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SubjectRe: [PATCH] lib/Kconfig.debug: Add check for non-constant .{s,u}leb128 support to DWARF5
On Wed, Sep 28, 2022 at 11:25 AM Nathan Chancellor <nathan@kernel.org> wrote:
>
> When building with a RISC-V kernel with DWARF5 debug info using clang
> and the GNU assembler, several instances of the following error appear:
>
> /tmp/vgettimeofday-48aa35.s:2963: Error: non-constant .uleb128 is not supported
>
> Dumping the .s file reveals these .uleb128 directives come from
> .debug_loc and .debug_ranges:
>
> .Ldebug_loc0:
> .byte 4 # DW_LLE_offset_pair
> .uleb128 .Lfunc_begin0-.Lfunc_begin0 # starting offset
> .uleb128 .Ltmp1-.Lfunc_begin0 # ending offset
> .byte 1 # Loc expr size
> .byte 90 # DW_OP_reg10
> .byte 0 # DW_LLE_end_of_list
>
> .Ldebug_ranges0:
> .byte 4 # DW_RLE_offset_pair
> .uleb128 .Ltmp6-.Lfunc_begin0 # starting offset
> .uleb128 .Ltmp27-.Lfunc_begin0 # ending offset
> .byte 4 # DW_RLE_offset_pair
> .uleb128 .Ltmp28-.Lfunc_begin0 # starting offset
> .uleb128 .Ltmp30-.Lfunc_begin0 # ending offset
> .byte 0 # DW_RLE_end_of_list
>
> There is an outstanding binutils issue to support a non-constant operand
> to .sleb128 and .uleb128 in GAS for RISC-V but there does not appear to
> be any movement on it, due to concerns over how it would work with
> linker relaxation.
>
> To avoid these build errors, prevent DWARF5 from being selected when
> using clang and an assembler that does not have support for these symbol
> deltas, which can be easily checked in Kconfig with as-instr plus the
> small test program from the dwz test suite from the binutils issue.
>
> Link: https://sourceware.org/bugzilla/show_bug.cgi?id=27215
> Link: https://github.com/ClangBuiltLinux/linux/issues/1719
> Tested-by: Conor Dooley <conor.dooley@microchip.com>
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
> lib/Kconfig.debug | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index d3e5f36bb01e..19de03ead2ed 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -231,6 +231,9 @@ config DEBUG_INFO
> in the "Debug information" choice below, indicating that debug
> information will be generated for build targets.
>
> +config AS_HAS_NON_CONST_LEB128
> + def_bool $(as-instr,.uleb128 .Lexpr_end4 - .Lexpr_start3\n.Lexpr_start3:\n.Lexpr_end4:)
> +
> choice
> prompt "Debug information"
> depends on DEBUG_KERNEL
> @@ -277,6 +280,10 @@ config DEBUG_INFO_DWARF5
> bool "Generate DWARF Version 5 debuginfo"
> select DEBUG_INFO
> depends on !CC_IS_CLANG || (CC_IS_CLANG && (AS_IS_LLVM || (AS_IS_GNU && AS_VERSION >= 23502)))
> + # Clang is known to generate .{s,u}leb128 with symbol deltas with
> + # DWARF5, which some targets may not support.
> + # https://sourceware.org/bugzilla/show_bug.cgi?id=27215
> + depends on !CC_IS_CLANG || AS_HAS_NON_CONST_LEB128

Reraising my concern from
https://github.com/ClangBuiltLinux/linux/issues/1719#issuecomment-1258678969

We've put a fair amount of work into getting CC=clang LLVM_IAS=0 to
work for DWARF v5 (both on the GNU binutils side, and Kbuild), I'd
hate to see that effectively knee-capped because of an issue in GNU
binutils that is only relevant for one architecture.

I'd concede support for ARCH=riscv, but not for all other
architectures, which this effectively does.

> help
> Generate DWARF v5 debug info. Requires binutils 2.35.2, gcc 5.0+ (gcc
> 5.0+ accepts the -gdwarf-5 flag but only had partial support for some
>
> base-commit: f76349cf41451c5c42a99f18a9163377e4b364ff
> --
> 2.37.3
>
>


--
Thanks,
~Nick Desaulniers

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