Re: [PATCH bpf-next] libbpf: fix internal USDT address translation logic for shared libraries

Andrii Nakryiko
2022-07-28T22:05:12Z

On Wed, Jul 20, 2022 at 11:37 AM Jakub Sitnicki <jakub@cloudflare.com> wrote:
>
> On Wed, Jul 20, 2022 at 05:32 PM +02, Jakub Sitnicki wrote:
>
> [...]
>
> >> diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
> >> index 8ad7a733a505..e08e8e34e793 100644
> >> --- a/tools/testing/selftests/bpf/Makefile
> >> +++ b/tools/testing/selftests/bpf/Makefile
> >> @@ -172,13 +172,15 @@ $(OUTPUT)/%:%.c
> >> # do not fail. Static builds leave urandom_read relying on system-wide shared libraries.
> >> $(OUTPUT)/liburandom_read.so: urandom_read_lib1.c urandom_read_lib2.c
> >> $(call msg,LIB,,$@)
> >> - $(Q)$(CC) $(filter-out -static,$(CFLAGS) $(LDFLAGS)) $^ $(LDLIBS) -fPIC -shared -o $@
> >> + $(Q)$(CLANG) $(filter-out -static,$(CFLAGS) $(LDFLAGS)) $^ $(LDLIBS) \
> >> + -fuse-ld=lld -Wl,-znoseparate-code -fPIC -shared -o $@
> >>
> >> $(OUTPUT)/urandom_read: urandom_read.c urandom_read_aux.c $(OUTPUT)/liburandom_read.so
> >> $(call msg,BINARY,,$@)
> >> - $(Q)$(CC) $(filter-out -static,$(CFLAGS) $(LDFLAGS)) $(filter %.c,$^) \
> >> - liburandom_read.so $(LDLIBS) \
> >> - -Wl,-rpath=. -Wl,--build-id=sha1 -o $@
> >> + $(Q)$(CLANG) $(filter-out -static,$(CFLAGS) $(LDFLAGS)) $(filter %.c,$^) \
> >> + liburandom_read.so $(LDLIBS) \
> >> + -fuse-ld=lld -Wl,-znoseparate-code \
> >> + -Wl,-rpath=. -Wl,--build-id=sha1 -o $@
> >
> > [...]
> >
> > Not sure if this was considered - adding a dependency on Clang for the
> > target platform makes cross-compiling bpf selftests much harder than it
> > was.
> >
> > Maybe we could use $(CLANG) only when not cross-compiling, and execute
> > $(CC) like before otherwise?
>
> OTOH, there seems to be nothing Clang specific about the build
> recipe. We just want t use LLVM lld. GCC accepts -fuse-ld as well (bfd
> vs lld).
>
> Perhaps we could partially revert to something as below? It "fixes" the
> cross-compilation build of bpf selftests for arm64 for me.
>
> WDYT?
>
> --8<--
> diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
> index 8d59ec7f4c2d..541e2b0de27a 100644
> --- a/tools/testing/selftests/bpf/Makefile
> +++ b/tools/testing/selftests/bpf/Makefile
> @@ -170,24 +170,24 @@ $(OUTPUT)/%:%.c
>
> # LLVM's ld.lld doesn't support all the architectures, so use it only on x86
> ifeq ($(SRCARCH),x86)
> -LLD := lld
> +ALT_LD := lld
> else
> -LLD := ld
> +ALT_LD := bfd
> endif
>
> # Filter out -static for liburandom_read.so and its dependent targets so that static builds
> # do not fail. Static builds leave urandom_read relying on system-wide shared libraries.
> $(OUTPUT)/liburandom_read.so: urandom_read_lib1.c urandom_read_lib2.c
> $(call msg,LIB,,$@)
> - $(Q)$(CLANG) $(filter-out -static,$(CFLAGS) $(LDFLAGS)) $^ $(LDLIBS) \
> - -fuse-ld=$(LLD) -Wl,-znoseparate-code -fPIC -shared -o $@
> + $(Q)$(CC) $(filter-out -static,$(CFLAGS) $(LDFLAGS)) $^ $(LDLIBS) \
> + -fuse-ld=$(ALT_LD) -Wl,-znoseparate-code -fPIC -shared -o $@
>
> $(OUTPUT)/urandom_read: urandom_read.c urandom_read_aux.c $(OUTPUT)/liburandom_read.so
> $(call msg,BINARY,,$@)
> - $(Q)$(CLANG) $(filter-out -static,$(CFLAGS) $(LDFLAGS)) $(filter %.c,$^) \
> - liburandom_read.so $(LDLIBS) \
> - -fuse-ld=$(LLD) -Wl,-znoseparate-code \
> - -Wl,-rpath=. -Wl,--build-id=sha1 -o $@
> + $(Q)$(CC) $(filter-out -static,$(CFLAGS) $(LDFLAGS)) $(filter %.c,$^) \
> + liburandom_read.so $(LDLIBS) \
> + -fuse-ld=$(ALT_LD) -Wl,-znoseparate-code \
> + -Wl,-rpath=. -Wl,--build-id=sha1 -o $@
>

I tried building with CC locally and I still see ELF layout that would
exercise the logic that was fixed with my original patchset:

$ readelf -l liburandom_read.so | grep LOAD -A1
LOAD 0x0000000000000000 0x0000000000000000 0x0000000000000000
0x00000000000005ec 0x00000000000005ec R 0x1000
LOAD 0x00000000000005f0 0x00000000000015f0 0x00000000000015f0
0x0000000000000130 0x0000000000000130 R E 0x1000
LOAD 0x0000000000000720 0x0000000000002720 0x0000000000002720
0x0000000000000200 0x0000000000000200 RW 0x1000
LOAD 0x0000000000000920 0x0000000000003920 0x0000000000003920
0x0000000000000028 0x0000000000000029 RW 0x1000

So sure, sounds good to me, please send a patch with a change.

> $(OUTPUT)/bpf_testmod.ko: $(VMLINUX_BTF) $(wildcard bpf_testmod/Makefile bpf_testmod/*.[ch])
> $(call msg,MOD,,$@)