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

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Message-ID: <202410281716.0C8F383@keescook>
Date: Mon, 28 Oct 2024 17:16:44 -0700
From: Kees Cook <kees@...nel.org>
To: Rong Xu <xur@...gle.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>,
	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

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>

-- 
Kees Cook