Re: [PATCH dwarves v2] dwarf_loader: Fix skipped encoding of function BTF on 32-bit systems

["Alexis Lothoré" <alexis.lothore@bootlin.com>]

Hello,

On Fri May 2, 2025 at 9:03 AM CEST, Tony Ambardar wrote:
> I encountered an issue building BTF kernels for 32-bit armhf, where many
> functions are missing in BTF data:

[...]

> Fixes: a53c58158b76 ("dwarf_loader: Mark functions that do not use expected registers for params")
> Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>

I encountered some issues with pahole 1.30 when trying to generate BTF data
for functions having some __int128 values ([1]), and have been redirected
here by Tony. I gave a try to the patch below and confirm that it fixes my
issue: BTF data is now properly generated for my target function, so:

Tested-by: Alexis Lothoré <alexis.lothore@bootlin.com>

While at it, to follow-up on Alan's request for more testing, I did the
following:
- build kernel and bpf selftests with pahole 1.30, extract BTF raw data
  with bpftool
- repeat with pahole 1.30 + Tony's patch
- I build my kernel for arm64, it is based on bpf-next_base and I use a
  defconfig very close to the one used in BPF CI (so based on
  tools/testing/selftests/bpf/config*)

I observe the following when comparing the resulting BTF data with/without
Tony's patch:
- There is no difference on vmlinux BTF data
- For bpf_testmod.ko, there is a slight shift in the first BTF ID (first ID
  is 46 with pristine pahole, 47 with patched pahole), which in turns makes
  a lot of noise in the diff, but the actual diff seems to be about two new
  BTF entries related to my custom function now being properly detected
  (BTF_KIND_FUNC and BTF_KIND_FUNC_PROTO)

Alexis

[1] https://lore.kernel.org/bpf/D9Q73OTLEOU4.LNAO9K4POETM@bootlin.com/

-- 
Alexis Lothoré, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com