[PATCH 6.12.y v2] kbuild: link-vmlinux.sh: improve detection of third pass requirement

[André Draszik]

commit 14b8b4bf2d7bd5d58c17adbeed0edfda1dfc1a7f upstream.

It can happen that symbol sizes within .tmp_vmlinux1.kallsyms.o and
.tmp_vmlinux2.kallsyms.o differ, without affecting file size on disk
because of section alignment and/or padding emitted by the assembler.

link-vmlinux.sh doesn't detect this case currently and keeps using
.tmp_vmlinux2.kallsyms.o to link the final vmlinux. Due to the
different symbol sizes, other symbols are shifted within the final
image compared to .tmp_vmlinux2, and the final comparison of System.map
against "${kallsyms_sysmap}" fails with the message:

    Inconsistent kallsyms data
    Try "make KALLSYMS_EXTRA_PASS=1" as a workaround

This can happen in particular if the linker emits additional symbols
that might have different names between our (re-)linking steps, e.g.
because those names depend on the virtual address of the symbol. Linker
stubs for ARM errata work-arounds are one such case.

These changed symbol names can cause the output of the token
compression of kallsyms.c to change due to the changed symbol substring
count, which in turn can change the size of the kallsyms_names symbol
itself, causing the potential shift of subsequent symbol addresses in
.tmp_vmlinux2.kallsyms.o and therefore the final image.

Update link-vmlinux.sh to not rely on file size of
.tmp_vmlinux?.kallsyms.o alone but to also consider symbol offsets
within to resolve this, and do a third pass if required.

Signed-off-by: André Draszik <andre.draszik@xxxxxxxxxx>
Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Reviewed-by: Nicolas Schier <nsc@xxxxxxxxxx>
Signed-off-by: Nicolas Schier <nsc@xxxxxxxxxx>
[AD: adjust context]
Signed-off-by: André Draszik <andre.draszik@xxxxxxxxxx>
---
This patch fixes an actual problem people are seeing in various
different kernel versions.

The version from 7.3 doesn't apply cleanly on 6.12 and a minor context
update was necessary.

The bug was introduced by 7e2b37c971a2 ("kbuild: kallsyms allow
3-pass generation if symbols size has changed") in v4.10.

Cheers,
Andre'
---
Changes in v2:
- drop a left-over sentence from the v6.18 version of this patch that
  doesn't apply here anymore
- Link to v1: https://lore.kernel.org/r/20261030-linux-kallsyms-fix-6-12-v1-1-9c43c892de04@xxxxxxxxxx
---
 scripts/link-vmlinux.sh | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index 94f794ca11ca..a5729419a452 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -131,6 +131,18 @@ gen_btf()
 	btf_vmlinux_bin_o=${btf_data}
 }
 
+# Check if kallsymso_prev and kallsymso differ
+# If symbol sizes within ${kallsymso} change, any symbols within vmlinux are
+# likely to shift, invalidating ${kallsymso}.
+# Since file size can remain unchanged even if symbol sizes change, compare the
+# actual symbols instead of relying on file size only.
+kallsymso_changed()
+{
+	${NM} -n "${kallsymso_prev}" > "${kallsymso_prev}.sym"
+	${NM} -n "${kallsymso}" > "${kallsymso}.sym"
+	! cmp -s "${kallsymso_prev}.sym" "${kallsymso}.sym"
+}
+
 # Create ${2}.o file with all symbols from the ${1} object file
 kallsyms()
 {
@@ -151,6 +163,7 @@ kallsyms()
 	${CC} ${NOSTDINC_FLAGS} ${LINUXINCLUDE} ${KBUILD_CPPFLAGS} \
 	      ${KBUILD_AFLAGS} ${KBUILD_AFLAGS_KERNEL} -c -o "${2}.o" "${2}.S"
 
+	kallsymso_prev="${kallsymso:-}"
 	kallsymso=${2}.o
 }
 
@@ -263,7 +276,12 @@ if is_enabled CONFIG_KALLSYMS; then
 	sysmap_and_kallsyms .tmp_vmlinux2
 	size2=$(${CONFIG_SHELL} "${srctree}/scripts/file-size.sh" ${kallsymso})
 
-	if [ $size1 -ne $size2 ] || [ -n "${KALLSYMS_EXTRA_PASS}" ]; then
+	# Due to alignment, file size of the kallsymso object file might remain
+	# unchanged even if individual symbols within change size. Changed
+	# symbol sizes can still shift other symbols, though. Therefore, don't
+	# rely on file size alone.
+	if [ $size1 -ne $size2 ] || kallsymso_changed || \
+			[ -n "${KALLSYMS_EXTRA_PASS}" ]; then
 		vmlinux_link .tmp_vmlinux3
 		sysmap_and_kallsyms .tmp_vmlinux3
 	fi

---
base-commit: 85d82e0d7c2dfd3e75d34f35b7ebbf41f45c9c4e
change-id: 20261030-linux-kallsyms-fix-6-12-2a67fae81111

Best regards,
-- 
André Draszik <andre.draszik@xxxxxxxxxx>