-tree modules)
* ${1}.btf_ids with .BTF_ids section data, if it exists in ${1}
The execution of resolve_btfids and consumption of its output is
orchestrated by scripts/gen-btf.sh introduced in this patch.
The rationale for this approach is that updating ELF in-place with
libelf API is complicated and bug-prone, especially in the context of
the kernel build. On the other hand applying objcopy to manipulate ELF
sections is simpler and more reliable.
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 (details below). The patch doesn't change this
approach, only the implementation is a little different.
Before this patch it worked like follows:
* pahole consumed .tmp_vmlinux1 [1] and added .BTF section with
llvm-objcopy [2] to it
* then everything except the .BTF section was stripped from .tmp_vmlinux1
into a .tmp_vmlinux1.bpf.o object [1], later linked into vmlinux
* resolve_btfids was executed later on vmlinux.unstripped [3],
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 the kernel modules creating special .bpf.o file is not necessary,
and so embedding of sections data produced by resolve_btfids is
straightforward with the objcopy.
With this patch an ELF file becomes effectively read-only within
resolve_btfids, which allows to delete elf_update() call and satelite
code (like compressed_section_fix [4]).
Endianness handling of .BTF_ids data is also changed. Previously the
"flags" part of the section was bswapped in sets_patch() [5], 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://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/tree/scripts/link-vmlinux.sh#n115
[2] https://git.kernel.org/pub/scm/devel/pahole/pahole.git/tree/btf_encoder.c#n1835
[3] https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/tree/scripts/link-vmlinux.sh#n285
[4] https://lore.kernel.org/bpf/[email protected]/
[5] https://lore.kernel.org/bpf/[email protected]/
Signed-off-by: Ihor Solodrai <[email protected]>
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Pull request for series with
subject: resolve_btfids: Support for BTF modifications
version: 2
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=1028416