[PATCH v6 1/8] kbuild: record needed exported symbols for modules

From: Nicolas Pitre
Date: Sun Mar 13 2016 - 22:45:11 EST

Kernel modules are partially linked object files with some undefined
symbols that are expected to be matched with EXPORT_SYMBOL() entries
from elsewhere.

Each .tmp_versions/*.mod file currently contains two line of text
separated by a newline character. The first line has the actual module
file name while the second line has a list of object files constituting
that module. Those files are parsed by modpost (scripts/mod/sumversion.c),
scripts/Makefile.modpost, scripts/Makefile.modsign, etc. Only the
modpost utility cares about the second line while the others retrieve
only the first line.

Therefore we can add a third line to record the list of undefined symbols
aka required EXPORT_SYMBOL() entries for each module into that file
without breaking anything. Like for the second line, symbols are separated
by a blank and the list is terminated with a newline character.

To avoid needless build overhead, the undefined symbols extraction is
performed only when CONFIG_TRIM_UNUSED_KSYMS is selected.

Signed-off-by: Nicolas Pitre <nico@xxxxxxxxxx>
Acked-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
scripts/Makefile.build | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 2c47f9c305..f4b4320e0d 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -253,6 +253,13 @@ define rule_cc_o_c
mv -f $(dot-target).tmp $(dot-target).cmd

+# List module undefined symbols (or empty line if not enabled)
+cmd_undef_syms = $(NM) $@ | sed -n 's/^ \+U //p' | xargs echo
+cmd_undef_syms = echo
# Built-in and composite module parts
$(obj)/%.o: $(src)/%.c $(recordmcount_source) FORCE
$(call cmd,force_checksrc)
@@ -263,7 +270,8 @@ $(obj)/%.o: $(src)/%.c $(recordmcount_source) FORCE
$(single-used-m): $(obj)/%.o: $(src)/%.c $(recordmcount_source) FORCE
$(call cmd,force_checksrc)
$(call if_changed_rule,cc_o_c)
- @{ echo $(@:.o=.ko); echo $@; } > $(MODVERDIR)/$(@F:.o=.mod)
+ @{ echo $(@:.o=.ko); echo $@; \
+ $(cmd_undef_syms); } > $(MODVERDIR)/$(@F:.o=.mod)

quiet_cmd_cc_lst_c = MKLST $@
cmd_cc_lst_c = $(CC) $(c_flags) -g -c -o $*.o $< && \
@@ -393,7 +401,8 @@ $(call multi_depend, $(multi-used-y), .o, -objs -y)

$(multi-used-m): FORCE
$(call if_changed,link_multi-m)
- @{ echo $(@:.o=.ko); echo $(link_multi_deps); } > $(MODVERDIR)/$(@F:.o=.mod)
+ @{ echo $(@:.o=.ko); echo $(link_multi_deps); \
+ $(cmd_undef_syms); } > $(MODVERDIR)/$(@F:.o=.mod)
$(call multi_depend, $(multi-used-m), .o, -objs -y -m)

targets += $(multi-used-y) $(multi-used-m)