On 25/09/2026 23:42, Jay Wang wrote: > > Make CONFIG_DEBUG_INFO_BTF a tristate. With =m the vmlinux BTF is not > part of the kernel image: it is carried by a new module, btf_vmlinux, and > loaded the first time something needs it. Nothing that works with =y > stops working; the 5.4 MiB of read-only data (distribution config) is > simply not there on systems where nothing uses it. > > The only way to save that memory today is CONFIG_DEBUG_INFO_BTF=n, which > a distribution cannot ship: one binary goes to every user, and off takes > BTF away from the users of CO-RE, fentry/fexit, kfuncs, struct_ops, > sched_ext or bpf-lsm. Whether BTF is used is a property of the > workload, not of the build, so let the first user decide. > > The BTF is generated as before, but with =m the .BTF section is linked > as a non-loadable section (like .comment), so the kernel image does not > load it, and the final step that makes vmlinux from vmlinux.unstripped > strips it: every boot image made from vmlinux, whether a raw binary or > an ELF copy, is without it. Module BTF is generated against > vmlinux.unstripped, which keeps it. .BTF_ids stays loadable, the > verifier needs it once the BTF is loaded. The object that carries .BTF > also carries .BTF.meta, the size and SHA-256 of the BTF (struct > btf_vmlinux_meta, checked by the module notifier); the first link, which > the BTF is generated from, gets a zeroed .BTF.meta of the same size. > kernel/bpf/btf_vmlinux.c is an empty carrier module; scripts/gen-btf.sh > gives it the vmlinux .BTF as its own .BTF section instead of generating > split BTF for it, so that one module depends on vmlinux with =m; with > CONFIG_DEBUG_INFO_BTF_MODULES all of them do, as before. > > CONFIG_BPF_PRELOAD is not selectable with =m: its iterator programs > attach through the vmlinux BTF, so every bpffs mount (systemd does one > at boot) would load it and defeat the point. Module BTF is still kept > when a module loads, as with =y; the saving is the vmlinux BTF only. > Programs that need kernel types before the root file system is mounted > need btf_vmlinux.ko in the initramfs; the Kconfig help says so. > > The module has no exit: once loaded the BTF stays, as with =y. The > runtime side -- loading the module on first use, checking it against > .BTF.meta, deferring kfunc and struct_ops registrations and module BTF > until it arrives -- and the IS_ENABLED()/$(subst m,y,...) preparation of > the existing checks are in the preceding patches; this one makes it > selectable. >
A few thoughts here; .BTF.meta seems too vague as a name. This is very like .gnu_debuglink ; should we call it .BTF.link ? It also could be nicely extended to .BTF.inline (.BTF.inlink.link) where inline info is delivered as a module. I also wonder if the right place to populate the section is in the script? We might run into endian issues etc; why not do it as part of resolve_btfids? Pass a --btf_link parameter or similar, and resolve_btfids (which has access to BTF size after carrying out various operations) can add it and the