linux-kernel - Re: [RFC PATCH] ftrace: Skip __fentry__ location of overridden weak functions

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Date: Tue, 11 Jun 2024 11:21:57 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Zheng Yejian <zhengyejian1@...wei.com>
Cc: rostedt@...dmis.org, mcgrof@...nel.org, mhiramat@...nel.org,
	mark.rutland@....com, mathieu.desnoyers@...icios.com,
	jpoimboe@...nel.org, linux-modules@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	bpf@...r.kernel.org
Subject: Re: [RFC PATCH] ftrace: Skip __fentry__ location of overridden weak
 functions

On Tue, Jun 11, 2024 at 09:56:51AM +0800, Zheng Yejian wrote:
> On 2024/6/7 23:02, Peter Zijlstra wrote:

> > Oh gawd, sodding weak functions again.
> > 
> > I would suggest changing scipts/kallsyms.c to emit readily identifiable
> > symbol names for all the weak junk, eg:
> > 
> >    __weak_junk_NNNNN
> > 
> 
> Sorry for the late reply, I just had a long noon holiday :>
> 
> scripts/kallsyms.c is compiled and used to handle symbols in vmlinux.o
> or vmlinux.a, see kallsyms_step() in scripts/link-vmlinux.sh, those
> overridden weak symbols has been removed from symbol table of vmlinux.o
> or vmlinux.a. But we can found those symbols from original xx/xx.o file,
> for example, the weak free_initmem() in in init/main.c is overridden,
> its symbol is not in vmlinx but is still in init/main.o .
> 
> How about traversing all origin xx/xx.o and finding all weak junk symbols ?

You don't need to. ELF symbl tables have an entry size for FUNC type
objects, this means that you can readily find holes in the text and fill
them with a symbol.

Specifically, you can check the mcount locations against the symbol
table and for every one that falls in a hole, generate a new junk
symbol.

Also see 4adb23686795 where objtool adds these holes to the
ignore/unreachable code check.


The lack of size for kallsyms is in a large part what is causing the
problems.