linux-kernel - Re: [PATCH 6/6] kallsyms: Prevent module removal when printing module name and buildid

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Message-ID: <aRHoHMJYAhSoEh1e@pathway.suse.cz>
Date: Mon, 10 Nov 2025 14:26:52 +0100
From: Petr Mladek <pmladek@...e.com>
To: Aaron Tomlin <atomlin@...mlin.com>
Cc: Petr Pavlu <petr.pavlu@...e.com>, Steven Rostedt <rostedt@...dmis.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <kees@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
	John Fastabend <john.fastabend@...il.com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Luis Chamberlain <mcgrof@...nel.org>,
	Daniel Gomez <da.gomez@...nel.org>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
	linux-modules@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] kallsyms: Prevent module removal when printing
 module name and buildid

On Fri 2025-11-07 19:36:35, Aaron Tomlin wrote:
> On Wed, Nov 05, 2025 at 03:23:18PM +0100, Petr Mladek wrote:
> > kallsyms_lookup_buildid() copies the symbol name into the given buffer
> > so that it can be safely read anytime later. But it just copies pointers
> > to mod->name and mod->build_id which might get reused after the related
> > struct module gets removed.
> > 
> > The lifetime of struct module is synchronized using RCU. Take the rcu
> > read lock for the entire __sprint_symbol().
> > 
> > Signed-off-by: Petr Mladek <pmladek@...e.com>
> > ---
> >  kernel/kallsyms.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> > index ff7017337535..1fda06b6638c 100644
> > --- a/kernel/kallsyms.c
> > +++ b/kernel/kallsyms.c
> > @@ -468,6 +468,9 @@ static int __sprint_symbol(char *buffer, unsigned long address,
> >  	unsigned long offset, size;
> >  	int len;
> >  
> > +	/* Prevent module removal until modname and modbuildid are printed */
> > +	guard(rcu)();
> > +
> >  	address += symbol_offset;
> >  	len = kallsyms_lookup_buildid(address, &size, &offset, &modname, &buildid,
> >  				       buffer);
> > -- 
> > 2.51.1
> > 
> > 
> 
> Hi Petr,
> 
> If I am not mistaken, this is handled safely within the context of
> module_address_lookup() since f01369239293e ("module: Use RCU in
> find_kallsyms_symbol()."), no?

The above mention commit fixed an API which is looking only for
the symbol name. It seems to be used, for example, in kprobe
or ftrace code.

This patch is fixing another API which is used in vsprintf() for
printing backtraces. It looks for more information: symbol name,
module name, and buildid. It needs its own RCU read protection.

Best Regards,
Petr