Re: [PATCH v3] purgatory: fix disabling debug info

From: Masahiro Yamada <masahiroy@kernel.org>
To: Nathan Chancellor <nathan@kernel.org>
Cc: Alyssa Ross <hi@alyssa.is>, Nick Cao <nickcao@nichi.co>,
	linux-kbuild@vger.kernel.org,
	Nick Desaulniers <ndesaulniers@google.com>,
	linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
	linux-riscv@lists.infradead.org, Tom Rix <trix@redhat.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	stable@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH v3] purgatory: fix disabling debug info
Date: Sat, 1 Apr 2023 00:42:13 +0900	[thread overview]
Message-ID: <CAK7LNARU444UrZXVodNftud-scy5KKUjdtTM0GOrxHB9pyKmkg@mail.gmail.com> (raw)
In-Reply-To: <20230330222928.GA644044@dev-arch.thelio-3990X>

On Fri, Mar 31, 2023 at 7:29 AM Nathan Chancellor <nathan@kernel.org> wrote:
>
> On Thu, Mar 30, 2023 at 06:22:24PM +0000, Alyssa Ross wrote:
> > Since 32ef9e5054ec, -Wa,-gdwarf-2 is no longer used in KBUILD_AFLAGS.
> > Instead, it includes -g, the appropriate -gdwarf-* flag, and also the
> > -Wa versions of both of those if building with Clang and GNU as.  As a
> > result, debug info was being generated for the purgatory objects, even
> > though the intention was that it not be.
> >
> > Fixes: 32ef9e5054ec ("Makefile.debug: re-enable debug info for .S files")
> > Signed-off-by: Alyssa Ross <hi@alyssa.is>
> > Cc: stable@vger.kernel.org
> > Acked-by: Nick Desaulniers <ndesaulniers@google.com>
>
> This is definitely more future proof.
>
> Reviewed-by: Nathan Chancellor <nathan@kernel.org>
> Tested-by: Nathan Chancellor <nathan@kernel.org>

I prefer v3 since it is cleaner, but unfortunately
it does not work for Clang+GAS.

With v3 applied, I still see the debug info.

$ make LLVM=1 LLVM_IAS=0  arch/x86/purgatory/setup-x86_64.o
  UPD     include/config/kernel.release
  UPD     include/generated/utsrelease.h
  CALL    scripts/checksyscalls.sh
  DESCEND objtool
  INSTALL libsubcmd_headers
  AS      arch/x86/purgatory/setup-x86_64.o
$ readelf -S arch/x86/purgatory/setup-x86_64.o
There are 18 section headers, starting at offset 0x14d8:

Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0
  [ 1] .text             PROGBITS         0000000000000000  00000040
       0000000000000027  0000000000000000  AX       0     0     16
  [ 2] .rela.text        RELA             0000000000000000  000012f8
       0000000000000060  0000000000000018   I      15     1     8
  [ 3] .data             PROGBITS         0000000000000000  00000067
       0000000000000000  0000000000000000  WA       0     0     1
  [ 4] .bss              NOBITS           0000000000000000  00001000
       0000000000001000  0000000000000000  WA       0     0     4096
  [ 5] .rodata           PROGBITS         0000000000000000  00001000
       0000000000000020  0000000000000000   A       0     0     16
  [ 6] .rela.rodata      RELA             0000000000000000  00001358
       0000000000000018  0000000000000018   I      15     5     8
  [ 7] .debug_line       PROGBITS         0000000000000000  00001020
       000000000000005f  0000000000000000           0     0     1
  [ 8] .rela.debug_line  RELA             0000000000000000  00001370
       0000000000000018  0000000000000018   I      15     7     8
  [ 9] .debug_info       PROGBITS         0000000000000000  0000107f
       0000000000000027  0000000000000000           0     0     1
  [10] .rela.debug_info  RELA             0000000000000000  00001388
       0000000000000090  0000000000000018   I      15     9     8
  [11] .debug_abbrev     PROGBITS         0000000000000000  000010a6
       0000000000000014  0000000000000000           0     0     1
  [12] .debug_aranges    PROGBITS         0000000000000000  000010c0
       0000000000000030  0000000000000000           0     0     16
  [13] .rela.debug_[...] RELA             0000000000000000  00001418
       0000000000000030  0000000000000018   I      15    12     8
  [14] .debug_str        PROGBITS         0000000000000000  000010f0
       0000000000000054  0000000000000001  MS       0     0     1
  [15] .symtab           SYMTAB           0000000000000000  00001148
       0000000000000168  0000000000000018          16    12     8
  [16] .strtab           STRTAB           0000000000000000  000012b0
       0000000000000041  0000000000000000           0     0     1
  [17] .shstrtab         STRTAB           0000000000000000  00001448
       000000000000008d  0000000000000000           0     0     1
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
  L (link order), O (extra OS processing required), G (group), T (TLS),
  C (compressed), x (unknown), o (OS specific), E (exclude),
  D (mbind), l (large), p (processor specific)

With -g0 given, GCC stops passing -g -gdwarf-4 down to GAS.

Clang does not do anything about -g0 for the external assembler.

I was thinking of dropping LLVM_IAS=0 support.
When we decide to give up -fno-integrated-as,
we can clean up the code in various places.

Anyway, v3 does not work in the current situation.

V2 works for all usecases.

-- 
Best Regards
Masahiro Yamada