linux-toradex.git/scripts/Makefile.btf, branch v7.0-rc5

resolve_btfids: Change in-place update with raw binary output

2025-12-19T18:55:40+00:00

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 
Signed-off-by: Andrii Nakryiko 
Tested-by: Alan Maguire 
Acked-by: Eduard Zingerman 
Link: https://lore.kernel.org/bpf/20251219181825.1289460-3-ihor.solodrai@linux.dev

lib/Kconfig.debug: Set the minimum required pahole version to v1.22

2025-12-19T18:55:40+00:00

Subsequent patches in the series change vmlinux linking scripts to
unconditionally pass --btf_encode_detached to pahole, which was
introduced in v1.22 [1][2].

This change allows to remove PAHOLE_HAS_SPLIT_BTF Kconfig option and
other checks of older pahole versions.

[1] https://github.com/acmel/dwarves/releases/tag/v1.22
[2] https://lore.kernel.org/bpf/cbafbf4e-9073-4383-8ee6-1353f9e5869c@oracle.com/

Signed-off-by: Ihor Solodrai 
Signed-off-by: Andrii Nakryiko 
Tested-by: Alan Maguire 
Acked-by: Eduard Zingerman 
Acked-by: Nicolas Schier 
Link: https://lore.kernel.org/bpf/20251219181825.1289460-1-ihor.solodrai@linux.dev

kbuild, bpf: Enable --btf_features=attributes

2025-04-15T18:17:50+00:00

pahole v1.30 has a BTF encoding feature for arbitrary attributes, used
in particular for tagging bpf_arena_alloc_pages and
bpf_arena_free_pages BPF kfuncs [1][2].

Enable it for the kernel build.

[1] https://lore.kernel.org/bpf/20250130201239.1429648-1-ihor.solodrai@linux.dev/
[2] https://lore.kernel.org/bpf/20250228194654.1022535-1-ihor.solodrai@linux.dev/

Signed-off-by: Ihor Solodrai 
Signed-off-by: Martin KaFai Lau 
Tested-by: Eduard Zingerman 
Link: https://patch.msgid.link/20250414185918.538195-1-ihor.solodrai@linux.dev

kbuild, bpf: Correct pahole version that supports distilled base btf feature

2025-02-24T22:14:52+00:00

pahole commit [0] of supporting distilled base btf feature released on
pahole v1.28 rather than v1.26. So let's correct this.

Signed-off-by: Pu Lehui 
Signed-off-by: Andrii Nakryiko 
Link: https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/?id=c7b1f6a29ba1 [0]
Link: https://lore.kernel.org/bpf/20250219063113.706600-1-pulehui@huaweicloud.com

kbuild,bpf: Pass make jobs' value to pahole

2024-11-12T04:00:21+00:00

Pass the value of make's -j/--jobs argument to pahole, to avoid out of
memory errors and make pahole respect the "jobs" value of make.

On systems with little memory but many cores, invoking pahole using -j
without argument potentially creates too many pahole instances,
causing an out-of-memory situation. Instead, we should pass make's
"jobs" value as an argument to pahole's -j, which is likely configured
to be (much) lower than the actual core count on such systems.

If make was invoked without -j, either via cmdline or MAKEFLAGS, then
JOBS will be simply empty, resulting in the existing behavior, as
expected.

Signed-off-by: Florian Schmaus 
Signed-off-by: Andrii Nakryiko 
Tested-by: Holger Hoffstätte 
Link: https://lore.kernel.org/bpf/20241102100452.793970-1-flo@geekplace.eu

kbuild,bpf: Add module-specific pahole flags for distilled base BTF

2024-06-21T21:45:07+00:00

Support creation of module BTF along with distilled base BTF;
the latter is stored in a .BTF.base ELF section and supplements
split BTF references to base BTF with information about base types,
allowing for later relocation of split BTF with a (possibly
changed) base.  resolve_btfids detects the presence of a .BTF.base
section and will use it instead of the base BTF it is passed in
BTF id resolution.

