Re: [PATCH bpf-next] libbpf: Add libbpf_version() function to get library version at runtime

[David Ahern <dsahern@gmail.com>]

On 11/18/20 10:07 AM, Toke Høiland-Jørgensen wrote:
> As a response to patches adding libbpf support to iproute2, an extensive
> discussion ensued about libbpf version visibility and enforcement in tools
> using the library[0]. In particular, two problems came to light:
> 
> 1. If a tool is statically linked against libbpf, there is no way for a user
>    to discover which version of libbpf the tool is using, unless the tool
>    takes particular care to embed the library version at build time and print
>    it.
> 
> 2. If a tool is dynamically linked against libbpf, but doesn't use any
>    symbols from the latest library version, the library version used at
>    runtime can be older than the one used at compile time, and the
>    application has no way to verify the version at runtime.
> 
> To make progress on resolving this, let's add a libbpf_version() function that
> will simply return a version string which is embedded into the library at
> compile time. This makes it possible for applications to unambiguously get the
> library version at runtime, resolving (2.) above, and as an added bonus makes it
> easy for applications to print the library version, which should help with (1.).
> 
> [0] https://lore.kernel.org/bpf/20201109070802.3638167-1-haliu@redhat.com/T/#t
> 
> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
> ---
>  tools/lib/bpf/Makefile   |  1 +
>  tools/lib/bpf/libbpf.c   | 12 ++++++++++++
>  tools/lib/bpf/libbpf.h   |  1 +
>  tools/lib/bpf/libbpf.map |  1 +
>  4 files changed, 15 insertions(+)
> 
> diff --git a/tools/lib/bpf/Makefile b/tools/lib/bpf/Makefile
> index 5f9abed3e226..c9999e09a0c8 100644
> --- a/tools/lib/bpf/Makefile
> +++ b/tools/lib/bpf/Makefile
> @@ -107,6 +107,7 @@ override CFLAGS += -Werror -Wall
>  override CFLAGS += $(INCLUDES)
>  override CFLAGS += -fvisibility=hidden
>  override CFLAGS += -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64
> +override CFLAGS += -DLIBBPF_VERSION="$(LIBBPF_VERSION)"
>  
>  # flags specific for shared library
>  SHLIB_FLAGS := -DSHARED -fPIC
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 313034117070..dc7bb3001fa6 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -136,6 +136,18 @@ static void pr_perm_msg(int err)
>  
>  #define STRERR_BUFSIZE  128
>  
> +#ifndef LIBBPF_VERSION
> +#define LIBBPF_VERSION unset
> +#endif
> +#define __str(s) #s
> +#define _str(s) __str(s)
> +static const char *_libbpf_version = _str(LIBBPF_VERSION);
> +
> +const char *libbpf_version(void)
> +{
> +	return _libbpf_version;
> +}
> +
>  /* Copied from tools/perf/util/util.h */
>  #ifndef zfree
>  # define zfree(ptr) ({ free(*ptr); *ptr = NULL; })
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index 6909ee81113a..d8256bc1e02e 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -45,6 +45,7 @@ enum libbpf_errno {
>  };
>  
>  LIBBPF_API int libbpf_strerror(int err, char *buf, size_t size);
> +LIBBPF_API const char *libbpf_version(void);
>  
>  enum libbpf_print_level {
>          LIBBPF_WARN,
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index 29ff4807b909..5f931bf1b5b0 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -345,4 +345,5 @@ LIBBPF_0.3.0 {
>  		btf__parse_split;
>  		btf__new_empty_split;
>  		btf__new_split;
> +                libbpf_version;
>  } LIBBPF_0.2.0;
> 

a good export for libraries in general to track not just the compiled
against version, but the run time version. It would be good to have the
option to add the git hash of the top commit to LIBBPF_VERSION as well
to make it easier to track dev builds.