kbuild: Add support for extra BTF

information

.BTF.extra sections wil be used to add additional BTF information
for inlines etc.  .BTF.extra sections are split BTF relative to
kernel/module BTF and are enabled via CONFIG_DEBUG_INFO_BTF_EXTRA.
It is bool for now but will become tristate in a later patch when
support for a separate module is added (vmlinux .BTF.extra is
9Mb so 'y' is not a good option for most cases since it will
bloat vmlinux size).

Signed-off-by: Alan Maguire <[email protected]>

Author: alan-maguire

include/asm-generic/vmlinux.lds.h

@@ -675,6 +675,10 @@ defined(CONFIG_AUTOFDO_CLANG) || defined(CONFIG_PROPELLER_CLANG)
 	. = ALIGN(PAGE_SIZE);						\
 	.BTF_ids : AT(ADDR(.BTF_ids) - LOAD_OFFSET) {			\
 		*(.BTF_ids)						\
+	}								\
+	. = ALIGN(PAGE_SIZE);						\
+	.BTF.extra : AT(ADDR(.BTF.extra) - LOAD_OFFSET) {		\
+		BOUNDED_SECTION_BY(.BTF.extra, _BTF_extra)		\
 	}
 #else
 #define BTF

lib/Kconfig.debug

@@ -409,6 +409,13 @@ config PAHOLE_HAS_LANG_EXCLUDE
 	  otherwise it would emit malformed kernel and module binaries when
 	  using DEBUG_INFO_BTF_MODULES.
 
+config PAHOLE_HAS_INLINE
+	def_bool PAHOLE_VERSION >= 130
+	help
+	   Support for the "inline" feature in BTF is available; it supports
+	   encoding information about inline sites in BTF; their location
+	   and information to help retrieve their associated parameters.
+
 config DEBUG_INFO_BTF_MODULES
 	bool "Generate BTF type information for kernel modules"
 	default y
@@ -426,6 +433,17 @@ config MODULE_ALLOW_BTF_MISMATCH
 	  this option will still load module BTF where possible but ignore
 	  it when a mismatch is found.
 
+config DEBUG_INFO_BTF_EXTRA
+	bool "Provide extra information about inline sites in BTF"
+	default n
+	depends on DEBUG_INFO_BTF && PAHOLE_HAS_INLINE
+	help
+	   Generate information about inline sites in .BTF.extra sections
+	   which consist of split BTF relative to kernel/module BTF;
+	   this BTF is made available to the user in /sys/kernel/btf.extra
+	   where the filename corresponds to the kernel (vmlinux) or the
+	   module the inline info refers to.
+
 config GDB_SCRIPTS
 	bool "Provide GDB scripts for kernel debugging"
 	help

scripts/Makefile.btf

@@ -2,6 +2,7 @@
 
 pahole-ver := $(CONFIG_PAHOLE_VERSION)
 pahole-flags-y :=
+btf-extra :=
 
 JOBS := $(patsubst -j%,%,$(filter -j%,$(MAKEFLAGS)))
 
@@ -25,8 +26,14 @@ pahole-flags-$(call test-ge, $(pahole-ver), 126)  = -j$(JOBS) --btf_features=enc
 
 pahole-flags-$(call test-ge, $(pahole-ver), 130) += --btf_features=attributes
 
+btf-extra =
 ifneq ($(KBUILD_EXTMOD),)
 module-pahole-flags-$(call test-ge, $(pahole-ver), 128) += --btf_features=distilled_base
+else
+ifneq ($(CONFIG_DEBUG_INFO_BTF_EXTRA),)
+pahole-flags-$(call test-ge, $(pahole-ver), 130) += --btf_features=inline.extra
+btf-extra := y
+endif
 endif
 
 endif
@@ -35,3 +42,4 @@ pahole-flags-$(CONFIG_PAHOLE_HAS_LANG_EXCLUDE)		+= --lang_exclude=rust
 
 export PAHOLE_FLAGS := $(pahole-flags-y)
 export MODULE_PAHOLE_FLAGS := $(module-pahole-flags-y)
+export BTF_EXTRA := $(btf-extra)

scripts/link-vmlinux.sh

@@ -105,22 +105,29 @@ vmlinux_link()
 		${kallsymso} ${btf_vmlinux_bin_o} ${arch_vmlinux_o} ${ldlibs}
 }
 
-# generate .BTF typeinfo from DWARF debuginfo
+# generate .BTF typeinfo from DWARF debuginfo.  Optionally add .BTF.extra
+# section if BTF_EXTRA is set.
 # ${1} - vmlinux image
 gen_btf()
 {
 	local btf_data=${1}.btf.o
+	local btf_extra_flags=
 
 	info BTF "${btf_data}"
 	LLVM_OBJCOPY="${OBJCOPY}" ${PAHOLE} -J ${PAHOLE_FLAGS} ${1}
 
-	# Create ${btf_data} which contains just .BTF section but no symbols. Add
+
+	if [ -n "${BTF_EXTRA}" ]; then
+		btf_extra_flags="--only-section=.BTF.extra --set-section-flags .BTF.extra=alloc,readonly"
+	fi
+
+	# Create ${btf_data} which contains just .BTF sections but no symbols. Add
 	# SHF_ALLOC because .BTF will be part of the vmlinux image. --strip-all
 	# deletes all symbols including __start_BTF and __stop_BTF, which will
 	# be redefined in the linker script. Add 2>/dev/null to suppress GNU
 	# objcopy warnings: "empty loadable segment detected at ..."
 	${OBJCOPY} --only-section=.BTF --set-section-flags .BTF=alloc,readonly \
-		--strip-all ${1} "${btf_data}" 2>/dev/null
+		${btf_extra_flags} --strip-all ${1} "${btf_data}" 2>/dev/null
 	# 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.
 	if is_enabled CONFIG_CPU_BIG_ENDIAN; then