linux-kernel - Re: [PATCH v6 2/7] objtool: Fix unreachable instruction warnings for weak functions

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Message-ID: <CAF1bQ=QDQ+ACsH2CchnRk2LDRpN_WYjJOAjr9FDspxiar+YbrQ@mail.gmail.com>
Date: Mon, 28 Oct 2024 22:51:21 -0700
From: Rong Xu <xur@...gle.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Kees Cook <kees@...nel.org>, 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>, 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>, 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 2/7] objtool: Fix unreachable instruction warnings for
 weak functions

I think the objtool works on individual object files (or vmlinux.o).
The -Wl,--gc-sections flag, on the other hand, is a linker flag that
acts on the final link -- it's applied after objtool invocations.
Therefore, even with -Wl,--gc-sections, we'll still encounter those
spurious warnings from objtool.

-Rong

On Mon, Oct 28, 2024 at 5:19 PM H. Peter Anvin <hpa@...or.com> wrote:
>
> On 10/28/24 17:16, Kees Cook wrote:
> > On Fri, Oct 25, 2024 at 10:14:04PM -0700, Rong Xu wrote:
> >> In the presence of both weak and strong function definitions, the
> >> linker drops the weak symbol in favor of a strong symbol, but
> >> leaves the code in place. Code in ignore_unreachable_insn() has
> >> some heuristics to suppress the warning, but it does not work when
> >> -ffunction-sections is enabled.
> >>
> >> Suppose function foo has both strong and weak definitions.
> >> Case 1: The strong definition has an annotated section name,
> >> like .init.text. Only the weak definition will be placed into
> >> .text.foo. But since the section has no symbols, there will be no
> >> "hole" in the section.
> >>
> >> Case 2: Both sections are without an annotated section name.
> >> Both will be placed into .text.foo section, but there will be only one
> >> symbol (the strong one). If the weak code is before the strong code,
> >> there is no "hole" as it fails to find the right-most symbol before
> >> the offset.
> >>
> >> The fix is to use the first node to compute the hole if hole.sym
> >> is empty. If there is no symbol in the section, the first node
> >> will be NULL, in which case, -1 is returned to skip the whole
> >> section.
> >>
> >> Co-developed-by: Han Shen <shenhan@...gle.com>
> >> Signed-off-by: Han Shen <shenhan@...gle.com>
> >
> > This seems logically correct to me, but I'd love to see review from Josh
> > and/or Peter Z on this change too.
> >
> > Reviewed-by: Kees Cook <kees@...nel.org>
> >
>
> Does this happen even with -Wl,--gc-sections?
>
>         -hpa
>