SHA. resolve_btfids is already updating other BTF-related sections so seems like a natural home for this. > Tested with 1 GiB of memory, same tree, =y vs =m, both with > CONFIG_DEBUG_INFO_BTF_MODULES=y: > > - MemTotal is ~5.4 MB higher with =m while the BTF is unused: the size > of the .BTF section. > - stat() of /sys/kernel/btf/vmlinux reports the BTF size before it is > loaded, as the btf_sysfs selftest expects. > - With BTF in use, MemFree is the same within run-to-run noise. > - Modules loaded before the trigger (ext4, nf_conntrack and its kfuncs, > xfrm_interface) appear in /sys/kernel/btf immediately and get BTF ids > once the BTF is loaded; a socket filter loads without loading the > module; a kprobe program calling bpf_get_current_task_btf(), opening > /sys/kernel/btf/vmlinux or BPF_BTF_GET_NEXT_ID each load it. > - A carrier module with one byte of its .BTF changed is refused with > "BTF does not match this kernel" and leaves no state behind. > - After the load: fstat/read/mmap of /sys/kernel/btf/vmlinux, a > struct_ops map for tcp_congestion_ops, a syscall program calling the > bpf_task_from_pid()/bpf_task_release() kfuncs, and modules loaded > afterwards (nf_nat) all work as with =y. > - =m without DEBUG_INFO_BTF_MODULES, and =y, build and pass the same > tests. > > Signed-off-by: Jay Wang <[email protected]> > --- > Documentation/bpf/btf.rst | 35 ++++++++++++ > Makefile | 5 +- > include/asm-generic/vmlinux.lds.h | 31 +++++++++- > kernel/bpf/Makefile | 4 ++ > kernel/bpf/btf_vmlinux.c | 23 ++++++++ > kernel/bpf/preload/Kconfig | 4 ++ > lib/Kconfig.debug | 22 ++++++- > scripts/Makefile.modfinal | 26 +++++++-- > scripts/Makefile.vmlinux | 5 ++ > scripts/gen-btf.sh | 95 +++++++++++++++++++++++++++++-- > scripts/link-vmlinux.sh | 25 ++++++-- > 11 files changed, 257 insertions(+), 18 deletions(-) > create mode 100644 kernel/bpf/btf_vmlinux.c > > diff --git a/Documentation/bpf/btf.rst b/Documentation/bpf/btf.rst > index 29de1222c3e7..1bd0d35cdb05 100644 > --- a/Documentation/bpf/btf.rst > +++ b/Documentation/bpf/btf.rst > @@ -1276,6 +1276,41 @@ format.:: > .long 58 > .long 8206 # Line 8 Col 14 > > +6.1 Kernel BTF > +-------------- > + > +With CONFIG_DEBUG_INFO_BTF=y the BTF of the kernel is generated at link time > +from its DWARF and placed in the .BTF section of vmlinux, which is read-only > +data of the kernel image. It is available as /sys/kernel/btf/vmlinux and, if > +CONFIG_DEBUG_INFO_BTF_MODULES is set, module BTF is generated as split BTF > +against it and available as /sys/kernel/btf/<module>. > + > +With CONFIG_DEBUG_INFO_BTF=m the same BTF is generated, but it is not part of > +the kernel image or of the vmlinux ELF file (vmlinux.unstripped in the build > +tree keeps it, for module BTF generation). It is delivered by the > +btf_vmlinux module, which the kernel loads on demand the first time the BTF > is > +needed: when /sys/kernel/btf/vmlinux is read or mmap()ed, when kernel BTF > +objects are enumerated (BPF_BTF_GET_NEXT_ID), or when a BPF program needs > +kernel type information (an attach_btf_id, a