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Date: Sun, 29 Sep 2019 23:06:01 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Yonghong Song <yhs@...com>
Cc: bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
Alexei Starovoitov <ast@...com>,
Daniel Borkmann <daniel@...earbox.net>,
Kernel Team <kernel-team@...com>,
Kevin Laatz <kevin.laatz@...el.com>,
Arnaldo Carvalho de Melo <acme@...hat.com>,
Andrii Nakryiko <andriin@...com>
Subject: Re: [PATCH bpf v2] libbpf: handle symbol versioning properly for libbpf.a
On Sat, Sep 28, 2019 at 4:23 PM Yonghong Song <yhs@...com> wrote:
>
> bcc uses libbpf repo as a submodule. It brings in libbpf source
> code and builds everything together to produce shared libraries.
> With latest libbpf, I got the following errors:
> /bin/ld: libbcc_bpf.so.0.10.0: version node not found for symbol xsk_umem__create@...BPF_0.0.2
> /bin/ld: failed to set dynamic section sizes: Bad value
> collect2: error: ld returned 1 exit status
> make[2]: *** [src/cc/libbcc_bpf.so.0.10.0] Error 1
>
> In xsk.c, we have
> asm(".symver xsk_umem__create_v0_0_2, xsk_umem__create@...BPF_0.0.2");
> asm(".symver xsk_umem__create_v0_0_4, xsk_umem__create@@LIBBPF_0.0.4");
> The linker thinks the built is for LIBBPF but cannot find proper version
> LIBBPF_0.0.2/4, so emit errors.
>
> I also confirmed that using libbpf.a to produce a shared library also
> has issues:
> -bash-4.4$ cat t.c
> extern void *xsk_umem__create;
> void * test() { return xsk_umem__create; }
> -bash-4.4$ gcc -c -fPIC t.c
> -bash-4.4$ gcc -shared t.o libbpf.a -o t.so
> /bin/ld: t.so: version node not found for symbol xsk_umem__create@...BPF_0.0.2
> /bin/ld: failed to set dynamic section sizes: Bad value
> collect2: error: ld returned 1 exit status
> -bash-4.4$
>
> Symbol versioning does happens in commonly used libraries, e.g., elfutils
> and glibc. For static libraries, for a versioned symbol, the old definitions
> will be ignored, and the symbol will be an alias to the latest definition.
> For example, glibc sched_setaffinity is versioned.
> -bash-4.4$ readelf -s /usr/lib64/libc.so.6 | grep sched_setaffinity
> 756: 000000000013d3d0 13 FUNC GLOBAL DEFAULT 13 sched_setaffinity@...BC_2.3.3
> 757: 00000000000e2e70 455 FUNC GLOBAL DEFAULT 13 sched_setaffinity@@GLIBC_2.3.4
> 1800: 0000000000000000 0 FILE LOCAL DEFAULT ABS sched_setaffinity.c
> 4228: 00000000000e2e70 455 FUNC LOCAL DEFAULT 13 __sched_setaffinity_new
> 4648: 000000000013d3d0 13 FUNC LOCAL DEFAULT 13 __sched_setaffinity_old
> 7338: 000000000013d3d0 13 FUNC GLOBAL DEFAULT 13 sched_setaffinity@...BC_2
> 7380: 00000000000e2e70 455 FUNC GLOBAL DEFAULT 13 sched_setaffinity@@GLIBC_
> -bash-4.4$
> For static library, the definition of sched_setaffinity aliases to the new definition.
> -bash-4.4$ readelf -s /usr/lib64/libc.a | grep sched_setaffinity
> File: /usr/lib64/libc.a(sched_setaffinity.o)
> 8: 0000000000000000 455 FUNC GLOBAL DEFAULT 1 __sched_setaffinity_new
> 12: 0000000000000000 455 FUNC WEAK DEFAULT 1 sched_setaffinity
>
> For both elfutils and glibc, additional macros are used to control different handling
> of symbol versioning w.r.t static and shared libraries.
> For elfutils, the macro is SYMBOL_VERSIONING
> (https://sourceware.org/git/?p=elfutils.git;a=blob;f=lib/eu-config.h).
> For glibc, the macro is SHARED
> (https://sourceware.org/git/?p=glibc.git;a=blob;f=include/shlib-compat.h;hb=refs/heads/master)
>
> This patch used SYMBOL_VERSIONING as the macro name as it clearly indicates the
> intention. The macro name can be changed later if necessary as it is internal
I actually like SHARED more, because it's really is a generic
indicator of whether we are linking as static or shared library. The
fact that we are using that flag for symbol versioning is specific to
NEW_VERSION/OLD_VERSION macros, but SHARED itself can be later used
for some other static vs shared logic. But it's subjective matter, so
I won't insist.
> to libbpf. After this patch, the libbpf.a has
> -bash-4.4$ readelf -s libbpf.a | grep xsk_umem__create
> 372: 0000000000017145 1190 FUNC GLOBAL DEFAULT 1 xsk_umem__create_v0_0_4
> 405: 0000000000017145 1190 FUNC WEAK DEFAULT 1 xsk_umem__create
> 499: 00000000000175eb 103 FUNC GLOBAL DEFAULT 1 xsk_umem__create_v0_0_2
> -bash-4.4$
> No versioned symbols for xsk_umem__create.
