linux-kernel - Re: [PATCH v5 4/6] scripts/sorttable: Zero out weak functions in mcount

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Message-ID: <5225b07b-a9b2-4558-9d5f-aa60b19f6317@sirena.org.uk>
Date: Mon, 24 Feb 2025 20:06:28 +0000
From: Mark Brown <broonie@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	linux-kbuild@...r.kernel.org, bpf <bpf@...r.kernel.org>,
	linux-arm-kernel@...ts.infradead.org, linux-s390@...r.kernel.org,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Vasily Gorbik <gor@...ux.ibm.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>
Subject: Re: [PATCH v5 4/6] scripts/sorttable: Zero out weak functions in
 mcount_loc table

On Tue, Feb 18, 2025 at 02:59:22PM -0500, Steven Rostedt wrote:
> From: Steven Rostedt <rostedt@...dmis.org>
> 
> When a function is annotated as "weak" and is overridden, the code is not
> removed. If it is traced, the fentry/mcount location in the weak function
> will be referenced by the "__mcount_loc" section. This will then be added
> to the available_filter_functions list. Since only the address of the
> functions are listed, to find the name to show, a search of kallsyms is
> used.

This breaks builds with ftrace on architectures without KASLR, one
affected configuration is bcm2835_defconfig:

/home/broonie/git/bisect/kernel/trace/ftrace.c: In function 'ftrace_process_locs':
/home/broonie/git/bisect/kernel/trace/ftrace.c:7057:24: error: implicit declaration of function 'kaslr_offset' [-Werror=implicit-function-declaration]
 7057 |         kaslr = !mod ? kaslr_offset() : 0;
      |                        ^~~~~~~~~~~~

since that happens to enable CONFIG_FUNCTION_TRACER but doesn't have
KASLR, we don't have stubs for KASLR on architectures that don't have
it.  It also looks like from a quick glance at least RISC-V will fail to
link since it only provides kaslr_offset() with RANDOMIZE_BASE enabled.
This all feels a bit footgunish.

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