kfunc call, a ksym, a map > pointer > +or a helper that takes or returns a kernel BTF pointer). Until then no memory > +is used for it, and afterwards nothing differs from =y. In particular: > + > + * /sys/kernel/btf/vmlinux exists from boot with its final size. > + * Modules loaded before the vmlinux BTF are exposed in /sys/kernel/btf > right > + away, their BTF is parsed and gets a BTF id once the vmlinux BTF is > + loaded, together with their kfunc and struct_ops registrations. > + * kfunc, dtor kfunc and struct_ops registrations of the kernel itself are > + applied before the BTF becomes visible. > + * The kernel only accepts the BTF it was built with: the size and SHA-256 > of > + the BTF are linked into the kernel and checked against the module. > + * Once loaded the BTF stays; the module cannot be unloaded. > + > +If the module is not available (not installed, or the root file system is not > +mounted yet), the kernel behaves as one built without BTF and retries next > +time. CONFIG_BPF_PRELOAD is not available with =m: its iterators attach > through > +the vmlinux BTF, so mounting bpffs would load it. bpf_snprintf_btf() and > bpf_seq_printf_btf() only use the BTF if it has > +already been parsed, as they run in program context. > + > 7. Testing > ========== > > diff --git a/Makefile b/Makefile > index f561516e1735..7ce5d478abd3 100644 > --- a/Makefile > +++ b/Makefile > @@ -1745,8 +1745,9 @@ endif > # > > # *.ko are usually independent of vmlinux, but CONFIG_DEBUG_INFO_BTF_MODULES > -# is an exception. > -ifdef CONFIG_DEBUG_INFO_BTF_MODULES > +# is an exception, and so is the btf_vmlinux module with > CONFIG_DEBUG_INFO_BTF=m, > +# which carries the vmlinux BTF. > +ifneq ($(CONFIG_DEBUG_INFO_BTF_MODULES)$(filter m,$(CONFIG_DEBUG_INFO_BTF)),) > KBUILD_BUILTIN := y > modules: vmlinux > endif > diff --git a/include/asm-generic/vmlinux.lds.h > b/include/asm-generic/vmlinux.lds.h > index b2988aa12f66..204a9ee171c6 100644 > --- a/include/asm-generic/vmlinux.lds.h > +++ b/include/asm-generic/vmlinux.lds.h > @@ -674,8 +674,18 @@ > > /* > * .BTF > + * > + * With CONFIG_DEBUG_INFO_BTF=y the vmlinux BTF is loaded as read-only data > and > + * bounded by __start_BTF/__stop_BTF. With CONFIG_DEBUG_INFO_BTF=m it is > + * linked as a non-loadable section (see BTF_NOLOAD in ELF_DETAILS), so that > + * module BTF generation can read it from vmlinux.unstripped; it is stripped > + * from vmlinux (scripts/Makefile.vmlinux), and the btf_vmlinux module > carries > + * a copy and provides it on demand at runtime. > + * What is loaded instead is .BTF.meta, the size and hash of that BTF (see > + * scripts/gen-btf.sh), empty in the first link that the BTF is generated > + * from. .BTF_ids is needed by the kernel in both cases. > */ > -#ifdef CONFIG_DEBUG_INFO_BTF > +#if IS_BUILTIN(CONFIG_DEBUG_INFO_BTF) > #define BTF \ > . = ALIGN(PAGE_SIZE); \ > .BTF : AT(ADDR(.BTF) - LOAD_OFFSET) { \ > @@ -685,10 +695,28 @@ > .BTF_ids : AT(ADDR(.BTF_ids) - LOAD_OFFSET) { \ > *(.BTF_ids) \ > } > +#elif IS_MODULE(CONFIG_DEBUG_INFO_BTF) > +#define