btf: move pahole check in scripts/link-vmlinux.sh to lib/Kconfig.debug

When DEBUG_INFO_DWARF5 is selected, pahole 1.21+ is required to enable
DEBUG_INFO_BTF.

When DEBUG_INFO_DWARF4 or DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT is selected,
DEBUG_INFO_BTF can be enabled without pahole installed, but a build error
will occur in scripts/link-vmlinux.sh:

LD .tmp_vmlinux1
BTF: .tmp_vmlinux1: pahole (pahole) is not available
Failed to generate BTF for vmlinux
Try to disable CONFIG_DEBUG_INFO_BTF

We did not guard DEBUG_INFO_BTF by PAHOLE_VERSION when previously
discussed [1].

However, commit 613fe1692377 ("kbuild: Add CONFIG_PAHOLE_VERSION")
added CONFIG_PAHOLE_VERSION after all. Now several CONFIG options, as
well as the combination of DEBUG_INFO_BTF and DEBUG_INFO_DWARF5, are
guarded by PAHOLE_VERSION.

The remaining compile-time check in scripts/link-vmlinux.sh now appears
to be an awkward inconsistency.

This commit adopts Nathan's original work.

[1]: https://lore.kernel.org/lkml/[e-mail]/

Signed-off-by: Masahiro Yamada <[e-mail]>
Reviewed-by: Alan Maguire <[e-mail]>
Acked-by: Andrii Nakryiko <[e-mail]>
Reviewed-by: Nathan Chancellor <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

lib/Kconfig.debug

···
 	depends on !DEBUG_INFO_SPLIT && !DEBUG_INFO_REDUCED
 	depends on !GCC_PLUGIN_RANDSTRUCT || COMPILE_TEST
 	depends on BPF_SYSCALL
+	depends on PAHOLE_VERSION >= 116
 	depends on !DEBUG_INFO_DWARF5 || PAHOLE_VERSION >= 121
 	# pahole uses elfutils, which does not have support for Hexagon relocations
 	depends on !HEXAGON
 	help
 	  Generate deduplicated BTF type information from DWARF debug info.
-	  Turning this on expects presence of pahole tool, which will convert
-	  DWARF type info into equivalent deduplicated BTF type info.
+	  Turning this on requires pahole v1.16 or later (v1.21 or later to
+	  support DWARF 5), which will convert DWARF type info into equivalent
+	  deduplicated BTF type info.
 
 config PAHOLE_HAS_SPLIT_BTF
 	def_bool PAHOLE_VERSION >= 119

scripts/link-vmlinux.sh

···
 # ${1} - vmlinux image
 gen_btf()
 {
-	local pahole_ver
 	local btf_data=${1}.btf.o
-
-	if ! [ -x "$(command -v ${PAHOLE})" ]; then
-		echo >&2 "BTF: ${1}: pahole (${PAHOLE}) is not available"
-		return 1
-	fi
-
-	pahole_ver=$(${PAHOLE} --version | sed -E 's/v([0-9]+)\.([0-9]+)/\1\2/')
-	if [ "${pahole_ver}" -lt "116" ]; then
-		echo >&2 "BTF: ${1}: pahole version $(${PAHOLE} --version) is too old, need at least v1.16"
-		return 1
-	fi
 
 	info BTF "${btf_data}"
 	LLVM_OBJCOPY="${OBJCOPY}" ${PAHOLE} -J ${PAHOLE_FLAGS} ${1}