[PATCH v2 3/3] kbuild: merge temporary vmlinux for BTF and kallsyms

[Masahiro Yamada]

CONFIG_DEBUG_INFO_BTF=y requires one additional link step.
(.tmp_vmlinux.btf)

CONFIG_KALLSYMS=y requires two additional link steps.
(.tmp_vmlinux.kallsyms1 and .tmp_vmlinux.kallsyms2)

Enabling both requires three additional link steps.

When CONFIG_DEBUG_INFO_BTF=y and CONFIG_KALLSYMS=y, the build steps are
as follows:

    KSYMS   .tmp_vmlinux.kallsyms0.S
    AS      .tmp_vmlinux.kallsyms0.o
    LD      .tmp_vmlinux.btf             # temporary vmlinux for BTF
    BTF     .btf.vmlinux.bin.o
    LD      .tmp_vmlinux.kallsyms1       # temporary vmlinux for kallsyms step 1
    NM      .tmp_vmlinux.kallsyms1.syms
    KSYMS   .tmp_vmlinux.kallsyms1.S
    AS      .tmp_vmlinux.kallsyms1.o
    LD      .tmp_vmlinux.kallsyms2       # temporary vmlinux for kallsyms step 2
    NM      .tmp_vmlinux.kallsyms2.syms
    KSYMS   .tmp_vmlinux.kallsyms2.S
    AS      .tmp_vmlinux.kallsyms2.o
    LD      vmlinux                      # final vmlinux

This is redundant because the BTF generation and the kallsyms step 1 can
be performed against the same temporary vmlinux.

When both CONFIG_DEBUG_INFO_BTF and CONFIG_KALLSYMS are enabled, we can
reduce the number of link steps.

The build steps will be changed as follows:

    KSYMS   .tmp_vmlinux0.kallsyms.S
    AS      .tmp_vmlinux0.kallsyms.o
    LD      .tmp_vmlinux1                # temporary vmlinux for BTF and kallsyms step 1
    BTF     .tmp_vmlinux1.btf.o
    NM      .tmp_vmlinux1.syms
    KSYMS   .tmp_vmlinux1.kallsyms.S
    AS      .tmp_vmlinux1.kallsyms.o
    LD      .tmp_vmlinux2                # temporary vmlinux for kallsyms step 2
    NM      .tmp_vmlinux2.syms
    KSYMS   .tmp_vmlinux2.kallsyms.S
    AS      .tmp_vmlinux2.kallsyms.o
    LD      vmlinux                      # final vmlinux

Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
---

Changes in v2:
 - Fix the handling --strip-debug flag

 scripts/link-vmlinux.sh | 38 ++++++++++++++++++++------------------
 1 file changed, 20 insertions(+), 18 deletions(-)

diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index 6f2fdc9e4fbf..14aedec224af 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -89,7 +89,8 @@ vmlinux_link()
 	ldflags="${ldflags} ${wl}--script=${objtree}/${KBUILD_LDS}"
 
 	# The kallsyms linking does not need debug symbols included.
-	if [ "$output" != "${output#.tmp_vmlinux.kallsyms}" ] ; then
+	if [ "$output" != vmlinux ] &&
+	   (! is_enabled CONFIG_DEBUG_INFO_BTF || [ "$output" != .tmp_vmlinux1 ]) then
 		ldflags="${ldflags} ${wl}--strip-debug"
 	fi
 
@@ -105,11 +106,10 @@ vmlinux_link()
 
 # generate .BTF typeinfo from DWARF debuginfo
 # ${1} - vmlinux image
-# ${2} - file to dump raw BTF data into
 gen_btf()
 {
 	local pahole_ver
-	local btf_data=${2}
+	local btf_data=${1}.btf.o
 
 	if ! [ -x "$(command -v ${PAHOLE})" ]; then
 		echo >&2 "BTF: ${1}: pahole (${PAHOLE}) is not available"
@@ -122,8 +122,6 @@ gen_btf()
 		return 1
 	fi
 
-	vmlinux_link ${1}
-
 	info BTF "${btf_data}"
 	LLVM_OBJCOPY="${OBJCOPY}" ${PAHOLE} -J ${PAHOLE_FLAGS} ${1}
 
@@ -177,15 +175,13 @@ kallsyms()
 	kallsymso=${2}.o
 }
 
-# Perform one step in kallsyms generation, including temporary linking of
-# vmlinux.
-kallsyms_step()
+# Perform kallsyms for the given temporary vmlinux.
+sysmap_and_kallsyms()
 {
-	kallsyms_vmlinux=.tmp_vmlinux.kallsyms${1}
+	mksysmap "${1}" "${1}.syms"
+	kallsyms "${1}.syms" "${1}.kallsyms"
 
-	vmlinux_link "${kallsyms_vmlinux}"
-	mksysmap "${kallsyms_vmlinux}" "${kallsyms_vmlinux}.syms"
-	kallsyms "${kallsyms_vmlinux}.syms" "${kallsyms_vmlinux}"
+	kallsyms_sysmap=${1}.syms
 }
 
 # Create map file with all symbols from ${1}
@@ -227,11 +223,15 @@ kallsymso=
 btf_vmlinux_bin_o=
 
 if is_enabled CONFIG_KALLSYMS; then
-	kallsyms /dev/null .tmp_vmlinux.kallsyms0
+	kallsyms /dev/null .tmp_vmlinux0.kallsyms
+fi
+
+if is_enabled CONFIG_KALLSYMS || is_enabled CONFIG_DEBUG_INFO_BTF; then
+	vmlinux_link .tmp_vmlinux1
 fi
 
 if is_enabled CONFIG_DEBUG_INFO_BTF; then
-	if ! gen_btf .tmp_vmlinux.btf .btf.vmlinux.bin.o ; then
+	if ! gen_btf .tmp_vmlinux1; then
 		echo >&2 "Failed to generate BTF for vmlinux"
 		echo >&2 "Try to disable CONFIG_DEBUG_INFO_BTF"
 		exit 1
@@ -264,14 +264,16 @@ if is_enabled CONFIG_KALLSYMS; then
 	# a)  Verify that the System.map from vmlinux matches the map from
 	#     ${kallsymso}.
 
-	kallsyms_step 1
+	sysmap_and_kallsyms .tmp_vmlinux1
 	size1=$(${CONFIG_SHELL} "${srctree}/scripts/file-size.sh" ${kallsymso})
 
-	kallsyms_step 2
+	vmlinux_link .tmp_vmlinux2
+	sysmap_and_kallsyms .tmp_vmlinux2
 	size2=$(${CONFIG_SHELL} "${srctree}/scripts/file-size.sh" ${kallsymso})
 
 	if [ $size1 -ne $size2 ] || [ -n "${KALLSYMS_EXTRA_PASS}" ]; then
-		kallsyms_step 3
+		vmlinux_link .tmp_vmlinux3
+		sysmap_and_kallsyms .tmp_vmlinux3
 	fi
 fi
 
@@ -295,7 +297,7 @@ fi
 
 # step a (see comment above)
 if is_enabled CONFIG_KALLSYMS; then
-	if ! cmp -s System.map ${kallsyms_vmlinux}.syms; then
+	if ! cmp -s System.map "${kallsyms_sysmap}"; then
 		echo >&2 Inconsistent kallsyms data
 		echo >&2 'Try "make KALLSYMS_EXTRA_PASS=1" as a workaround'
 		exit 1
-- 
2.43.0