BTF \ > + . = ALIGN(8); \ > + .BTF.meta : AT(ADDR(.BTF.meta) - LOAD_OFFSET) { \ > + BOUNDED_SECTION_BY(.BTF.meta, _BTF_meta) \ > + } \ > + . = ALIGN(PAGE_SIZE); \ > + .BTF_ids : AT(ADDR(.BTF_ids) - LOAD_OFFSET) { \ > + *(.BTF_ids) \ > + } > #else > #define BTF > #endif > > +#if IS_MODULE(CONFIG_DEBUG_INFO_BTF) > +/* quoted: BTF is a macro, an unquoted .BTF here would expand it */ > +#define BTF_NOLOAD \ > + ".BTF" 0 : { *(".BTF") } > +#else > +#define BTF_NOLOAD > +#endif > + > /* > * Init task > */ > @@ -849,6 +877,7 @@ > /* Required sections not related to debugging. */ > #define ELF_DETAILS \ > .comment 0 : { *(.comment) } \ > + BTF_NOLOAD \ > .symtab 0 : { *(.symtab) } \ > .strtab 0 : { *(.strtab) } \ > .shstrtab 0 : { *(.shstrtab) } \ > diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile > index 0b7db88f1bed..8a7ad4303c0d 100644 > --- a/kernel/bpf/Makefile > +++ b/kernel/bpf/Makefile > @@ -43,6 +43,10 @@ endif > ifeq ($(CONFIG_SYSFS),y) > obj-$(subst m,y,$(CONFIG_DEBUG_INFO_BTF)) += sysfs_btf.o > endif > +# With CONFIG_DEBUG_INFO_BTF=m the vmlinux BTF is carried by this module > +ifeq ($(CONFIG_DEBUG_INFO_BTF),m) > +obj-m += btf_vmlinux.o > +endif > ifeq ($(CONFIG_BPF_JIT),y) > obj-$(CONFIG_BPF_SYSCALL) += bpf_struct_ops.o > obj-$(CONFIG_BPF_SYSCALL) += cpumask.o > diff --git a/kernel/bpf/btf_vmlinux.c b/kernel/bpf/btf_vmlinux.c > new file mode 100644 > index 000000000000..8d89b4bb3c43 > --- /dev/null > +++ b/kernel/bpf/btf_vmlinux.c > @@ -0,0 +1,23 @@ > +// SPDX-License-Identifier: GPL-2.0 > +/* > + * Carrier module for the vmlinux BTF when CONFIG_DEBUG_INFO_BTF=m. > + * > + * This module has no code of its own. Its .BTF section is a copy of the > + * vmlinux BTF (see scripts/gen-btf.sh), which the BTF module notifier in > + * kernel/bpf/btf.c recognizes by module name and installs as the vmlinux > BTF. > + * The kernel loads it on demand, the first time the vmlinux BTF is needed. > + * > + * There is deliberately no module_exit(): once the BTF is in use it cannot > + * be taken away again, exactly as with CONFIG_DEBUG_INFO_BTF=y. > + */ > +#include <linux/init.h> > +#include <linux/module.h> > + > +static int __init btf_vmlinux_init(void) > +{ > + return 0; > +} > +module_init(btf_vmlinux_init); > + > +MODULE_DESCRIPTION("BTF type information for vmlinux"); > +MODULE_LICENSE("GPL"); > diff --git a/kernel/bpf/preload/Kconfig b/kernel/bpf/preload/Kconfig > index aef7b0bc96d6..b1600bdce7a0 100644 > --- a/kernel/bpf/preload/Kconfig > +++ b/kernel/bpf/preload/Kconfig > @@ -6,6 +6,10 @@ menuconfig BPF_PRELOAD > # The dependency on !COMPILE_TEST prevents it from being enabled > # in allmodconfig or allyesconfig configurations > depends on !COMPILE_TEST > + # The preloaded iterators attach through the vmlinux BTF, so with > + # CONFIG_DEBUG_INFO_BTF=m every bpffs mount would load the BTF, which > + # defeats the point of =m on any system that mounts bpffs at boot. > + depends on DEBUG_INFO_BTF!