Modules will be built with a distilled .BTF.base section for external
module build, i.e.

make -C. -M=path2/module

...while in-tree module build as part of a normal kernel build will
not generate distilled base BTF; this is because in-tree modules
change with the kernel and do not require BTF relocation for the
running vmlinux.

Signed-off-by: Alan Maguire 
Signed-off-by: Andrii Nakryiko 
Reviewed-by: Eduard Zingerman 
Link: https://lore.kernel.org/bpf/20240620091733.1967885-6-alan.maguire@oracle.com

kbuild: bpf: Tell pahole to DECL_TAG kfuncs

2024-06-12T18:01:30+00:00

With [0], pahole can now discover kfuncs and inject DECL_TAG
into BTF. With this commit, we will start shipping said DECL_TAGs
to downstream consumers if pahole supports it.

This is useful for feature probing kfuncs as well as generating
compilable prototypes. This is particularly important as kfuncs
do not have stable ABI.

[0]: https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/?id=72e88f29c6f7e14201756e65bd66157427a61aaf

Signed-off-by: Daniel Xu 
Link: https://lore.kernel.org/r/324aac5c627bddb80d9968c30df6382846994cc8.1718207789.git.dxu@dxuuu.xyz
Signed-off-by: Alexei Starovoitov

kbuild, bpf: Use test-ge check for v1.25-only pahole

2024-05-18T17:46:16+00:00

There is no need to set the pahole v1.25-only flags in an
"ifeq" version clause; we are already in a <= v1.25 branch
of "ifeq", so that combined with a "test-ge" v1.25 ensures the
flags will be applied for v1.25 only.

Suggested-by: Masahiro Yamada 
Signed-off-by: Alan Maguire 
Signed-off-by: Andrii Nakryiko 
Link: https://lore.kernel.org/bpf/20240514162716.2448265-1-alan.maguire@oracle.com
Signed-off-by: Alexei Starovoitov

kbuild,bpf: Switch to using --btf_features for pahole v1.26 and later

2024-05-09T21:44:35+00:00

The btf_features list can be used for pahole v1.26 and later -
it is useful because if a feature is not yet implemented it will
not exit with a failure message.  This will allow us to add feature
requests to the pahole options without having to check pahole versions
in future; if the version of pahole supports the feature it will be
added.

Signed-off-by: Alan Maguire 
Signed-off-by: Andrii Nakryiko 
Tested-by: Eduard Zingerman 
Acked-by: Andrii Nakryiko 
Link: https://lore.kernel.org/bpf/20240507135514.490467-1-alan.maguire@oracle.com

kbuild: avoid too many execution of scripts/pahole-flags.sh

2023-10-28T12:10:08+00:00

scripts/pahole-flags.sh is executed so many times.

You can confirm it, as follows:

  $ cat <> scripts/pahole-flags.sh
  > echo "scripts/pahole-flags.sh was executed" >&2
  > EOF

  $ make -s
  scripts/pahole-flags.sh was executed
  scripts/pahole-flags.sh was executed
  scripts/pahole-flags.sh was executed
  scripts/pahole-flags.sh was executed
  scripts/pahole-flags.sh was executed
    [ lots of repeated lines... ]

This scripts is executed more than 20 times during the kernel build
because PAHOLE_FLAGS is a recursively expanded variable and exported
to sub-processes.

With GNU Make >= 4.4, it is executed more than 60 times because
exported variables are also passed to other $(shell ) invocations.
Without careful coding, it is known to cause an exponential fork
explosion. [1]

The use of $(shell ) in an exported recursive variable is likely wrong
because $(shell ) is always evaluated due to the 'export' keyword, and
the evaluation can occur multiple times by the nature of recursive
variables.

Convert the shell script to a Makefile, which is included only when
CONFIG_DEBUG_INFO_BTF=y.

[1]: https://savannah.gnu.org/bugs/index.php?64746

Signed-off-by: Masahiro Yamada 
Reviewed-by: Alan Maguire 
Tested-by: Alan Maguire 
Reviewed-by: Nicolas Schier 
Tested-by: Miguel Ojeda 
Acked-by: Miguel Ojeda 
Acked-by: Jiri Olsa 
Reviewed-by: Martin Rodriguez Reboredo