Re: [PATCH dwarves v2] btf: Add support for the floating-point types

[Arnaldo Carvalho de Melo <acme@kernel.org>]

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.
+
 .TP
 .B \-l, \-\-show_first_biggest_size_base_type_member
 Show first biggest size base_type member.
[acme@five pahole]$