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

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Message-ID: <0605fa9c-0e48-48ec-b04d-c2ef1c48fdd9@zytor.com>
Date: Mon, 28 Oct 2024 17:18:09 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Kees Cook <kees@...nel.org>, 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>, 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 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