> The libbpf.a can be used to build a shared library succesfully.
> -bash-4.4$ cat t.c
> extern void *xsk_umem__create;
> void * test() { return xsk_umem__create; }
> -bash-4.4$ gcc -c -fPIC t.c
> -bash-4.4$ gcc -shared t.o libbpf.a -o t.so
> -bash-4.4$
>
> Fixes: 10d30e301732 ("libbpf: add flags to umem config")
> Cc: Kevin Laatz <kevin.laatz@...el.com>
> Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
> Cc: Andrii Nakryiko <andriin@...com>
> Signed-off-by: Yonghong Song <yhs@...com>
> ---
I only have few minor nits, otherwise this looks good, thanks!
Acked-by: Andrii Nakryiko <andriin@...com>
> tools/lib/bpf/Makefile | 27 ++++++++++++++++++---------
> tools/lib/bpf/libbpf_internal.h | 16 ++++++++++++++++
> tools/lib/bpf/xsk.c | 4 ++--
> 3 files changed, 36 insertions(+), 11 deletions(-)
>
> diff --git a/tools/lib/bpf/Makefile b/tools/lib/bpf/Makefile
> index c6f94cffe06e..9533e185d9b6 100644
> --- a/tools/lib/bpf/Makefile
> +++ b/tools/lib/bpf/Makefile
> @@ -110,6 +110,9 @@ override CFLAGS += $(INCLUDES)
> override CFLAGS += -fvisibility=hidden
> override CFLAGS += -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64
>
> +# flags specific for shared library
> +SHLIB_FLAGS = -DSYMBOL_VERSIONING
Nit: use :=, there is no need for expansions at later point.
> +
> ifeq ($(VERBOSE),1)
> Q =
> else
> @@ -126,14 +129,17 @@ all:
> export srctree OUTPUT CC LD CFLAGS V
> include $(srctree)/tools/build/Makefile.include
>
> -BPF_IN := $(OUTPUT)libbpf-in.o
> +SHARED_OBJDIR := $(OUTPUT)sharedobjs/
> +STATIC_OBJDIR := $(OUTPUT)staticobjs/
> +BPF_IN_SHARED := $(SHARED_OBJDIR)libbpf-in.o
> +BPF_IN_STATIC := $(STATIC_OBJDIR)libbpf-in.o
> VERSION_SCRIPT := libbpf.map
>
> LIB_TARGET := $(addprefix $(OUTPUT),$(LIB_TARGET))
> LIB_FILE := $(addprefix $(OUTPUT),$(LIB_FILE))
> PC_FILE := $(addprefix $(OUTPUT),$(PC_FILE))
>
> -GLOBAL_SYM_COUNT = $(shell readelf -s --wide $(BPF_IN) | \
> +GLOBAL_SYM_COUNT = $(shell readelf -s --wide $(BPF_IN_SHARED) | \
> cut -d "@" -f1 | sed 's/_v[0-9]_[0-9]_[0-9].*//' | \
> awk '/GLOBAL/ && /DEFAULT/ && !/UND/ {print $$8}' | \
> sort -u | wc -l)
> @@ -155,7 +161,7 @@ all: fixdep
>
> all_cmd: $(CMD_TARGETS) check
>
> -$(BPF_IN): force elfdep bpfdep
> +$(BPF_IN_SHARED): force elfdep bpfdep
> @(test -f ../../include/uapi/linux/bpf.h -a -f ../../../include/uapi/linux/bpf.h && ( \
> (diff -B ../../include/uapi/linux/bpf.h ../../../include/uapi/linux/bpf.h >/dev/null) || \
> echo "Warning: Kernel ABI header at 'tools/include/uapi/linux/bpf.h' differs from latest version at 'include/uapi/linux/bpf.h'" >&2 )) || true
> @@ -171,17 +177,20 @@ $(BPF_IN): force elfdep bpfdep
> @(test -f ../../include/uapi/linux/if_xdp.h -a -f ../../../include/uapi/linux/if_xdp.h && ( \
> (diff -B ../../include/uapi/linux/if_xdp.h ../../../include/uapi/linux/if_xdp.h >/dev/null) || \
> echo "Warning: Kernel ABI header at 'tools/include/uapi/linux/if_xdp.h' differs from latest version at 'include/uapi/linux/if_xdp.h'" >&2 )) || true
> - $(Q)$(MAKE) $(build)=libbpf
> + $(Q)$(MAKE) $(build)=libbpf OUTPUT=$(SHARED_OBJDIR) CFLAGS="$(CFLAGS) $(SHLIB_FLAGS)"
> +
> +$(BPF_IN_STATIC): force elfdep bpfdep
> + $(Q)$(MAKE) $(build)=libbpf OUTPUT=$(STATIC_OBJDIR)
>
> $(OUTPUT)libbpf.so: $(OUTPUT)libbpf.so.$(LIBBPF_VERSION)
>
> -$(OUTPUT)libbpf.so.$(LIBBPF_VERSION): $(BPF_IN)
> +$(OUTPUT)libbpf.so.$(LIBBPF_VERSION): $(BPF_IN_SHARED)
> $(QUIET_LINK)$(CC) --shared -Wl,-soname,libbpf.so.$(LIBBPF_MAJOR_VERSION) \
