Re: [PATCH v2 2/4] Kbuild: do not emit debug info for assembly with LLVM_IAS=1

[Fangrui Song]

On 2020-11-04, Nathan Chancellor wrote:
> On Tue, Nov 03, 2020 at 04:53:41PM -0800, Nick Desaulniers wrote:
> > Clang's integrated assembler produces the warning for assembly files:
> >
> > warning: DWARF2 only supports one section per compilation unit
> >
> > If -Wa,-gdwarf-* is unspecified, then debug info is not emitted.  This
>
> Is this something that should be called out somewhere? If I understand
> this correctly, LLVM_IAS=1 + CONFIG_DEBUG_INFO=y won't work? Maybe this
> should be handled in Kconfig?
>
> > will be re-enabled for new DWARF versions in a follow up patch.
> >
> > Enables defconfig+CONFIG_DEBUG_INFO to build cleanly with
> > LLVM=1 LLVM_IAS=1 for x86_64 and arm64.
> >
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Link: https://github.com/ClangBuiltLinux/linux/issues/716
> > Reported-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> > Suggested-by: Dmitry Golovin <dima@xxxxxxxxxx>
>
> If you happen to respin, Dmitry deserves a Reported-by tag too :)
>
> > Suggested-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
> > Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
>
> Regardless of the other two comments, this is fine as is as a fix for
> stable to unblock Android + CrOS since we have been running something
> similar to it in CI:
>
> Reviewed-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
>
> > ---
> >  Makefile | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/Makefile b/Makefile
> > index f353886dbf44..75b1a3dcbf30 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -826,7 +826,9 @@ else
> >  DEBUG_CFLAGS	+= -g
> >  endif
> >
> > +ifndef LLVM_IAS
>
> Nit: this should probably match the existing LLVM_IAS check
>
> ifneq ($(LLVM_IAS),1)
>
> >  KBUILD_AFLAGS	+= -Wa,-gdwarf-2
> > +endif
> >
> >  ifdef CONFIG_DEBUG_INFO_DWARF4
> >  DEBUG_CFLAGS	+= -gdwarf-4
> > --
> > 2.29.1.341.ge80a0c044ae-goog

The root cause is that DWARF v2 has no DW_AT_ranges, so it cannot
represent non-contiguous address ranges. It seems that GNU as -gdwarf-3
emits DW_AT_ranges as well and emits an entry for a non-executable section.
In any case, the option is of very low value, at least for LLVM.


Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>