On Tue, 2021-03-09 at 13:37 -0800, Andrii Nakryiko wrote:
> 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.
BTF version seems to exist: btf_header.version. Should we maybe bump
this?
>
> 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.
I thought about just adding --btf-allow-floats, but if new features
will be added in the future, the list of options will become unwieldy.
So I thought it would be good to settle for something that increases
monotonically.
