[PATCH 2/2] riscv: Restrict DWARF5 when building with LLVM to known working versions

[Fangrui Song]

On Wed, Nov 29, 2023 at 3:30 PM Nathan Chancellor <nathan at kernel.org> wrote:
>
> LLVM prior to 18.0.0 would generate incorrect debug info for DWARF5 due
> to linker relaxation, which was worked around in clang by defaulting
> RISC-V to DWARF4 [1]. Unfortunately, this workaround does not work for
> the kernel because the DWARF version can be independently changed from
> the default in Kconfig.
>
> Do not allow DWARF5 to be selected for RISC-V when using linker
> relaxation (ld.lld >= 15.0.0) and a version of LLVM that does not have
> the fixes (the integrated assembler [2] and ld.lld [3] < 18.0.0)
> necessary to generate the correct debug info.
>
> Link: https://github.com/llvm/llvm-project/commit/bbc0f99f3bc96f1db16f649fc21dd18e5b0918f6 [1]
> Link: https://github.com/llvm/llvm-project/commit/1df5ea29b43690b6622db2cad7b745607ca4de6a [2]
> Link: https://github.com/llvm/llvm-project/commit/7ffabb61a5569444b5ac9322e22e5471cc5e4a77 [3]
> Signed-off-by: Nathan Chancellor <nathan at kernel.org>
> ---
>  arch/riscv/Kconfig | 9 +++++++++
>  lib/Kconfig.debug  | 1 +
>  2 files changed, 10 insertions(+)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 72be1d8122a3..81b473cb47b0 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -186,6 +186,15 @@ config RISCV_USE_LINKER_RELAXATION
>         # https://github.com/llvm/llvm-project/commit/6611d58f5bbcbec77262d392e2923e1d680f6985
>         depends on !LD_IS_LLD || LLD_VERSION >= 150000
>
> +# https://github.com/llvm/llvm-project/commit/bbc0f99f3bc96f1db16f649fc21dd18e5b0918f6
> +config ARCH_HAS_BROKEN_DWARF5
> +       def_bool y
> +       depends on RISCV_USE_LINKER_RELAXATION
> +       # https://github.com/llvm/llvm-project/commit/1df5ea29b43690b6622db2cad7b745607ca4de6a
> +       depends on AS_IS_LLVM && AS_VERSION < 180000
> +       # https://github.com/llvm/llvm-project/commit/7ffabb61a5569444b5ac9322e22e5471cc5e4a77
> +       depends on LD_IS_LLD && LLD_VERSION < 180000
> +
>  config ARCH_MMAP_RND_BITS_MIN
>         default 18 if 64BIT
>         default 8
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index cc7d53d9dc01..a0ebce05a368 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -282,6 +282,7 @@ config DEBUG_INFO_DWARF4
>  config DEBUG_INFO_DWARF5
>         bool "Generate DWARF Version 5 debuginfo"
>         select DEBUG_INFO
> +       depends on !ARCH_HAS_BROKEN_DWARF5
>         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
>
> --
> 2.43.0
>

A comment and "depends on" condition introduced by commit
0a6de78cff600cb991f2a1b7ed376935871796a0 "lib/Kconfig.debug: Add check
for non-constant .{s,u}leb128 support to DWARF5"
can now be refined.

> ib/Kconfig.debug
> (AS_IS_GNU && AS_VERSION >= 23502 && AS_HAS_NON_CONST_LEB128)

AS_VERSION can now be 24100.

Thanks for the comment. I just marked
https://sourceware.org/bugzilla/show_bug.cgi?id=27215 (" as: Error:
non-constant .uleb128 is not supported on riscv64") as resolved :)

> +# 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

This comment can be updated:)


-- 
宋方睿