Re: [PATCH v2] lib/Kconfig.debug: Add check for non-constant .{s,u}leb128 support to DWARF5

[Nick Desaulniers]

On Fri, Oct 14, 2022 at 1:48 PM Nathan Chancellor <nathan@xxxxxxxxxx> 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
> Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>

Thanks for keeping these LLVM_IAS=0 builds alive a little longer.  My
hope is the GNU binutils can relax their requirement for debug info
sections to improve support for DWARF v5.

Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>


> ---
>
> v2:
>     - Rebase on commit bb1435f3f575 ("Kconfig.debug: add toolchain
>       checks for DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT") from the kbuild
>       tree.
>     - Limit CONFIG_AS_HAS_NON_CONST_LEB128 dependency to GNU as. There
>       is no point to applying this dependency to the integrated
>       assembler because it will always pass.
>     - Apply the CONFIG_AS_HAS_NON_CONST_LEB128 dependency to
>       CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT as well, due to the
>       aforementioned kbuild change.
>     - Move comment block to above CONFIG_AS_HAS_NON_CONST_LEB128, as the
>       configuration is now used in two places.
>     - Drop Conor's tested by, as the patch is different enough to
>       potentially require new testing.
>
> v1: https://lore.kernel.org/20220928182523.3105953-1-nathan@xxxxxxxxxx/
>
>  lib/Kconfig.debug | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index db8d9271cabf..5c1c63575895 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -231,6 +231,11 @@ config DEBUG_INFO
>           in the "Debug information" choice below, indicating that debug
>           information will be generated for build targets.
>
> +# 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
> +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
> @@ -253,7 +258,7 @@ config DEBUG_INFO_NONE
>  config DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
>         bool "Rely on the toolchain's implicit default DWARF version"
>         select DEBUG_INFO
> -       depends on !CC_IS_CLANG || AS_IS_LLVM || CLANG_VERSION < 140000 || (AS_IS_GNU && AS_VERSION >= 23502)
> +       depends on !CC_IS_CLANG || AS_IS_LLVM || CLANG_VERSION < 140000 || (AS_IS_GNU && AS_VERSION >= 23502 && AS_HAS_NON_CONST_LEB128)
>         help
>           The implicit default version of DWARF debug info produced by a
>           toolchain changes over time.
> @@ -277,7 +282,7 @@ config DEBUG_INFO_DWARF4
>  config DEBUG_INFO_DWARF5
>         bool "Generate DWARF Version 5 debuginfo"
>         select DEBUG_INFO
> -       depends on !CC_IS_CLANG || AS_IS_LLVM || (AS_IS_GNU && AS_VERSION >= 23502)
> +       depends on !CC_IS_CLANG || AS_IS_LLVM || (AS_IS_GNU && AS_VERSION >= 23502 && AS_HAS_NON_CONST_LEB128)
>         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: bb1435f3f575b5213eaf27434efa3971f51c01de
> --
> 2.38.0
>


--
Thanks,
~Nick Desaulniers