linux-kernel - Re: [PATCH] printk: Userspace format enumeration support

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Date:   Fri, 5 Feb 2021 12:47:48 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Chris Down <chris@...isdown.name>, linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        John Ogness <john.ogness@...utronix.de>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>, kernel-team@...com,
        Alexey Dobriyan <adobriyan@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jason Baron <jbaron@...mai.com>,
        Kees Cook <keescook@...omium.org>, linux-api@...r.kernel.org
Subject: Re: [PATCH] printk: Userspace format enumeration support

On Fri, 5 Feb 2021 17:42:55 +0100
Petr Mladek <pmladek@...e.com> wrote:

> Hi,
> 
> I would like to hear opinion from a bigger audience. It is an
> userspace interface that we might need to maintain forewer.
> Adding few more people in to CC:
> 
> Steven Rostedt <rostedt@...dmis.org>: printk co-maintainer

Thanks for Cc'ing me.

> Alexey Dobriyan <adobriyan@...il.com>: fs/proc maintainer
> Greg Kroah-Hartman <gregkh@...uxfoundation.org>: sysfs maintainer
> Jason Baron <jbaron@...mai.com>: dynamic_debug maintainer
> Kees Cook <keescook@...omium.org>: security POV
> linux-api@...r.kernel.org: Linux API mailing list
> 
> Of course, we should also ask if this is the right approach
> for the think that you want to achieve.
> 
> The motivation for this patch is that the strings printed by kernels
> are not reliable and you want a simple way to compare differences
> bethween versions. Do I get it right?
> 
> See more comments below.
> 
> 


> Also this is yet another style how the format is displayed. We already have
> 
> 	+ console/syslog: formated by record_print_text()
> 	+ /dev/kmsg: formatted by info_print_ext_header(),  msg_print_ext_body().
> 	+ /sys/kernel/debug/dynamic_debug/control


> 	+ /sys/kernel/debug/tracing/printk_formats
> 
> We should get some inspiration from the existing interfaces.

Interesting, because when I was looking at the original patch (looked at
the lore link before reading your reply), I thought to myself "this looks
exactly like what I did for trace_printk formats", which the above file is
where it is shown. I'm curious if this work was inspired by that?



> > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> > index 34b7e0d2346c..0ca6e28e05d6 100644
> > --- a/include/asm-generic/vmlinux.lds.h
> > +++ b/include/asm-generic/vmlinux.lds.h
> > @@ -309,6 +309,17 @@
> >  #define ACPI_PROBE_TABLE(name)
> >  #endif
> >  
> > +#ifdef CONFIG_PRINTK_ENUMERATION
> > +#define PRINTK_FMTS							\
> > +	.printk_fmts : AT(ADDR(.printk_fmts) - LOAD_OFFSET) {		\
> > +		__start_printk_fmts = .;				\
> > +		*(.printk_fmts)						\
> > +		__stop_printk_fmts = .;					\
> > +	}
> > +#else
> > +#define PRINTK_FMTS
> > +#endif  
> 
> It should be defined after #define TRACEDATA to follow the existing
> style.
> 
> But honestly I am not much familiar with the sections definitions.
> I am curious why TRACE_PRINTKS() and __dyndbg are defined
> a bit different way.
> 

I'm not sure what difference you mean.

> > +static int proc_pf_show(struct seq_file *s, void *v)
> > +{
> > +	const struct printk_fmt_sec *ps = NULL;
> > +	const char **fptr = NULL;
> > +
> > +	mutex_lock(&printk_fmts_mutex);
> > +
> > +	list_for_each_entry(ps, &printk_fmts_list, list) {
> > +		const char *mod_name = ps_get_module_name(ps);
> > +
> > +		for (fptr = ps->start; fptr < ps->end; fptr++) {
> > +			seq_puts(s, mod_name);
> > +			seq_putc(s, ',');
> > +			seq_puts(s, *fptr);
> > +			seq_putc(s, '\0');
> > +		}  
> 
> You probably should get inspiration from t_show() in trace_printk.c.
> It handles newlines, ...
> 
> Or by ddebug_proc_show(). It uses seq_escape().
> 
> Anyway, there is something wrong at the moment. The output looks fine
> with cat. But "less" says that it is a binary format and the output
> is a bit messy:

Hmm, that's usually the case when lseek gets messed up. Not sure how that
happened.

> 
> $> less /proc/printk_formats   
> "/proc/printk_formats" may be a binary file.  See it anyway? 
> vmlinux,^A3Warning: unable to open an initial console.
> ^@...inux,^A3Failed to execute %s (error %d)
> ^@...inux,^A6Kernel memory protection disabled.
> ^@...inux,^A3Starting init: %s exists but couldn't execute it (error %d)
> 
> 
> That is for now. I still have to think about it. And I am also curious
> about what others thing about this idea.
> 

I'm not against the idea. I don't think it belongs in /proc. Perhaps
debugfs is a better place to put it.

-- Steve