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

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Message-ID: <kubk2a4ydmja45dfnwxkkhpdbov27m6errnenc6eljbgdmidzl@is24eqefukit>
Date: Fri, 7 Nov 2025 19:36:35 -0500
From: Aaron Tomlin <atomlin@...mlin.com>
To: Petr Mladek <pmladek@...e.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 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?


Kind regards,
-- 
Aaron Tomlin