Re: [Qemu-devel] [PULL 05/21] rules.mak: Fix DSO build by pulling in archive symbols

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 09/12/2014 03:58 PM, Paolo Bonzini wrote:
> From: Fam Zheng <famz@redhat.com>
> 
> This fixes an issue with module build system. block/iscsi.so is
> currently broken:
> 
>     $ ~/build/last/qemu-img
>     Failed to open module: /home/fam/build/master/block-iscsi.so:
>     undefined symbol: qmp_query_uuid
>     qemu-img: Not enough arguments
>     Try 'qemu-img --help' for more information
> 
> To fix this, we should (at least) let qemu-img link qmp_query_uuid from
> libqemustub.a. (There are a few other symbols missing, as well.)
> 
> This patch changes the linking rules to:
> 
> 1) Build ".mo" with "ld -r -o $@ $^" for each ".so", and later build .so
>    with it.
> 
> 2) Always build all the .mo before linking the executables. This is
>    achieved by adding those .mo files to the executables' "-y"
>    variables.
> 
> 3) When linking an executable, those .mo files in its "-y" variables are
>    filtered out, and replaced by one or more -Wl,-u,$symbol flags. This
>    is done in the added macro "process-archive-undefs".
> 
>    These "-Wl,-u,$symbol" flags will force ld to pull in the function
>    definition from the archives when linking.
> 
>    Note that the .mo objects, that are actually meant to be linked in
>    the executables, are already expanded in unnest-vars, before the
>    linking command. So we are safe to simply filter out .mo for the
>    purpose of pulling undefined symbols.
> 
>    process-archive-undefs works as this: For each ".mo", find all the
>    undefined symbols in it, filter ones that are defined in the
>    archives. For each of these symbols, generate a "-Wl,-u,$symbol" in
>    the link command, and put them before archive names in the command
>    line.
> 
> Suggested-by: H.J. Lu <hjl.tools@gmail.com>
> Signed-off-by: Fam Zheng <famz@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>


This patch breaks building the mo files in po/ for me:

make -C po install
make[1]: Entering directory `/home/dailybuild/builder/packages/BUILD/qemu-2.1.0/po'
  LD -r tr.mo
/usr/bin/ld: no input file


Christian

> ---
>  rules.mak | 43 ++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 40 insertions(+), 3 deletions(-)
> 
> diff --git a/rules.mak b/rules.mak
> index ba2f4c1..1d73293 100644
> --- a/rules.mak
> +++ b/rules.mak
> @@ -22,6 +22,32 @@ QEMU_DGFLAGS += -MMD -MP -MT $@ -MF $(*D)/$(*F).d
>  # Same as -I$(SRC_PATH) -I., but for the nested source/object directories
>  QEMU_INCLUDES += -I$(<D) -I$(@D)
> 
> +WL_U := -Wl,-u,
> +find-symbols = $(if $1, $(sort $(shell nm -P -g $1 | $2)))
> +defined-symbols = $(call find-symbols,$1,awk '$$2!="U"{print $$1}')
> +undefined-symbols = $(call find-symbols,$1,awk '$$2=="U"{print $$1}')
> +
> +# All the .mo objects in -m variables are also added into corresponding -y
> +# variable in unnest-vars, but filtered out here, when LINK is called.
> +#
> +# The .mo objects are supposed to be linked as a DSO, for module build. So here
> +# they are only used as a placeholders to generate those "archive undefined"
> +# symbol options (-Wl,-u,$symbol_name), which are the archive functions
> +# referenced by the code in the DSO.
> +#
> +# Also the presence in -y variables will also guarantee they are built before
> +# linking executables that will load them. So we can look up symbol reference
> +# in LINK.
> +#
> +# This is necessary because the exectuable itself may not use the function, in
> +# which case the function would not be linked in. Then the DSO loading will
> +# fail because of the missing symbol.
> +process-archive-undefs = $(filter-out %.a %.mo,$1) \
> +                $(addprefix $(WL_U), \
> +                     $(filter $(call defined-symbols,$(filter %.a, $1)), \
> +                              $(call undefined-symbols,$(filter %.mo,$1)))) \
> +                $(filter %.a,$1)
> +
>  extract-libs = $(strip $(foreach o,$1,$($o-libs)))
>  expand-objs = $(strip $(sort $(filter %.o,$1)) \
>                    $(foreach o,$(filter %.mo,$1),$($o-objs)) \
> @@ -38,7 +64,8 @@ LINKPROG = $(or $(CXX),$(CC))
> 
>  ifeq ($(LIBTOOL),)
>  LINK = $(call quiet-command, $(LINKPROG) $(QEMU_CFLAGS) $(CFLAGS) $(LDFLAGS) -o $@ \
> -       $1 $(version-obj-y) $(call extract-libs,$1) $(LIBS),"  LINK  $(TARGET_DIR)$@")
> +       $(call process-archive-undefs, $1) \
> +       $(version-obj-y) $(call extract-libs,$1) $(LIBS),"  LINK  $(TARGET_DIR)$@")
>  else
>  LIBTOOL += $(if $(V),,--quiet)
>  %.lo: %.c
> @@ -50,7 +77,8 @@ LIBTOOL += $(if $(V),,--quiet)
> 
>  LINK = $(call quiet-command,\
>         $(if $(filter %.lo %.la,$1),$(LIBTOOL) --mode=link --tag=CC \
> -       )$(LINKPROG) $(QEMU_CFLAGS) $(CFLAGS) $(LDFLAGS) -o $@ $1 \
> +       )$(LINKPROG) $(QEMU_CFLAGS) $(CFLAGS) $(LDFLAGS) -o $@ \
> +       $(call process-archive-undefs, $1)\
>         $(if $(filter %.lo %.la,$1),$(version-lobj-y),$(version-obj-y)) \
>         $(if $(filter %.lo %.la,$1),$(LIBTOOLFLAGS)) \
>         $(call extract-libs,$(1:.lo=.o)) $(LIBS),$(if $(filter %.lo %.la,$1),"lt LINK ", "  LINK  ")"$(TARGET_DIR)$@")
> @@ -76,11 +104,17 @@ endif
> 
>  %$(DSOSUF): CFLAGS += -fPIC -DBUILD_DSO
>  %$(DSOSUF): LDFLAGS += $(LDFLAGS_SHARED)
> -%$(DSOSUF):
> +%$(DSOSUF): %.mo
>  	$(call LINK,$^)
>  	@# Copy to build root so modules can be loaded when program started without install
>  	$(if $(findstring /,$@),$(call quiet-command,cp $@ $(subst /,-,$@), "  CP    $(subst /,-,$@)"))
> 
> +
> +LD_REL := $(CC) -nostdlib -Wl,-r
> +
> +%.mo:
> +	$(call quiet-command,$(LD_REL) -o $@ $^,"  LD -r $(TARGET_DIR)$@")
> +
>  .PHONY: modules
>  modules:
> 
> @@ -306,6 +340,9 @@ define unnest-vars
>          # For module build, build shared libraries during "make modules"
>          # For non-module build, add -m to -y
>          $(if $(CONFIG_MODULES),
> +             $(foreach o,$($v),
> +                   $(eval $o: $($o-objs)))
> +             $(eval $(patsubst %-m,%-y,$v) += $($v))
>               $(eval modules: $($v:%.mo=%$(DSOSUF))),
>               $(eval $(patsubst %-m,%-y,$v) += $(call expand-objs, $($v)))))
>