Re: [PATCH/RFC] Re: 'perf test BPF' failing, libbpf regression wrt "basic API for BPF obj name"

[Martin KaFai Lau]

On Thu, Nov 30, 2017 at 01:53:58PM -0300, Arnaldo Carvalho de Melo wrote:
> Em Thu, Nov 30, 2017 at 12:01:10AM -0300, Arnaldo Carvalho de Melo escreveu:
> > Em Wed, Nov 29, 2017 at 02:31:36PM -0800, Martin KaFai Lau escreveu:
> > > On Wed, Nov 29, 2017 at 06:15:43PM -0300, Arnaldo Carvalho de Melo wrote:
> > > > Em Wed, Nov 29, 2017 at 01:07:34PM -0800, Martin KaFai Lau escreveu:
> > > > > On Tue, Nov 28, 2017 at 04:05:19PM -0300, Arnaldo Carvalho de Melo wrote:
> > > > > > [root@jouet ~]# perf test -v bpf
> > > > > > 39: BPF filter                                            :
> > > > > > 39.1: Basic BPF filtering                                 :
> > > > > > Kernel build dir is set to /lib/modules/4.14.0+/build
> > > > > [ ... ]
> > > > > > libbpf: failed to create map (name: 'flip_table'): Invalid argument
> > > > > > libbpf: failed to load object '[basic_bpf_test]'
> > > > > > bpf: load objects failed
> > > > > 88cda1c9da02 ("bpf: libbpf: Provide basic API support to specify BPF obj name")
> > > > > is introduced in 4.15.
> > 
> > > > > I think the perf@xxxxxxxxxxx broke on older kernels like 4.14 because
> > > > > the new bpf prog/map name is only introduced since 4.15.
> > 
> > > > > The newer perf needs to be compatible with an older kernel?
> > 
> > > > Sure :-)
>  
> > > Would the latest features introduced in perf/libbpf supposed to be
> > > available in the latest kernel only?  What may be the reason that the
> > 
> > Yes, then the new perf binary should try to use the new stuff, if it
> > fails, use the old one, there is no requirement that one uses perf 4.14
> > in lockstep with the kernel 4.14 (or any other version), perf 4.15
> > should work with the 4.14 kernel as well as with 4.15 (or any other
> > future kernel), only limited by what it can grok up to when it was
> > released.
> 
> So, see the patch below, that makes a 'perf test bpf' and my other test
> cases, including that one for probe_read_str() work again, it just
> fallbacks to a behaviour the older kernels can accept.
Thanks for the patch.

> 
> We can improve it so that that EINVAL fallback happens only for
> MAP_CREATE, and probably we don't need to change the size arg, just zero
> the unused fields, but I haven't checked that.
> 
> - Arnaldo
> 
> diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
> index 5128677e4117..3084f07c7c33 100644
> --- a/tools/lib/bpf/bpf.c
> +++ b/tools/lib/bpf/bpf.c
> @@ -19,6 +19,7 @@
>   * License along with this program; if not,  see <http://www.gnu.org/licenses>
>   */
>  
> +#include <errno.h>
>  #include <stdlib.h>
>  #include <memory.h>
>  #include <unistd.h>
> @@ -53,10 +54,26 @@ static inline __u64 ptr_to_u64(const void *ptr)
>  	return (__u64) (unsigned long) ptr;
>  }
>  
> -static inline int sys_bpf(enum bpf_cmd cmd, union bpf_attr *attr,
> -			  unsigned int size)
> +static int sys_bpf(enum bpf_cmd cmd, union bpf_attr *attr, unsigned int size)
>  {
> -	return syscall(__NR_bpf, cmd, attr, size);
> +	int err = syscall(__NR_bpf, cmd, attr, size);
> +	if (err == -1 && (errno == EINVAL || errno == E2BIG)) {
I would add a check to the length of map_name/prog_name.

> +		const unsigned int old_union_size = offsetof(union bpf_attr, prog_name);
> +		/*
> +		 * These were the ones that added fields after the old bpf_attr
> +		 * layout in commit 88cda1c9da02 ("bpf: libbpf: Provide basic
> +		 * API support to specify BPF obj name") so zero that out to
> +		 * pass the CHECK_ATTR() test in kernel/bpf/syscall.c in older
> +		 * kernels.
> +		 */
> +		if (cmd == BPF_MAP_CREATE)
> +			memset(&attr->map_name, 0, size - offsetof(union bpf_attr, map_name));
> +		else
> +			memset(&attr->prog_name, 0, size - old_union_size);
If bpf_attr is extended in the future,  map_name/prog_name will still be
used as the anchor for backward compatibility instead of trial and error
attribute by attribute?

Instead of sinking all future bpf_attr's backward compatibility
requirements to sys_bpf,  I would push it up to its own BPF_* command
helper which has a better sense of its bpf_attr, i.e. push it up
to bpf_create_map_node() and bpf_load_program_name() in this case.

> +
> +		err = syscall(__NR_bpf, cmd, attr, old_union_size);
> +	}
> +	return err;
>  }
>  
>  int bpf_create_map_node(enum bpf_map_type map_type, const char *name,