[PATCH] kbuild: Strip trailing padding bytes from modules.builtin.modinfo

[Nathan Chancellor <nathan@kernel.org>]

After commit d50f21091358 ("kbuild: align modinfo section for Secureboot
Authenticode EDK2 compat"), running modules_install with certain
versions of kmod (such as 29.1 in Ubuntu Jammy) in certain
configurations may fail with:

  depmod: ERROR: kmod_builtin_iter_next: unexpected string without modname prefix

The additional padding bytes to ensure .modinfo is aligned within
vmlinux.unstripped are unexpected by kmod, as this section has always
just been null-terminated strings.

Strip the trailing padding bytes from modules.builtin.modinfo after it
has been extracted from vmlinux.unstripped to restore the format that
kmod expects while keeping .modinfo aligned within vmlinux.unstripped to
avoid regressing the Authenticode calculation fix for EDK2.

Cc: stable@vger.kernel.org
Fixes: d50f21091358 ("kbuild: align modinfo section for Secureboot Authenticode EDK2 compat")
Reported-by: Omar Sandoval <osandov@fb.com>
Reported-by: Samir M <samir@linux.ibm.com>
Reported-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Closes: https://lore.kernel.org/7fef7507-ad64-4e51-9bb8-c9fb6532e51e@linux.ibm.com/
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Tested-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 scripts/Makefile.vmlinux | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/scripts/Makefile.vmlinux b/scripts/Makefile.vmlinux
index ced4379550d7..cd788cac9d91 100644
--- a/scripts/Makefile.vmlinux
+++ b/scripts/Makefile.vmlinux
@@ -102,11 +102,24 @@ vmlinux: vmlinux.unstripped FORCE
 # modules.builtin.modinfo
 # ---------------------------------------------------------------------------
 
+# .modinfo in vmlinux.unstripped is aligned to 8 bytes for compatibility with
+# tools that expect vmlinux to have sufficiently aligned sections but the
+# additional bytes used for padding .modinfo to satisfy this requirement break
+# certain versions of kmod with
+#
+#   depmod: ERROR: kmod_builtin_iter_next: unexpected string without modname prefix
+#
+# Strip the trailing padding bytes after extracting .modinfo to comply with
+# what kmod expects to parse.
+quiet_cmd_modules_builtin_modinfo = GEN     $@
+      cmd_modules_builtin_modinfo = $(cmd_objcopy); \
+                                    sed -i 's/\x00\+$$/\x00/g' $@
+
 OBJCOPYFLAGS_modules.builtin.modinfo := -j .modinfo -O binary
 
 targets += modules.builtin.modinfo
 modules.builtin.modinfo: vmlinux.unstripped FORCE
-	$(call if_changed,objcopy)
+	$(call if_changed,modules_builtin_modinfo)
 
 # modules.builtin
 # ---------------------------------------------------------------------------

---
base-commit: 6146a0f1dfae5d37442a9ddcba012add260bceb0
change-id: 20251105-kbuild-fix-builtin-modinfo-for-kmod-5cc1984719d3

Best regards,
--  
Nathan Chancellor <nathan@kernel.org>