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

From: Nathan Chancellor
Date: Tue Jan 31 2023 - 16:17:18 EST

On Tue, Jan 31, 2023 at 12:59:04PM -0800, Ian Rogers wrote:
> On Tue, Jan 31, 2023 at 11:33 AM Nathan Chancellor <nathan@xxxxxxxxxx> wrote:
> >
> > On Tue, Jan 31, 2023 at 11:25:38AM -0800, Ian Rogers wrote:
> > > On Tue, Jan 31, 2023 at 10:08 AM Ian Rogers <irogers@xxxxxxxxxx> wrote:
> > > >
> > > > On Tue, Jan 31, 2023 at 9:21 AM Nathan Chancellor <nathan@xxxxxxxxxx> wrote:
> > > > >
> > > > > 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@xxxxxxxxxx>
> > > > > > ---
> > > > > > 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
> > > >
> > > > Thanks Nathan, and thanks for all the details in the bug report. I'm
> > > > looking into this.
> > > >
> > > > Ian
> > >
> > > Given the somewhat complicated cross compile I wasn't able to get a
> > > reproduction. Could you see if the following addresses the problem:
> >
> > As long as you have an LLVM toolchain that targets AArch64 and your
> > host, you should be able to reproduce this issue with those commands
> > verbatim, as that command should not use any GNU binutils. I am pretty
> > sure I tried it in a fresh container before reporting it but it is
> > possible that I did not.
>
> Thanks, do you have instructions on setting up the container?

Sure thing, this worked for me in the Linux source you would like to
build (I marked it readonly to make sure any modifications were done in
my host environment, remove the ':ro' if you would like to make edits to
the source within the container). Docker should work as well but I did
not test it.

$ podman run --rm -ti -v $PWD:/linux:ro docker.io/archlinux:base-devel
# pacman -Syyu --noconfirm bc clang lib32-glibc lld llvm python
...

# To turn on CONFIG_DEBUG_INFO_BTF
# make -C /linux -skj"$(nproc)" ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- HOSTLDFLAGS=-fuse-ld=lld LLVM=1 O=/build defconfig menuconfig

# make -C /linux -skj"$(nproc)" ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- HOSTLDFLAGS=-fuse-ld=lld LLVM=1 O=/build prepare
ld.lld: error: /build/tools/bpf/resolve_btfids//resolve_btfids-in.o is incompatible with elf64-x86-64
clang-15: error: linker command failed with exit code 1 (use -v to see invocation)

> > > ```
> > > diff --git a/tools/bpf/resolve_btfids/Makefile
> > > b/tools/bpf/resolve_btfids/Makefile
> > > index daed388aa5d7..a06966841df4 100644
> > > --- a/tools/bpf/resolv
>
> Ian
> e_btfids/Makefile
> > > +++ b/tools/bpf/resolve_btfids/Makefile
> > > @@ -19,10 +19,9 @@ endif
> > >
> > > # always use the host compiler
> > > HOST_OVERRIDES := AR="$(HOSTAR)" CC="$(HOSTCC)" LD="$(HOSTLD)"
> > > ARCH="$(HOSTARCH)" \
> > > - EXTRA_CFLAGS="$(HOSTCFLAGS) $(KBUILD_HOSTCFLAGS)"
> > > + EXTRA_CFLAGS="$(HOSTCFLAGS) $(KBUILD_HOSTCFLAGS)"
> > > CROSS_COMPILE=""
> > >
> > > RM ?= rm
> > > -CROSS_COMPILE =
> > >
> > > OUTPUT ?= $(srctree)/tools/bpf/resolve_btfids/
> > > ```
> > >
> >
> > Unfortunately, it does not. I still see '--target=' end up in the
> > CFLAGS of those files.
> >
> > Cheers,
> > Nathan
>
> Hmm.. I can see the issue. We avoid this in places like libsubcmd
> because the Makefile there includes tools/scripts/Makefile.include,
> but here the Makefile.build inclusion is direct and we have CC set up
> for cross compilation. This is messy and it will take some playing
> around to come up with a simple fix.

Ack, if you have any issues with reproducing it, please let me know.

Cheers,
Nathan