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From:   Masahiro Yamada <masahiroy@kernel.org>
Date:   Mon, 17 Oct 2022 03:00:56 +0900
Message-ID: <CAK7LNAQvpSFUiX9=92UnoZpJs=uzaZqpGG-r1dyeWsRXpS5fmw@mail.gmail.com>
Subject: Re: [PATCH v2] lib/Kconfig.debug: Add check for non-constant
 .{s,u}leb128 support to DWARF5
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Nathan Chancellor <nathan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Tom Rix <trix@redhat.com>, Palmer Dabbelt <palmer@dabbelt.com>,
        Conor Dooley <conor@kernel.org>, linux-kbuild@vger.kernel.org,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        patches@lists.linux.dev, llvm@lists.linux.dev
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On Sat, Oct 15, 2022 at 6:52 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Fri, Oct 14, 2022 at 1:48 PM 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
> > Signed-off-by: Nathan Chancellor <nathan@kernel.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@google.com>


Applied to linux-kbuild.
Thanks.


> > ---
> >
> > 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


--
Best Regards
Masahiro Yamada