linux-kernel - Re: [POC][RFC][PATCH] build: Make weak functions visible in kallsyms

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Message-ID: <20250103110045.GA22934@noisy.programming.kicks-ass.net>
Date: Fri, 3 Jan 2025 12:00:45 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>,
	bpf <bpf@...r.kernel.org>, linux-kbuild@...r.kernel.org,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Masahiro Yamada <masahiroy@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nicolas Schier <nicolas@...sle.eu>,
	Zheng Yejian <zhengyejian1@...wei.com>,
	Martin Kelly <martin.kelly@...wdstrike.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Mark Rutland <mark.rutland@....com>
Subject: Re: [POC][RFC][PATCH] build: Make weak functions visible in kallsyms

On Thu, Dec 26, 2024 at 07:35:18PM -0800, Linus Torvalds wrote:

> Btw, does this actually happen when the compiler does the mcount thing for us?

Yeah, so the linker literally just drops the symbol name on the floor,
nothing else. So all the section information -- either compiler
generated like mcount locations, or manually inserted like alternatives
-- stick around for the 'dead' code.

The only time the toolchain has enough visibility to do the right thing
is when doing a full LTO build, at that point can the compiler see the
weak function is actually unused and do 'normal' DCE, including all the
resulting extra section stuff.