linux-kernel - Re: [PATCH v6 3/7] Adjust symbol ordering in text output section

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Message-ID: <CAF1bQ=R18HLC2vjCGj+M=VYidrVzz3RT=U8cckXgpgrxc0kG0Q@mail.gmail.com>
Date: Mon, 11 Nov 2024 21:38:57 -0800
From: Rong Xu <xur@...gle.com>
To: Klara Modin <klarasmodin@...il.com>
Cc: Alice Ryhl <aliceryhl@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Arnd Bergmann <arnd@...db.de>, Bill Wendling <morbo@...gle.com>, Borislav Petkov <bp@...en8.de>, 
	Breno Leitao <leitao@...ian.org>, Brian Gerst <brgerst@...il.com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, David Li <davidxl@...gle.com>, 
	Han Shen <shenhan@...gle.com>, Heiko Carstens <hca@...ux.ibm.com>, "H. Peter Anvin" <hpa@...or.com>, 
	Ingo Molnar <mingo@...hat.com>, Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, Juergen Gross <jgross@...e.com>, 
	Justin Stitt <justinstitt@...gle.com>, Kees Cook <kees@...nel.org>, 
	Masahiro Yamada <masahiroy@...nel.org>, "Mike Rapoport (IBM)" <rppt@...nel.org>, 
	Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Nicolas Schier <nicolas@...sle.eu>, "Paul E. McKenney" <paulmck@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Sami Tolvanen <samitolvanen@...gle.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Wei Yang <richard.weiyang@...il.com>, 
	workflows@...r.kernel.org, Miguel Ojeda <miguel.ojeda.sandonis@...il.com>, 
	Maksim Panchenko <max4bolt@...il.com>, "David S. Miller" <davem@...emloft.net>, 
	Andreas Larsson <andreas@...sler.com>, Yonghong Song <yonghong.song@...ux.dev>, 
	Yabin Cui <yabinc@...gle.com>, Krzysztof Pszeniczny <kpszeniczny@...gle.com>, 
	Sriraman Tallam <tmsriram@...gle.com>, Stephane Eranian <eranian@...gle.com>, x86@...nel.org, 
	linux-arch@...r.kernel.org, sparclinux@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-kbuild@...r.kernel.org, 
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH v6 3/7] Adjust symbol ordering in text output section

I compared the System.map files from Klara Modin. The linker script is
doing what I expected: relocating the unlikely executed functions to the
beginning of the .text section.

However, the problem is with the _stext symbol. It belongs to the
.text section, so
it is positioned after the unlikely (or hot) functions. But it really
needs to be
the start of the text section.

I checked all vmlinux.lds.S in arch/, I found that most archs
explicitly assign _stext to the same address as _text, with the
following 3 exceptions:
  arch/sh/kernel/vmlinux.lds.S
  arch/mips/kernel/vmlinux.lds.S
  arch/sparc/kernel/vmlinux.lds.S

Note that we already partially handled arch/sparc/kernel/vmlinux.lds.S
for sparc64.
But we need to handle sparc32 also.

Additionally, the boot/compressed/vmlinux.lds.S also the TEXT_TEXT
template. However,
I presume these files do not generate the .text.unlikely. or
.text.hot.* sections.

I sent the following patch to Klara because I don't have an
environment to build and test.
====================
diff --git a/arch/mips/kernel/vmlinux.lds.S b/arch/mips/kernel/vmlinux.lds.S
index 9ff55cb80a64..5f130af44247 100644
--- a/arch/mips/kernel/vmlinux.lds.S
+++ b/arch/mips/kernel/vmlinux.lds.S
@@ -61,6 +61,7 @@ SECTIONS
        /* read-only */
        _text = .;      /* Text and read-only data */
        .text : {
+               _stext = .;
                TEXT_TEXT
                SCHED_TEXT
                LOCK_TEXT
======================

If Klara confirms the fix, I will send the patch for review.

Thanks,

-Rong


On Mon, Nov 11, 2024 at 2:39 PM Rong Xu <xur@...gle.com> wrote:
>
> In the new System.map, we have:
> ffffffff81112400 T _stext
>
> This looks wrong. It should point to the beginning of the text, like
> ffffffff81100400 T _stext
>
> I'll do some debugging on this.
>
> -Rong
>
> On Mon, Nov 11, 2024 at 1:32 PM Klara Modin <klarasmodin@...il.com> wrote:
> >
> > On 2024-11-11 21:43, Rong Xu wrote:
> > > Thanks for reporting this issue!
> > >
> > > I'm assuming your kernel build enables dead code elimination and
> > > uses the --ffunction-sections compiler flag. Without this patch, all
> > > the functions
> > > -- I think there are only .text.unlikely.* and .text.* are grouped
> > > together in the
> > > final vmlinux. This patch modifies the linker script to place
> > > .text.unlikely.* functions
> > >   before .text.* functions. I've examined arch/mips/kernel/vmlinux.lds.S, and
> > > haven't found any obvious issue.
> > >
> > > Can you send me the following?
> > > (1) the kernel build command
> > > (2) System.map without the patch
> > > (3) System.map with the patch
> > >
> > > Best regards,
> > >
> > > -Rong
> > >
> > I don't set -ffunction-sections explicitly but it seems to be used when
> > I look at the .cmd files. The build command is nothing fancy, I just set
> > ARCH=mips CROSS_COMPILE=mips64-unknown-linux-gnuabin32- and build with
> > make -j24.
> >
> > I've attached the System.map, built on next-20241111 as well as it with
> > this series reverted.
> >
> > Regards,
> > Klara Modin