> -Wl,--version-script=$(VERSION_SCRIPT) $^ -lelf -o $@
> @ln -sf $(@F) $(OUTPUT)libbpf.so
> @ln -sf $(@F) $(OUTPUT)libbpf.so.$(LIBBPF_MAJOR_VERSION)
>
> -$(OUTPUT)libbpf.a: $(BPF_IN)
> +$(OUTPUT)libbpf.a: $(BPF_IN_STATIC)
> $(QUIET_LINK)$(RM) $@; $(AR) rcs $@ $^
>
> $(OUTPUT)test_libbpf: test_libbpf.cpp $(OUTPUT)libbpf.a
> @@ -197,7 +206,7 @@ check: check_abi
>
> check_abi: $(OUTPUT)libbpf.so
> @if [ "$(GLOBAL_SYM_COUNT)" != "$(VERSIONED_SYM_COUNT)" ]; then \
> - echo "Warning: Num of global symbols in $(BPF_IN)" \
> + echo "Warning: Num of global symbols in $(BPF_IN_SHARED)" \
> "($(GLOBAL_SYM_COUNT)) does NOT match with num of" \
> "versioned symbols in $^ ($(VERSIONED_SYM_COUNT))." \
> "Please make sure all LIBBPF_API symbols are" \
> @@ -255,9 +264,9 @@ config-clean:
> $(Q)$(MAKE) -C $(srctree)/tools/build/feature/ clean >/dev/null
>
> clean:
> - $(call QUIET_CLEAN, libbpf) $(RM) $(TARGETS) $(CXX_TEST_TARGET) \
> + $(call QUIET_CLEAN, libbpf) $(RM) -rf $(TARGETS) $(CXX_TEST_TARGET) \
> *.o *~ *.a *.so *.so.$(LIBBPF_MAJOR_VERSION) .*.d .*.cmd \
> - *.pc LIBBPF-CFLAGS
> + *.pc LIBBPF-CFLAGS $(SHARED_OBJDIR) $(STATIC_OBJDIR)
> $(call QUIET_CLEAN, core-gen) $(RM) $(OUTPUT)FEATURE-DUMP.libbpf
>
>
> diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
> index 2e83a34f8c79..40a6d376de9a 100644
> --- a/tools/lib/bpf/libbpf_internal.h
> +++ b/tools/lib/bpf/libbpf_internal.h
> @@ -34,6 +34,22 @@
> (offsetof(TYPE, FIELD) + sizeof(((TYPE *)0)->FIELD))
> #endif
>
> +/* Symbol versoining is different between static and shared library.
typo: versioning
> + * Properly versioned symbols are needed for shared library, but
> + * only the symbol of the new version is needed.
> + */
> +#ifdef SYMBOL_VERSIONING
I like that those projects that are building libbpf from sources as
submodule won't have to define anything to get correctly linked static
library!
> +# define OLD_VERSION(internal_name, api_name, version) \
> + asm(".symver " #internal_name "," #api_name "@" #version);
> +# define NEW_VERSION(internal_name, api_name, version) \
Again, subjective, but CUR_VERSION or DEFAULT_VERSION name seems more
precise to me.
> + asm(".symver " #internal_name "," #api_name "@@" #version);
> +#else
> +# define OLD_VERSION(internal_name, api_name, version)
> +# define NEW_VERSION(internal_name, api_name, version) \
> + extern typeof(internal_name) api_name \
> + __attribute__ ((weak, alias (#internal_name)));
nit: is extra space after alias necessary?
> +#endif
> +
> extern void libbpf_print(enum libbpf_print_level level,
> const char *format, ...)
> __attribute__((format(printf, 2, 3)));
> diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
> index 24fa313524fb..6b983a4b7664 100644
> --- a/tools/lib/bpf/xsk.c
> +++ b/tools/lib/bpf/xsk.c
> @@ -261,8 +261,8 @@ int xsk_umem__create_v0_0_2(struct xsk_umem **umem_ptr, void *umem_area,
> return xsk_umem__create_v0_0_4(umem_ptr, umem_area, size, fill, comp,
> &config);
> }
> -asm(".symver xsk_umem__create_v0_0_2, xsk_umem__create@...BPF_0.0.2");
> -asm(".symver xsk_umem__create_v0_0_4, xsk_umem__create@@LIBBPF_0.0.4");
> +OLD_VERSION(xsk_umem__create_v0_0_2, xsk_umem__create, LIBBPF_0.0.2)
> +NEW_VERSION(xsk_umem__create_v0_0_4, xsk_umem__create, LIBBPF_0.0.4)
>
> static int xsk_load_xdp_prog(struct xsk_socket *xsk)
> {
> --
> 2.17.1
>
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