linux-kernel - Re: [PATCH 0/2] ftrace/recordmcount: Handle object files without section symbols

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Message-ID: <20220816100423.5ee2e17c@gandalf.local.home>
Date:   Tue, 16 Aug 2022 10:04:23 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        <linux-kernel@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org>,
        llvm@...ts.linux.dev
Subject: Re: [PATCH 0/2] ftrace/recordmcount: Handle object files without
 section symbols

On Wed, 27 Apr 2022 15:01:20 +0530
"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> wrote:

> This solves a build issue on powerpc with binutils v2.36 and newer [1].
> Since commit d1bcae833b32f1 ("ELF: Don't generate unused section
> symbols") [2], binutils started dropping section symbols that it thought
> were unused. Due to this, in certain scenarios, recordmcount is unable 
> to find a non-weak symbol to generate a relocation record against.
> 
> Clang integrated assembler is also aggressive in dropping section 
> symbols [3].
> 
> In the past, there have been various workarounds to address this. See 
> commits 55d5b7dd6451b5 ("initramfs: fix clang build failure") and 
> 6e7b64b9dd6d96 ("elfcore: fix building with clang") and a recent patch:
> https://lore.kernel.org/linuxppc-dev/20220425174128.11455-1-naveen.n.rao@linux.vnet.ibm.com/T/#u
> 
> Fix this issue by using the weak symbol in the relocation record. This 
> can result in duplicate locations in the mcount table if those weak 
> functions are overridden, so have ftrace skip dupicate entries.
> 
> Objtool already follows this approach, so patch 2 updates recordmcount 
> to do the same. Patch 1 updates ftrace to skip duplicate entries.
> 
> - Naveen
> 
> 
> [1] https://github.com/linuxppc/issues/issues/388
> [2] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=d1bcae833b32f1
> [3] https://github.com/ClangBuiltLinux/linux/issues/981
> 
>

There's been work to handle weak functions, but I'm not sure that work
handled the issues here. Are these patches still needed, or was there
another workaround to handle the problems this addressed?

-- Steve