linux-kernel - Re: [PATCH v4 05/13] module: Add printk formats to add module build ID to stacktraces

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Message-ID: <YHWyOhYZuLsbt2gB@alley>
Date:   Tue, 13 Apr 2021 17:01:14 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Jiri Olsa <jolsa@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Jessica Yu <jeyu@...nel.org>,
        Evan Green <evgreen@...omium.org>,
        Hsin-Yi Wang <hsinyi@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-doc@...r.kernel.org, Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v4 05/13] module: Add printk formats to add module build
 ID to stacktraces

On Fri 2021-04-09 18:52:52, Stephen Boyd wrote:
> Let's make kernel stacktraces easier to identify by including the build
> ID[1] of a module if the stacktrace is printing a symbol from a module.
> This makes it simpler for developers to locate a kernel module's full
> debuginfo for a particular stacktrace. Combined with
> scripts/decode_stracktrace.sh, a developer can download the matching
> debuginfo from a debuginfod[2] server and find the exact file and line
> number for the functions plus offsets in a stacktrace that match the
> module. This is especially useful for pstore crash debugging where the
> kernel crashes are recorded in something like console-ramoops and the
> recovery kernel/modules are different or the debuginfo doesn't exist on
> the device due to space concerns (the debuginfo can be too large for
> space limited devices).
> 
> diff --git a/include/linux/module.h b/include/linux/module.h
> index 59f094fa6f74..4bf869f6c944 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -11,6 +11,7 @@
>  
>  #include <linux/list.h>
>  #include <linux/stat.h>
> +#include <linux/buildid.h>
>  #include <linux/compiler.h>
>  #include <linux/cache.h>
>  #include <linux/kmod.h>
> @@ -367,6 +368,9 @@ struct module {
>  	/* Unique handle for this module */
>  	char name[MODULE_NAME_LEN];
>  
> +	/* Module build ID */
> +	unsigned char build_id[BUILD_ID_SIZE_MAX];

Do we want to initialize/store the ID even when
CONFIG_STACKTRACE_BUILD_ID is disabled and nobody would
use it?

Most struct module members are added only when the related feature
is enabled.

I am not sure how it would complicate the code. It is possible
that it is not worth it. Well, I could imagine that the API
will always pass the buildid parameter and
module_address_lookup() might do something like

#ifndef CONFIG_STACKTRACE_BUILD_ID
static char empty_build_id[BUILD_ID_SIZE_MAX];
#endif

		if (modbuildid) {
			if (IS_ENABLED(CONFIG_STACKTRACE_BUILD_ID))
				*modbuildid = mod->build_id;
			else
				*modbuildid = empty_build_id;

IMHO, this is primary a call for Jessica as the module code maintainer.

Otherwise, I am fine with this patch. And it works as expected.

Best Regards,
Petr