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

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Message-ID: <CAHk-=wgTFSqiMvbGYqFLQaERoeXR5nK1Y=-L3SN7rB3UtzG0PQ@mail.gmail.com>
Date: Thu, 26 Dec 2024 15:01:07 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: 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>, Peter Zijlstra <peterz@...radead.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>, 
	Mark Rutland <mark.rutland@....com>
Subject: Re: [POC][RFC][PATCH] build: Make weak functions visible in kallsyms

On Thu, 26 Dec 2024 at 13:49, Steven Rostedt <rostedt@...dmis.org> wrote:
>
> But then, when the linker removes these functions because they were
> overridden, the code does not disappear, leaving the pointers in the
> __mcount_loc locations.

This seems entirely unrelated to weak functions, and will be true for
any other "linker removed it" (which can happen for other reasons
too).

So your "fix" seems to be hacking around a symptom.

And honestly, the kallsyms argument seems bogus too. The problem with
kallsyms is that it looks up the size the wrong way. Making up new
function names doesn't fix the problem, it - once again - just hacks
around the symptom of doing something wrong.

Christ, kallsyms looking at nm output and going by "next symbol" was
always bogus, but I think that's how the old a.out format worked
originally.

But "nm" literally takes a "-S" argument. We just don't use it.

So I think the fix is literally to just make kallsysms have the size
data. Of course, very annoyingly out /proc/kallsyms file format also
tracks the (legacy) nm output that doesn't have size information.

But I do think that if you hit real problems, you need to fix the
*source* of the issue, not add another ugly hack around things.

             Linus