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Date: Fri, 14 Oct 2022 14:51:51 -0700
From: Nick Desaulniers <ndesaulniers@...gle.com>
To: Nathan Chancellor <nathan@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Masahiro Yamada <masahiroy@...nel.org>,
Tom Rix <trix@...hat.com>, Palmer Dabbelt <palmer@...belt.com>,
Conor Dooley <conor@...nel.org>, linux-kbuild@...r.kernel.org,
linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
patches@...ts.linux.dev, llvm@...ts.linux.dev
Subject: Re: [PATCH v2] lib/Kconfig.debug: Add check for non-constant
.{s,u}leb128 support to DWARF5
On Fri, Oct 14, 2022 at 1:48 PM Nathan Chancellor <nathan@...nel.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
> Signed-off-by: Nathan Chancellor <nathan@...nel.org>
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@...gle.com>
> ---
>
> 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@kernel.org/
>
> 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
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