=m > help > This builds kernel module with several embedded BPF programs that are > pinned into BPF FS mount point as human readable files that are > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug > index 134b15a44625..418025c5e657 100644 > --- a/lib/Kconfig.debug > +++ b/lib/Kconfig.debug > @@ -396,7 +396,7 @@ config DEBUG_INFO_SPLIT > Incompatible with older versions of ccache. > > config DEBUG_INFO_BTF > - bool "Generate BTF type information" > + tristate "Generate BTF type information" > depends on !DEBUG_INFO_SPLIT && !DEBUG_INFO_REDUCED > depends on !GCC_PLUGIN_RANDSTRUCT || COMPILE_TEST > depends on BPF_SYSCALL > @@ -408,6 +408,26 @@ config DEBUG_INFO_BTF > Turning this on requires pahole v1.22 or later, which will convert > DWARF type info into equivalent deduplicated BTF type info. > > + If built as a module (=m), the vmlinux BTF is not part of the > + kernel image. It is carried by the btf_vmlinux module, which is > + loaded on demand the first time the BTF is needed: when a BPF > + program requires kernel type information, or when > + /sys/kernel/btf/vmlinux is opened. Until then, no memory is > + spent on it. The vmlinux ELF file does not carry the BTF > + either; module BTF is generated against vmlinux.unstripped, and > + tools that read the BTF from a file can use that or > + /sys/kernel/btf/vmlinux. > + > + Module BTF (DEBUG_INFO_BTF_MODULES) is kept when a module loads, > + as with =y, and registered once the vmlinux BTF is available; the > + saving is the vmlinux BTF only. > + > + If BPF programs that use kernel types run before the root file > + system is mounted, put btf_vmlinux.ko into the initramfs: until > + the module can be loaded, such programs fail as on a kernel > + without BTF. Not compatible with BPF_PRELOAD, whose iterators > + would load the BTF at every bpffs mount. > + > config PAHOLE_HAS_BTF_TAG > def_bool PAHOLE_VERSION >= 123 > depends on CC_IS_CLANG > diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal > index 01a37ec872b9..6a0958cf0e6f 100644 > --- a/scripts/Makefile.modfinal > +++ b/scripts/Makefile.modfinal > @@ -38,20 +38,34 @@ quiet_cmd_ld_ko_o = LD [M] $@ > $(KBUILD_LDFLAGS_MODULE) $(LDFLAGS_MODULE) \ > -T $(objtree)/scripts/module.lds -o $@ $(filter %.o, $^) > > +# The ELF file with the vmlinux BTF: with CONFIG_DEBUG_INFO_BTF=m the BTF is > +# stripped from vmlinux (scripts/Makefile.vmlinux), vmlinux.unstripped keeps > it. > +btf-vmlinux := $(objtree)/vmlinux$(if $(filter > m,$(CONFIG_DEBUG_INFO_BTF)),.unstripped) > + > quiet_cmd_btf_ko = BTF [M] $@ > cmd_btf_ko = \ > - if [ ! -f $(objtree)/vmlinux ]; then \ > + if [ ! -f $(btf-vmlinux) ]; then \ > printf "Skipping BTF generation for %s due to unavailability of > vmlinux\n" $@ 1>&2; \ > else \ > - $(CONFIG_SHELL) $(srctree)/scripts/gen-btf.sh --btf_base > $(objtree)/vmlinux $@; \ > + $(CONFIG_SHELL) $(srctree)/scripts/gen-btf.sh --btf_base > $(btf-vmlinux) $@; \ > fi; > > -# Re-generate module BTFs if either module's .ko or vmlinux changed > -%.ko: %.o %.mod.o .module-common.o $(objtree)/scripts/module.lds $(and > $(CONFIG_DEBUG_INFO_BTF_MODULES),$(KBUILD_BUILTIN),$(objtree)/vmlinux) FORCE > - +$(call if_changed,ld_ko_o) > +# Modules that get a .BTF section: all of them with > CONFIG_DEBUG_INFO_BTF_MODULES, > +# otherwise only the vmlinux BTF carrier module with CONFIG_DEBUG_INFO_BTF=m. > ifdef CONFIG_DEBUG_INFO_BTF_MODULES > - +$(if $(newer-prereqs),$(call cmd,btf_ko)) > +btf-modules := $(modules:%.o=%.ko) > +else ifeq ($(CONFIG_DEBUG_INFO_BTF),m) > +btf-modules := $(filter %/btf_vmlinux.ko,$(modules:%.o=%.ko)) > +# Only the carrier depends on vmlinux, not every module > +ifdef KBUILD_BUILTIN > +$(btf-modules): $(btf-vmlinux) > +endif > endif > + > +# Re-generate module BTFs if either module's .ko or vmlinux changed > +%.ko: %.o %.mod.o .module-common.o $(objtree)/scripts/module.lds $(and > $(CONFIG_DEBUG_INFO_BTF_MODULES),$(KBUILD_BUILTIN),$(btf-vmlinux)) FORCE > + +$(call if_changed,ld_ko_o) > + +$(if $(and $(filter $@,$(btf-modules)),$(newer-prereqs)),$(call > cmd,btf_ko)) > +$(call cmd,check_tracepoint) > > targets += $(modules:%.o=%.ko) $(modules:%.o=%.mod.o) .module-common.o > diff --git a/scripts/Makefile.vmlinux b/scripts/Makefile.vmlinux > index fcae1e432d9a..557db1ee1f3b 100644 > --- a/scripts/Makefile.vmlinux > +++ b/scripts/Makefile.vmlinux > @@ -86,6 +86,11 @@ remove-section-$(CONFIG_ARCH_VMLINUX_NEEDS_RELOCS) += > '.rel*' '!.rel*.dyn' > # for compatibility with binutils < 2.32 > # > https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=c12d9fa2afe7abcbe407a00e15719e1a1350c2a7 > remove-section-$(CONFIG_ARCH_VMLINUX_NEEDS_RELOCS) += '.rel.*' > +# With CONFIG_DEBUG_INFO_BTF=m the btf_vmlinux module carries the vmlinux > BTF; > +# only vmlinux.unstripped keeps it, for module BTF generation. > +ifeq ($(CONFIG_DEBUG_INFO_BTF),m) > +remove-section-y += .BTF > +endif > > remove-symbols := -w --strip-unneeded-symbol='__mod_device_table__*' > > diff --git a/scripts/gen-btf.sh b/scripts/gen-btf.sh > index 8ca96eb10a69..d808d0006d32 100755 > --- a/scripts/gen-btf.sh > +++ b/scripts/gen-btf.sh > @@ -22,16 +22,27 @@ > # - ${1}.btf.o ready for linking into vmlinux > # - ${1}.BTF_ids with .BTF_ids data blob > # This output is consumed by scripts/link-vmlinux.sh > +# > +# With CONFIG_DEBUG_INFO_BTF=m the .BTF section in ${1}.btf.o is not > +# allocatable, so the kernel image does not carry the BTF; vmlinux.unstripped > +# does, for module BTF generation, and scripts/Makefile.vmlinux strips it > from > +# vmlinux. ${1}.btf.o then also carries .BTF.meta, the size and SHA-256 of > +# the BTF for the kernel (struct btf_vmlinux_meta); "--placeholder ${1}" > +# produces a ${1}.btf.o with a zeroed .BTF.meta and no .BTF for the first > +# vmlinux link, which the BTF is generated from. The btf_vmlinux module gets > +# no BTF of its own; its .BTF section is a copy of the vmlinux BTF, extracted > +# from --btf_base. > > set -e > > usage() > { > - echo "Usage: $0 [--btf_base <file>] <target ELF file>" > + echo "Usage: $0 [--btf_base <file>] [--placeholder] <target ELF file>" > exit 1 > } > > BTF_BASE="" > +PLACEHOLDER="" > > while [ $# -gt 0 ]; do > case "$1" in > @@ -39,6 +50,10 @@ while [ $# -gt 0 ]; do > BTF_BASE="$2" > shift 2 > ;; > + --placeholder) > + PLACEHOLDER=1 > + shift > + ;; > -*) > echo "Unknown option: $1" >&2 > usage > @@ -60,6 +75,10 @@ is_enabled() { > grep -q "^$1=y" ${objtree}/include/config/auto.conf > } > > +is_module() { > + grep -q "^$1=m" ${objtree}/include/config/auto.conf > +} > + > case "${KBUILD_VERBOSE}" in > *1*) > set -x > @@ -79,6 +98,30 @@ gen_btf_data() > --btf ${btf1} "${ELF_FILE}" > } > > +# Write one byte with value $1 (0..255) > +put_byte() > +{ > + printf "\\$(printf '%03o' "$1")" > +} > + > +# CONFIG_DEBUG_INFO_BTF=m: write struct btf_vmlinux_meta { u32 size; u8 > +# sha256[32]; } for the BTF in $1 to $2, in the target's byte order. > +gen_btf_meta() > +{ > + size=$(${CONFIG_SHELL} "${srctree}/scripts/file-size.sh" "$1") > + sha256=$(sha256sum < "$1" | cut -d' ' -f1) > + { > + if is_enabled CONFIG_CPU_BIG_ENDIAN; then > + for shift in 24 16 8 0; do put_byte $(( (size >> shift) > & 255 )); done > + else > + for shift in 0 8 16 24; do put_byte $(( (size >> shift) > & 255 )); done > + fi > + for byte in $(echo "${sha256}" | sed 's/../& /g'); do > + put_byte $(( 0x${byte} )) > + done > + } > "$2" > +} > + > gen_btf_o() > { > btf_data=${ELF_FILE}.btf.o > @@ -88,9 +131,23 @@ gen_btf_o() > # deletes all symbols including __start_BTF and __stop_BTF, which will > # be redefined in the linker script. > echo "" | ${CC} ${CLANG_FLAGS} ${KBUILD_CPPFLAGS} ${KBUILD_CFLAGS} > -fno-lto -c -x c -o ${btf_data} - > - ${OBJCOPY} --add-section .BTF=${ELF_FILE}.BTF \ > - --set-section-flags .BTF=alloc,readonly ${btf_data} > - ${OBJCOPY} --only-section=.BTF --strip-all ${btf_data} > + if is_module CONFIG_DEBUG_INFO_BTF; then > + # CONFIG_DEBUG_INFO_BTF=m: .BTF stays non-allocatable, kept in > + # vmlinux.unstripped for module BTF but not loaded; the > btf_vmlinux > + # module provides it at runtime. What is loaded is .BTF.meta, > + # its size and hash, so that /sys/kernel/btf/vmlinux has the > right > + # size from boot and only the matching BTF is accepted. > + gen_btf_meta ${ELF_FILE}.BTF ${ELF_FILE}.BTF.meta > + ${OBJCOPY} --add-section .BTF=${ELF_FILE}.BTF \ > + --set-section-flags .BTF=readonly \ > + --add-section .BTF.meta=${ELF_FILE}.BTF.meta \ > + --set-section-flags .BTF.meta=alloc,readonly ${btf_data} > + ${OBJCOPY} --only-section=.BTF --only-section=.BTF.meta > --strip-all ${btf_data} > + else > + ${OBJCOPY} --add-section .BTF=${ELF_FILE}.BTF \ > + --set-section-flags .BTF=alloc,readonly ${btf_data} > + ${OBJCOPY} --only-section=.BTF --strip-all ${btf_data} > + fi > > # Change e_type to ET_REL so that it can be used to link final vmlinux. > # GNU ld 2.35+ and lld do not allow an ET_EXEC input. > @@ -121,6 +178,7 @@ cleanup() > { > rm -f "${ELF_FILE}.BTF.1" > rm -f "${ELF_FILE}.BTF" > + rm -f "${ELF_FILE}.BTF.meta" > if [ "${BTFGEN_MODE}" = "module" ]; then > rm -f "${ELF_FILE}.BTF.base" > rm -f "${ELF_FILE}.BTF_ids" > @@ -133,6 +191,35 @@ if [ -n "${BTF_BASE}" ]; then > BTFGEN_MODE="module" > fi > > +if [ -n "${PLACEHOLDER}" ]; then > + btf_data=${ELF_FILE}.btf.o > + echo "" | ${CC} ${CLANG_FLAGS} ${KBUILD_CPPFLAGS} ${KBUILD_CFLAGS} > -fno-lto -c -x c -o ${btf_data} - > + dd if=/dev/zero of=${ELF_FILE}.BTF.meta bs=36 count=1 2>/dev/null > + ${OBJCOPY} --add-section .BTF.meta=${ELF_FILE}.BTF.meta \ > + --set-section-flags .BTF.meta=alloc,readonly ${btf_data} > + ${OBJCOPY} --only-section=.BTF.meta --strip-all ${btf_data} > + exit 0 > +fi > + > +# CONFIG_DEBUG_INFO_BTF=m: the btf_vmlinux module carries the vmlinux BTF > +# itself. Its own types are of no interest, so instead of generating split > +# BTF for it, copy the (non-loadable) .BTF section of --btf_base > +# (vmlinux.unstripped) into the module. > +# The kernel recognizes the module by name and treats its .BTF as base BTF. > +case "${BTFGEN_MODE}:${ELF_FILE}" in > +module:*/btf_vmlinux.ko) > + if is_module CONFIG_DEBUG_INFO_BTF; then > + # -O binary only emits allocatable sections; make .BTF one for > + # the extraction. ${BTF_BASE} itself is not modified. > + ${OBJCOPY} -O binary --only-section=.BTF > \ > + --set-section-flags .BTF=alloc,load,readonly \ > + "${BTF_BASE}" "${ELF_FILE}.BTF" > + ${OBJCOPY} --add-section .BTF="${ELF_FILE}.BTF" "${ELF_FILE}" > + exit 0 > + fi > + ;; > +esac > + > gen_btf_data > > case "${BTFGEN_MODE}" in > diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh > index ab0b8125c8cb..a9a066267ef6 100755 > --- a/scripts/link-vmlinux.sh > +++ b/scripts/link-vmlinux.sh > @@ -37,6 +37,15 @@ is_enabled() { > grep -q "^$1=y" include/config/auto.conf > } > > +is_module() { > + grep -q "^$1=m" include/config/auto.conf > +} > + > +# =y or =m > +is_set() { > + grep -q "^$1=[ym]" include/config/auto.conf > +} > + > # Nice output in kbuild format > # Will be suppressed by "make -s" > info() > @@ -211,17 +220,25 @@ if is_enabled CONFIG_KALLSYMS; then > kallsyms .tmp_vmlinux0.syms .tmp_vmlinux0.kallsyms > fi > > -if is_enabled CONFIG_KALLSYMS || is_enabled CONFIG_DEBUG_INFO_BTF; then > +if is_module CONFIG_DEBUG_INFO_BTF; then > + # The kernel refers to the size and hash of its BTF, which only the > + # BTF generated from the first link can provide; link a placeholder > + # of the same layout until then. > + ${CONFIG_SHELL} ${srctree}/scripts/gen-btf.sh --placeholder > .tmp_vmlinux0 > + btf_vmlinux_bin_o=.tmp_vmlinux0.btf.o > +fi > + > +if is_enabled CONFIG_KALLSYMS || is_set CONFIG_DEBUG_INFO_BTF; then > > # The kallsyms linking does not need debug symbols, but the BTF does. > - if ! is_enabled CONFIG_DEBUG_INFO_BTF; then > + if ! is_set CONFIG_DEBUG_INFO_BTF; then > strip_debug=1 > fi > > vmlinux_link .tmp_vmlinux1 > fi > > -if is_enabled CONFIG_DEBUG_INFO_BTF; then > +if is_set CONFIG_DEBUG_INFO_BTF; then > info BTF .tmp_vmlinux1 > if ! ${CONFIG_SHELL} ${srctree}/scripts/gen-btf.sh .tmp_vmlinux1; then > echo >&2 "Failed to generate BTF for vmlinux" > @@ -287,7 +304,7 @@ fi > > vmlinux_link "${VMLINUX}" > > -if is_enabled CONFIG_DEBUG_INFO_BTF; then > +if is_set CONFIG_DEBUG_INFO_BTF; then > info BTFIDS ${VMLINUX} > ${RESOLVE_BTFIDS} --patch_btfids ${btfids_vmlinux} ${VMLINUX} > fi > -- > 2.47.3 >
