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Date:   Sun, 16 Oct 2022 19:20:36 +0100
From:   Conor Dooley <conor@kernel.org>
To:     Sedat Dilek <sedat.dilek@gmail.com>
Cc:     Masahiro Yamada <masahiroy@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Tom Rix <trix@redhat.com>, Palmer Dabbelt <palmer@dabbelt.com>,
        linux-kbuild@vger.kernel.org, linux-riscv@lists.infradead.org,
        linux-kernel@vger.kernel.org, patches@lists.linux.dev,
        llvm@lists.linux.dev
Subject: Re: [PATCH v2] lib/Kconfig.debug: Add check for non-constant
 .{s,u}leb128 support to DWARF5
Message-ID: <Y0xLdOM0TAbcJHop@spud>
References: <20221014204210.383380-1-nathan@kernel.org>
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On Sun, Oct 16, 2022 at 08:10:30PM +0200, Sedat Dilek wrote:
> On Sun, Oct 16, 2022 at 8:04 PM Masahiro Yamada <masahiroy@kernel.org> wrote:
> >
> > 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.
> >
> 
> [1] says:
> 
> "This looks good to me! Feel free to submit with a:
> Tested-by: Conor Dooley <conor.dooley@microchip.com>"

Hey Sedat,

I actually didn't take a proper look at the v2 at all... I didn't realise
that Nathan dropped by T-b for v2 since he'd changed the patch enough to
feel that he should.

I'm not too worried about the removed T-b (espcially since Linus has
just merged Masahiro's PR containing this change).

Thanks though!
Conor.

> 
> -sed@-
> 
> [1] https://github.com/ClangBuiltLinux/linux/issues/1719#issuecomment-1261158627
> 
> >
> >
> > > > ---
> > > >
> > > > 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