Re: [PATCH v4 05/39] x86/asm: Avoid emitting DWARF CFI for non-VDSO

Jens Remus
2025-01-24T16:09:28Z

On 22.01.2025 03:30, Josh Poimboeuf wrote:
> It was decided years ago that .cfi_* annotations aren't maintainable in
> the kernel. They were replaced by objtool unwind hints. For the kernel
> proper, ensure the CFI_* macros don't do anything.
>
> On the other hand the VDSO library *does* use them, so user space can
> unwind through it.
>
> Make sure these macros only work for VDSO. They aren't actually being
> used outside of VDSO anyway, so there's no functional change.
>
> Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

> diff --git a/arch/x86/include/asm/dwarf2.h b/arch/x86/include/asm/dwarf2.h

> -#ifndef BUILD_VDSO
> - /*
> - * Emit CFI data in .debug_frame sections, not .eh_frame sections.
> - * The latter we currently just discard since we don't do DWARF
> - * unwinding at runtime. So only the offline DWARF information is
> - * useful to anyone. Note we should not use this directive if we
> - * ever decide to enable DWARF unwinding at runtime.
> - */
> - .cfi_sections .debug_frame
> -#else
> - /*
> - * For the vDSO, emit both runtime unwind information and debug
> - * symbols for the .dbg file.
> - */
> - .cfi_sections .eh_frame, .debug_frame
> -#endif
> +#else /* !BUILD_VDSO */
> +

Did you remove ".cfi_sections .debug_frame" on purpose from the
!BUILD_VDSO path compared to V3? Presumably to not only not emit
DWARF CFI from assembler, but any source?

> +/*
> + * On x86, these macros aren't used outside VDSO. As well they shouldn't be:
> + * they're fragile and very difficult to maintain.
> + */

Thanks and regards,
Jens
--
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@de.ibm.com

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