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1bd6352459
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:
|
||
---|---|---|
.. | ||
.gitignore | ||
bpf_prog_linfo.c | ||
bpf.c | ||
bpf.h | ||
btf_dump.c | ||
btf.c | ||
btf.h | ||
Build | ||
hashmap.c | ||
hashmap.h | ||
libbpf_errno.c | ||
libbpf_internal.h | ||
libbpf_probes.c | ||
libbpf_util.h | ||
libbpf.c | ||
libbpf.h | ||
libbpf.map | ||
libbpf.pc.template | ||
Makefile | ||
netlink.c | ||
nlattr.c | ||
nlattr.h | ||
README.rst | ||
str_error.c | ||
str_error.h | ||
test_libbpf.cpp | ||
xsk.c | ||
xsk.h |
.. SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) libbpf API naming convention ============================ libbpf API provides access to a few logically separated groups of functions and types. Every group has its own naming convention described here. It's recommended to follow these conventions whenever a new function or type is added to keep libbpf API clean and consistent. All types and functions provided by libbpf API should have one of the following prefixes: ``bpf_``, ``btf_``, ``libbpf_``, ``xsk_``, ``perf_buffer_``. System call wrappers -------------------- System call wrappers are simple wrappers for commands supported by sys_bpf system call. These wrappers should go to ``bpf.h`` header file and map one-on-one to corresponding commands. For example ``bpf_map_lookup_elem`` wraps ``BPF_MAP_LOOKUP_ELEM`` command of sys_bpf, ``bpf_prog_attach`` wraps ``BPF_PROG_ATTACH``, etc. Objects ------- Another class of types and functions provided by libbpf API is "objects" and functions to work with them. Objects are high-level abstractions such as BPF program or BPF map. They're represented by corresponding structures such as ``struct bpf_object``, ``struct bpf_program``, ``struct bpf_map``, etc. Structures are forward declared and access to their fields should be provided via corresponding getters and setters rather than directly. These objects are associated with corresponding parts of ELF object that contains compiled BPF programs. For example ``struct bpf_object`` represents ELF object itself created from an ELF file or from a buffer, ``struct bpf_program`` represents a program in ELF object and ``struct bpf_map`` is a map. Functions that work with an object have names built from object name, double underscore and part that describes function purpose. For example ``bpf_object__open`` consists of the name of corresponding object, ``bpf_object``, double underscore and ``open`` that defines the purpose of the function to open ELF file and create ``bpf_object`` from it. Another example: ``bpf_program__load`` is named for corresponding object, ``bpf_program``, that is separated from other part of the name by double underscore. All objects and corresponding functions other than BTF related should go to ``libbpf.h``. BTF types and functions should go to ``btf.h``. Auxiliary functions ------------------- Auxiliary functions and types that don't fit well in any of categories described above should have ``libbpf_`` prefix, e.g. ``libbpf_get_error`` or ``libbpf_prog_type_by_name``. AF_XDP functions ------------------- AF_XDP functions should have an ``xsk_`` prefix, e.g. ``xsk_umem__get_data`` or ``xsk_umem__create``. The interface consists of both low-level ring access functions and high-level configuration functions. These can be mixed and matched. Note that these functions are not reentrant for performance reasons. Please take a look at Documentation/networking/af_xdp.rst in the Linux kernel source tree on how to use XDP sockets and for some common mistakes in case you do not get any traffic up to user space. libbpf ABI ========== libbpf can be both linked statically or used as DSO. To avoid possible conflicts with other libraries an application is linked with, all non-static libbpf symbols should have one of the prefixes mentioned in API documentation above. See API naming convention to choose the right name for a new symbol. Symbol visibility ----------------- libbpf follow the model when all global symbols have visibility "hidden" by default and to make a symbol visible it has to be explicitly attributed with ``LIBBPF_API`` macro. For example: .. code-block:: c LIBBPF_API int bpf_prog_get_fd_by_id(__u32 id); This prevents from accidentally exporting a symbol, that is not supposed to be a part of ABI what, in turn, improves both libbpf developer- and user-experiences. ABI versionning --------------- To make future ABI extensions possible libbpf ABI is versioned. Versioning is implemented by ``libbpf.map`` version script that is passed to linker. Version name is ``LIBBPF_`` prefix + three-component numeric version, starting from ``0.0.1``. Every time ABI is being changed, e.g. because a new symbol is added or semantic of existing symbol is changed, ABI version should be bumped. This bump in ABI version is at most once per kernel development cycle. For example, if current state of ``libbpf.map`` is: .. code-block:: LIBBPF_0.0.1 { global: bpf_func_a; bpf_func_b; local: \*; }; , and a new symbol ``bpf_func_c`` is being introduced, then ``libbpf.map`` should be changed like this: .. code-block:: LIBBPF_0.0.1 { global: bpf_func_a; bpf_func_b; local: \*; }; LIBBPF_0.0.2 { global: bpf_func_c; } LIBBPF_0.0.1; , where new version ``LIBBPF_0.0.2`` depends on the previous ``LIBBPF_0.0.1``. Format of version script and ways to handle ABI changes, including incompatible ones, described in details in [1]. Stand-alone build ================= Under https://github.com/libbpf/libbpf there is a (semi-)automated mirror of the mainline's version of libbpf for a stand-alone build. However, all changes to libbpf's code base must be upstreamed through the mainline kernel tree. License ======= libbpf is dual-licensed under LGPL 2.1 and BSD 2-Clause. Links ===== [1] https://www.akkadia.org/drepper/dsohowto.pdf (Chapter 3. Maintaining APIs and ABIs).