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libbpf: handle symbol versioning properly for libbpf.a
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@LIBBPF_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@LIBBPF_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@LIBBPF_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@GLIBC_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@GLIBC_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 SHARED as the macro name. 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 GLOBAL 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@intel.com> Cc: Arnaldo Carvalho de Melo <acme@redhat.com> Cc: Andrii Nakryiko <andriin@fb.com> Acked-by: Andrii Nakryiko <andriin@fb.com> Signed-off-by: Yonghong Song <yhs@fb.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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@ -34,6 +34,22 @@
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(offsetof(TYPE, FIELD) + sizeof(((TYPE *)0)->FIELD))
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#endif
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/* Symbol versioning is different between static and shared library.
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* Properly versioned symbols are needed for shared library, but
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* only the symbol of the new version is needed for static library.
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*/
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#ifdef SHARED
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# define COMPAT_VERSION(internal_name, api_name, version) \
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asm(".symver " #internal_name "," #api_name "@" #version);
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# define DEFAULT_VERSION(internal_name, api_name, version) \
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asm(".symver " #internal_name "," #api_name "@@" #version);
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#else
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# define COMPAT_VERSION(internal_name, api_name, version)
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# define DEFAULT_VERSION(internal_name, api_name, version) \
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extern typeof(internal_name) api_name \
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__attribute__((alias(#internal_name)));
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#endif
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extern void libbpf_print(enum libbpf_print_level level,
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const char *format, ...)
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__attribute__((format(printf, 2, 3)));
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@ -261,8 +261,8 @@ int xsk_umem__create_v0_0_2(struct xsk_umem **umem_ptr, void *umem_area,
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return xsk_umem__create_v0_0_4(umem_ptr, umem_area, size, fill, comp,
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&config);
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}
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asm(".symver xsk_umem__create_v0_0_2, xsk_umem__create@LIBBPF_0.0.2");
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asm(".symver xsk_umem__create_v0_0_4, xsk_umem__create@@LIBBPF_0.0.4");
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COMPAT_VERSION(xsk_umem__create_v0_0_2, xsk_umem__create, LIBBPF_0.0.2)
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DEFAULT_VERSION(xsk_umem__create_v0_0_4, xsk_umem__create, LIBBPF_0.0.4)
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static int xsk_load_xdp_prog(struct xsk_socket *xsk)
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{
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