Re: [PATCH dwarves 0/1] LLD .BTF section patch

[Bill Wendling <morbo@google.com>]

On Mon, Nov 21, 2022 at 4:00 PM Bill Wendling <morbo@google.com> wrote:
>
> This patch is an attempted fix for the issue discussed in [1]. I believe it's
> actually caused by pahole generating a corrupted ELF file.
>
> LLD generates a zero-sized .BTF section, while BFD doesn't. When pahole goes to
> add the .BTF data to an LLD-generated file, the section exists, but is too
> small. The ELF calls don't appear to adjust following sections' addresses in
> this case.
>
> To avoid this, the patch does some magic with objcopy to generate a separate
> .BTF section and perform some renaming.
>
> NOTE: This patch resembles a hack. If it's possible to adjust a section's size
> with ELF calls we should do it that way.
>
For instance, this could be done in the kernel like the following. I
don't know which is the more acceptable alternative.

diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index b808ef0..e6bfe51 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -212,6 +212,24 @@

  info "BTF" ${2}
  vmlinux_link ${1}
+ if ${LD_vmlinux} --version | grep -q LLD ; then
+ # LLD will generate a zero-sized .BTF section (the BFD linker
+ # doesn't). In that case, pahole tries to use the zero-sized
+ # section to house the BTF information. However, pahole doesn't
+ # adjust the addresses of sections around the .BTF section,
+ # resulting in an ELF file that appears invalid to further
+ # linkers. Avoid this issue by renaming the .BTF to .BTF_old
+ # and letting pahole add a new .BTF section.
+ # TODO(b/182271653): If/when pahole is fixed, we should remove
+ # this code.
+ BTF_SECTION=$(${READELF} -SW ${1} | grep -v .BTF_ids | grep .BTF)
+ if [[ -n "${BTF_SECTION}" ]]; then
+ BTF_SIZE=$(echo ${BTF_SECTION} | awk '{print $6}')
+ if [[ "0x${BTF_SIZE}" -eq 0 ]]; then
+ ${OBJCOPY} --rename-section .BTF=.BTF.old ${1}
+ fi
+ fi
+ fi
  LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}

  # Create ${2} which contains just .BTF section but no symbols. Add

> [1]: https://lore.kernel.org/dwarves/20210317232657.mdnsuoqx6nbddjgt@google.com/
>
> Bill Wendling (1):
>   btf_encoder: Generate a new .BTF section even if one exists
>
>  btf_encoder.c | 88 +++++++++++++++++++++++++++++++--------------------
>  1 file changed, 54 insertions(+), 34 deletions(-)
>
> --
> 2.38.1.584.g0f3c55d4c2-goog
>