On Tue, Mar 9, 2021 at 3:48 AM Arnaldo Carvalho de Melo <acme@xxxxxxxxxx> wrote:
>
> Em Tue, Mar 09, 2021 at 12:59:13AM +0100, Ilya Leoshkevich escreveu:
> > Some BPF programs compiled on s390 fail to load, because s390
> > arch-specific linux headers contain float and double types.
> >
> > Fix as follows:
> >
> > - Make the DWARF loader fill base_type.float_type.
> >
> > - Introduce libbpf compatibility level command-line parameter, so that
> > pahole could be used to build both the older and the newer kernels.
> >
> > - libbpf introduced the support for the floating-point types in commit
> > 986962fade5, so update the libbpf submodule to that version and use
> > the new btf__add_float() function in order to emit the floating-point
> > types when not in the compatibility mode and base_type.float_type is
> > set.
> >
> > - Make the BTF loader recognize the new BTF kind.
> >
> > Example of the resulting entry in the vmlinux BTF:
> >
> > [7164] FLOAT 'double' size=8
> >
> > when building with:
> >
> > LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1} --libbpf_compat=0.4.0
>
> I'm testing it now, and added as a followup patch the man page entry,
> please check that the wording is appropriate.
>
> Thanks,
>
> - Arnaldo
>
> [acme@five pahole]$ vim man-pages/pahole.1
> [acme@five pahole]$ git diff
> diff --git a/man-pages/pahole.1 b/man-pages/pahole.1
> index 352bb5e45f319da4..787771753d1933b1 100644
> --- a/man-pages/pahole.1
> +++ b/man-pages/pahole.1
> @@ -199,6 +199,12 @@ Path to the base BTF file, for instance: vmlinux when encoding kernel module BTF
> This may be inferred when asking for a /sys/kernel/btf/MODULE, when it will be autoconfigured
> to "/sys/kernel/btf/vmlinux".
>
> +.TP
> +.B \-\-libbpf_compat=LIBBPF_VERSION
> +Produce output compatible with this libbpf version. For instance, specifying 0.4.0 as
> +the version would encode BTF_KIND_FLOAT entries in systems where the vmlinux DWARF
> +information has float types.
TBH, I think it's not exactly right to call out libbpf version here.
It's BTF "version" (if we had such a thing) that determines the set of
supported BTF kinds. There could be other libraries that might want to
parse BTF. So I don't know what this should be called, but
libbpf_compat is probably a wrong name for it.
If we do want to teach pahole to not emit some parts of BTF, it should
probably be a set of BPF features, not some arbitrary library
versions.
> +
> .TP
> .B \-l, \-\-show_first_biggest_size_base_type_member
> Show first biggest size base_type member.
> [acme@five pahole]$
