Re: [PATCH v4 2/2] tools/resolve_btfids: Alter how HOSTCC is forced

Nathan Chancellor
2023-01-31T17:26:12Z

Hi Ian,

On Mon, Jan 23, 2023 at 10:43:24PM -0800, Ian Rogers wrote:
> HOSTCC is always wanted when building. Setting CC to HOSTCC happens
> after tools/scripts/Makefile.include is included, meaning flags are
> set assuming say CC is gcc, but then it can be later set to HOSTCC
> which may be clang. tools/scripts/Makefile.include is needed for host
> set up and common macros in objtool's Makefile. Rather than override
> CC to HOSTCC, just pass CC as HOSTCC to Makefile.build, the libsubcmd
> builds and the linkage step. This means the Makefiles don't see things
> like CC changing and tool flag determination, and similar, work
> properly.
>
> Also, clear the passed subdir as otherwise an outer build may break by
> inadvertently passing an inappropriate value.
>
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
> tools/bpf/resolve_btfids/Makefile | 17 +++++++----------
> 1 file changed, 7 insertions(+), 10 deletions(-)
>
> diff --git a/tools/bpf/resolve_btfids/Makefile b/tools/bpf/resolve_btfids/Makefile
> index 1fe0082b2ecc..daed388aa5d7 100644
> --- a/tools/bpf/resolve_btfids/Makefile
> +++ b/tools/bpf/resolve_btfids/Makefile
> @@ -18,14 +18,11 @@ else
> endif
>
> # always use the host compiler
> -AR = $(HOSTAR)
> -CC = $(HOSTCC)
> -LD = $(HOSTLD)
> -ARCH = $(HOSTARCH)
> +HOST_OVERRIDES := AR="$(HOSTAR)" CC="$(HOSTCC)" LD="$(HOSTLD)" ARCH="$(HOSTARCH)" \
> + EXTRA_CFLAGS="$(HOSTCFLAGS) $(KBUILD_HOSTCFLAGS)"
> +
> RM ?= rm
> CROSS_COMPILE =
> -CFLAGS := $(KBUILD_HOSTCFLAGS)
> -LDFLAGS := $(KBUILD_HOSTLDFLAGS)
>
> OUTPUT ?= $(srctree)/tools/bpf/resolve_btfids/
>
> @@ -56,12 +53,12 @@ $(OUTPUT) $(OUTPUT)/libsubcmd $(LIBBPF_OUT):
>
> $(SUBCMDOBJ): fixdep FORCE | $(OUTPUT)/libsubcmd
> $(Q)$(MAKE) -C $(SUBCMD_SRC) OUTPUT=$(SUBCMD_OUT) \
> - DESTDIR=$(SUBCMD_DESTDIR) prefix= \
> + DESTDIR=$(SUBCMD_DESTDIR) $(HOST_OVERRIDES) prefix= subdir= \
> $(abspath $@) install_headers
>
> $(BPFOBJ): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(LIBBPF_OUT)
> $(Q)$(MAKE) $(submake_extras) -C $(LIBBPF_SRC) OUTPUT=$(LIBBPF_OUT) \
> - DESTDIR=$(LIBBPF_DESTDIR) prefix= EXTRA_CFLAGS="$(CFLAGS)" \
> + DESTDIR=$(LIBBPF_DESTDIR) $(HOST_OVERRIDES) prefix= subdir= \
> $(abspath $@) install_headers
>
> LIBELF_FLAGS := $(shell $(HOSTPKG_CONFIG) libelf --cflags 2>/dev/null)
> @@ -80,11 +77,11 @@ export srctree OUTPUT CFLAGS Q
> include $(srctree)/tools/build/Makefile.include
>
> $(BINARY_IN): fixdep FORCE prepare | $(OUTPUT)
> - $(Q)$(MAKE) $(build)=resolve_btfids
> + $(Q)$(MAKE) $(build)=resolve_btfids $(HOST_OVERRIDES)
>
> $(BINARY): $(BPFOBJ) $(SUBCMDOBJ) $(BINARY_IN)
> $(call msg,LINK,$@)
> - $(Q)$(CC) $(BINARY_IN) $(LDFLAGS) -o $@ $(BPFOBJ) $(SUBCMDOBJ) $(LIBS)
> + $(Q)$(HOSTCC) $(BINARY_IN) $(KBUILD_HOSTLDFLAGS) -o $@ $(BPFOBJ) $(SUBCMDOBJ) $(LIBS)
>
> clean_objects := $(wildcard $(OUTPUT)/*.o \
> $(OUTPUT)/.*.o.cmd \
> --
> 2.39.0.246.g2a6d74b583-goog
>

I just bisected a linking failure when building resolve_btfids with
clang to this change as commit 13e07691a16f ("tools/resolve_btfids:
Alter how HOSTCC is forced") in the bpf-next tree.

It appears to be related to whether or not CROSS_COMPILE is specified,
which we have to do for certain architectures and configurations still.
arm64 is not one of those but it helps demonstrate the issue.

# Turn off CONFIG_DEBUG_INFO_REDUCED and turn on CONFIG_DEBUG_INFO_BTF
$ make -skj"$(nproc)" ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- HOSTLDFLAGS=-fuse-ld=lld LLVM=1 defconfig menuconfig

$ make -skj"$(nproc)" ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- HOSTLDFLAGS=-fuse-ld=lld LLVM=1 prepare
ld.lld: error: $LINUX_SRC/tools/bpf/resolve_btfids//resolve_btfids-in.o is incompatible with elf64-x86-64
clang-17: error: linker command failed with exit code 1 (use -v to see invocation)
...

Before your change, with V=1, I see:

clang -Wp,-MD,$LINUX_SRC/tools/bpf/resolve_btfids/.main.o.d -Wp,-MT,$LINUX_SRC/tools/bpf/resolve_btfids/main.o -Wall -Wmissing-prototypes -Wstrict-prototypes -O2 -fomit-frame-pointer -std=gnu11 -Wdeclaration-after-statement -g -I$LINUX_SRC/tools/include -I$LINUX_SRC/tools/include/uapi -I$LINUX_SRC/tools/bpf/resolve_btfids/libbpf/include -I$LINUX_SRC/tools/bpf/resolve_btfids/libsubcmd/include -D"BUILD_STR(s)=#s" -c -o $LINUX_SRC/tools/bpf/resolve_btfids/main.o main.c

After, I see:

clang -Wp,-MD,$LINUX_SRC/tools/bpf/resolve_btfids/.main.o.d -Wp,-MT,$LINUX_SRC/tools/bpf/resolve_btfids/main.o --target=aarch64-linux-gnu -g -I$LINUX_SRC/tools/include -I$LINUX_SRC/tools/include/uapi -I$LINUX_SRC/tools/bpf/resolve_btfids/libbpf/include -I$LINUX_SRC/tools/bpf/resolve_btfids/libsubcmd/include -D"BUILD_STR(s)=#s" -c -o $LINUX_SRC/tools/bpf/resolve_btfids/main.o main.c

We seem to have taken on a '--target=aarch64-linux-gnu' (changing the
target of resolve_btfids-in.o) and we dropped the warning flags.

I think this comes from the clang block in
tools/scripts/Makefile.include, which is included into the
resolve_btfids Makefile via tools/lib/bpf/Makefile.

I am not super familiar with the tools build system, otherwise I would
try to provide a patch. I tried moving CROSS_COMPILE from a recursive to
simple variable ('=' -> ':=') and moving it to HOST_OVERRIDES but those
did not appear to resolve it for me.

If there is any other information I can provide or patches I can test,
please let me know.

Cheers,
Nathan