After commit 600605853f ("scripts/gen-btf.sh: Fix .btf.o generation
when compiling for RISCV"), there is an error from llvm-objcopy when
CONFIG_LTO_CLANG is enabled:
llvm-objcopy: error: '.tmp_vmlinux1.btf.o': The file was not recognized as a valid object file
Failed to generate BTF for vmlinux
KBUILD_CFLAGS includes CC_FLAGS_LTO, which makes clang emit an LLVM IR
object, rather than an ELF one as expected by llvm-objcopy.
Most areas of the kernel deal with this by filtering out CC_FLAGS_LTO
from KBUILD_CFLAGS for the particular object or directory but this is
not so easy to do in bash. Just include '-fno-lto' after KBUILD_CFLAGS
to ensure an ELF object is consistently created as the initial .o file.
Additionally, while there is no reported or discovered bug yet, the
absence of KBUILD_CPPFLAGS from this command could result in incorrect
endianness because KBUILD_CPPFLAGS typically contains '-mbig-endian' and
'-mlittle-endian' so that biendian toolchains can be used. Include it in
this ${CC} command to hopefully limit necessary changes to this command
for the foreseeable future.
Fixes: 600605853f ("scripts/gen-btf.sh: Fix .btf.o generation when compiling for RISCV")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260106-fix-gen-btf-sh-lto-v2-1-01d3e1c241c4@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Recent changes in BTF generation [1] rely on ${OBJCOPY} command to
update .BTF_ids section data in target ELF files.
This exposed a bug in llvm-objcopy --update-section code path, that
may lead to corruption of a target ELF file. Specifically, because of
the bug st_shndx of some symbols may be (incorrectly) set to 0xffff
(SHN_XINDEX) [2][3].
While there is a pending fix for LLVM, it'll take some time before it
lands (likely in 22.x). And the kernel build must keep working with
older LLVM toolchains in the foreseeable future.
Using GNU objcopy for .BTF_ids update would work, but it would require
changes to LLVM-based build process, likely breaking existing build
environments as discussed in [2].
To work around llvm-objcopy bug, implement --patch_btfids code path in
resolve_btfids as a drop-in replacement for:
${OBJCOPY} --update-section .BTF_ids=${btf_ids} ${elf}
Which works specifically for .BTF_ids section:
${RESOLVE_BTFIDS} --patch_btfids ${btf_ids} ${elf}
This feature in resolve_btfids can be removed at some point in the
future, when llvm-objcopy with a relevant bugfix becomes common.
[1] https://lore.kernel.org/bpf/20251219181321.1283664-1-ihor.solodrai@linux.dev/
[2] https://lore.kernel.org/bpf/20251224005752.201911-1-ihor.solodrai@linux.dev/
[3] https://github.com/llvm/llvm-project/issues/168060#issuecomment-3533552952
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20251231012558.1699758-1-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
gen-btf.sh emits a .btf.o file with BTF sections to be linked into
vmlinux in link-vmlinux.sh
This .btf.o file is created by compiling an emptystring with ${CC},
and then adding BTF sections into it with ${OBJCOPY}.
To ensure the .btf.o is linkable when cross-compiling with LLVM, we
have to also pass ${KBUILD_FLAGS}, which in particular control the
target word size.
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202512240559.2M06DSX7-lkp@intel.com/
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20251229202823.569619-1-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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
Ihor Solodrai