[PATCH] Improve diagnostic on "Inconsistent kallsyms data"

[Domenico Andreoli <domenico.andreoli@linux.com>]

Hi,

  linking kallsyms into the kernel occasionally triggers edge conditions
of the linker heuristic and results in inconsistent System.map generation.

This patch adds a first-aid analysis of such inconsistency in form of
unified diff between the two generated symbol maps, where every symbol's
address is replaced by the offset from the previous symbol.

Becomes then trivial to spot symbols reordering without incurring in
the noise of other symbols shifting, as shown below.

$ make
...
  LD      vmlinux.o
  MODPOST vmlinux.o
  KSYM    .tmp_kallsyms1.o
  KSYM    .tmp_kallsyms2.o
  KSYM    .tmp_kallsyms3.o
  LD      vmlinux
  SORTEX  vmlinux
  SYSMAP  System.map
Inconsistent kallsyms data
Try "make KALLSYMS_EXTRA_PASS=1" as a workaround
---- .tmp_System.map.relative   2019-05-18 02:13:19.197282498 +0200
++++ System.map.relative        2019-05-18 02:13:16.817219696 +0200
@@ -7,10 +7,10 @@
 0000000000013cf78 A __rela_size
 0000000000012da88 A __pecoff_data_rawsize
 0000000000004e600 A __pecoff_data_size
-000000000004b7000 A __efistub_stext_offset
+000000000004a7000 A __efistub_stext_offset
 000000000001719d8 A __rela_offset
 0000000000026b628 A _kernel_size_le_lo32
-0ffff00000f4b3000 t __efistub__text
+0ffff00000f4c3000 t __efistub__text
 00000000000000000 t _head
 00000000000000000 T _text
 00000000000000040 t pe_header
@@ -28619,10 +28619,10 @@
 00000000000000008 r bus_spec.64271
 00000000000000008 r str_spec.64272
 00000000000000008 R kallsyms_offsets
-00000000000019bd0 R kallsyms_relative_base
+00000000000019bc8 R kallsyms_relative_base
 00000000000000008 R kallsyms_num_syms
 00000000000000008 R kallsyms_names
-0000000000004c468 R kallsyms_markers
+0000000000004c460 R kallsyms_markers
 000000000000001a0 R kallsyms_token_table
 00000000000000368 R kallsyms_token_index
 000000000000838c8 R __start_ro_after_init
@@ -43349,9 +43349,9 @@
 00000000000000000 R __start___modver
 00000000000000000 R __stop___param
 00000000000000008 r __modver_attr
-00000000000000008 R __stop___modver
-00000000000000ff0 R __end_rodata
+00000000000000008 R __end_rodata
 00000000000000000 R __start___ex_table
+00000000000000000 R __stop___modver
 00000000000001c78 R __start_notes
 00000000000000000 R __stop___ex_table
 00000000000000024 r _note_54
@@ -43359,11 +43359,11 @@
 0000000000000034c T idmap_pg_dir
 00000000000003000 T tramp_pg_dir
 00000000000001000 T swapper_pg_dir
-00000000000001000 T swapper_pg_end
-0000000000000f000 T __init_begin
+00000000000001000 T __init_begin
 00000000000000000 T __inittext_begin
 00000000000000000 T _sinittext
 00000000000000000 T stext
+00000000000000000 T swapper_pg_end
 00000000000000020 t preserve_boot_args
 00000000000000020 t __create_page_tables
 000000000000002e4 t __primary_switched
...

Signed-off-by: Domenico Andreoli <domenico.andreoli@linux.com>

---
 scripts/link-vmlinux.sh |   27 +++++++++++++++++++++++----
 1 file changed, 23 insertions(+), 4 deletions(-)

Index: b/scripts/link-vmlinux.sh
===================================================================
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -124,6 +124,25 @@ kallsyms()
 mksysmap()
 {
 	${CONFIG_SHELL} "${srctree}/scripts/mksysmap" ${1} ${2}
+
+	local KSYM_ADDR
+	local KSYM_ADDR_SIZE
+	local KSYM_ADDR_PREV
+	local KSYM_REMAINDER
+
+	# Replace the symbol's address with the offset from the previous
+	# so that in case of "Inconsistent kallsyms data" it's easier
+	# to spot symbols moving around
+	while read KSYM_ADDR KSYM_REMAINDER; do
+		if [ -z "${KSYM_ADDR_PREV}" ]; then
+			KSYM_ADDR_SIZE=`echo ${KSYM_ADDR} | wc -c`
+			KSYM_ADDR_PREV=${KSYM_ADDR}
+		fi
+		printf "%0${KSYM_ADDR_SIZE}x "                     \
+		       $(( 0x${KSYM_ADDR} - 0x${KSYM_ADDR_PREV} ))
+		echo ${KSYM_REMAINDER}
+		KSYM_ADDR_PREV=${KSYM_ADDR}
+	done <${2} >${2}.relative
 }
 
 sortextable()
@@ -134,10 +153,10 @@ sortextable()
 # Delete output files in case of error
 cleanup()
 {
-	rm -f .tmp_System.map
+	rm -f .tmp_System.map*
 	rm -f .tmp_kallsyms*
 	rm -f .tmp_vmlinux*
-	rm -f System.map
+	rm -f System.map*
 	rm -f vmlinux
 	rm -f vmlinux.o
 }
@@ -240,7 +259,6 @@ if [ -n "${CONFIG_KALLSYMS}" ]; then
 		kallsyms_vmlinux=.tmp_vmlinux3
 
 		vmlinux_link .tmp_kallsyms2.o .tmp_vmlinux3
-
 		kallsyms .tmp_vmlinux3 .tmp_kallsyms3.o
 	fi
 fi
@@ -262,7 +280,8 @@ if [ -n "${CONFIG_KALLSYMS}" ]; then
 
 	if ! cmp -s System.map .tmp_System.map; then
 		echo >&2 Inconsistent kallsyms data
-		echo >&2 Try "make KALLSYMS_EXTRA_PASS=1" as a workaround
+		echo >&2 Try \"make KALLSYMS_EXTRA_PASS=1\" as a workaround
+		diff -u .tmp_System.map.relative System.map.relative
 		exit 1
 	fi
 fi