Re: [PATCH] ARC: Strip libgcc debug information when generating BTF

[Alan Maguire]

On 10/15/26 4:40 PM, Sasha Levin wrote:
> ARC links against the compiler's libgcc archive. The ARC GNU 2023.03
> toolchain ships this library with debug information, including complex
> floating-point types in integer arithmetic helpers such as __udivdi3.
> These types reach vmlinux even though the kernel does not use complex
> floating-point arithmetic.
> 
> When floating-point BTF encoding is enabled, pahole rejects those types:
> 
>   Complex, interval and imaginary float types are not supported
>   Error while encoding BTF.
>   FAILED: load BTF from .tmp_vmlinux1.BTF.1: No such file or directory
> 
> KernelCI's upgrade to pahole 1.31 exposed this long-standing problem as
> failed ARC builds. As of 1.32, pahole still rejects these types, and
> older versions will stay in use, so the kernel build has to keep them
> out of vmlinux. Commit f122668ddcce ("ARC: Add eBPF JIT support")
> documented the same failure along with a workaround of disabling
> floating-point BTF encoding, but that also discards supported
> floating-point types from kernel BTF.
> 
> For CONFIG_DEBUG_INFO_BTF, generate a debug-stripped copy of the selected
> libgcc archive in the output tree and link that copy. Preserve the archive
> structure so that only needed helpers are linked, and use the normal
> Kbuild objcopy rule to track input and command changes. The trade-off is
> that vmlinux in BTF-enabled builds no longer carries DWARF for the libgcc
> helpers. Debug information for kernel code, the installed toolchain and
> non-BTF builds stay unchanged.
> 
> Reproduced and tested with ARC GCC 12.2.1 and pahole 1.31 and 1.32 using
> an ARCv2 tinyconfig with BPF_SYSCALL and DWARF4/BTF enabled. Before and
> after vmlinux.o files are identical after stripping debug information.
> 
> Fixes: db16c1fe92d7 ("bpf: Generate BTF_KIND_FLOAT when linking vmlinux")
> Link: https://github.com/kernelci/kernelci-core/commit/e71b8c9f42253180596e734c18965a5aad80c8c5
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

Acked-by: Alan Maguire (Oracle) <almagui@xxxxxxxxxx>

Thanks for the fix Sasha! More complete floating point representations for BTF
should definitely be on the to-do list..