kbuild: Strip trailing padding bytes from modules.builtin.modinfo

commit a26a6c93edfeee82cb73f55e87d995eea59ddfe8 upstream.

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: [email protected]
Fixes: d50f21091358 ("kbuild: align modinfo section for Secureboot Authenticode EDK2 compat")
Reported-by: Omar Sandoval <[email protected]>
Reported-by: Samir M <[email protected]>
Reported-by: Venkat Rao Bagalkote <[email protected]>
Closes: https://lore.kernel.org/[email protected]/
Tested-by: Omar Sandoval <[email protected]>
Tested-by: Samir M <[email protected]>
Tested-by: Venkat Rao Bagalkote <[email protected]>
Reviewed-by: Nicolas Schier <[email protected]>
Link: https://patch.msgid.link/20251105-kbuild-fix-builtin-modinfo-for-kmod-v1-1-b419d8ad4606@kernel.org
Signed-off-by: Nathan Chancellor <[email protected]>
[nathan: Apply to scripts/Makefile.vmlinux_o, location of
         modules.builtin.modinfo rule prior to 39cfd5b12160]
Signed-off-by: Nathan Chancellor <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: nathanchance

scripts/Makefile.vmlinux_o

@@ -76,11 +76,24 @@ targets += vmlinux.o
 # 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.o FORCE
-	$(call if_changed,objcopy)
+	$(call if_changed,modules_builtin_modinfo)
 
 # modules.builtin
 # ---------------------------------------------------------------------------