Re: [PATCH v4 dwarves] btf: Add support for the floating-point types

[Ilya Leoshkevich <iii@linux.ibm.com>]

On Wed, 2021-03-10 at 12:25 -0800, Andrii Nakryiko wrote:
> On Wed, Mar 10, 2021 at 12:16 PM Ilya Leoshkevich <iii@linux.ibm.com>
> wrote:
> > 
> > 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 the --btf_gen_floats 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.
> > 
> > - 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} --btf_gen_floats
> > 
> > Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> > ---
> 
> So it looks good to me overall, but here's the question about using
> this --btf-gen-floats flag from link-vmlinux.sh script. If you
> specify
> that flag for an old pahole, it will probably error out, right? So
> that means we'll need to do feature detection for pahole supported
> features, right?..

I was planning to just bump the version in this check:

    if [ "${pahole_ver}" -lt "116" ]; then

But we could also keep allowing 1.16-1.20 and pass the new flag on
1.21+ only.

What do you think?