linux-kernel - Re: [PATCH 1/3] ring-buffer: Add uname to match criteria for persistent ring buffer

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Message-ID: <20241217175301.03d25799@gandalf.local.home>
Date: Tue, 17 Dec 2024 17:53:01 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, Masami
 Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Andrew Morton
 <akpm@...ux-foundation.org>, stable@...r.kernel.org
Subject: Re: [PATCH 1/3] ring-buffer: Add uname to match criteria for
 persistent ring buffer

On Tue, 17 Dec 2024 14:24:08 -0800
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> (Aside: are those binary buffers actually exported to user space (that
> "u32 *bin_buf, size_t size" thing), or could we fix the binary printf
> code to really use a whole word for char/short values? The difference
> between '%hhd' and '%d' should be how it's printed out, not how the
> data is accessed)

libtraceevent is able to parse the raw trace_printk() events:

  https://git.kernel.org/pub/scm/libs/libtrace/libtraceevent.git/tree/src/event-parse.c#n5155

The format it reads is from /sys/kernel/tracing/events/ftrace/bprint/format:

name: bprint
ID: 6
format:
	field:unsigned short common_type;	offset:0;	size:2;	signed:0;
	field:unsigned char common_flags;	offset:2;	size:1;	signed:0;
	field:unsigned char common_preempt_count;	offset:3;	size:1;	signed:0;
	field:int common_pid;	offset:4;	size:4;	signed:1;

	field:unsigned long ip;	offset:8;	size:8;	signed:0;
	field:const char * fmt;	offset:16;	size:8;	signed:0;
	field:u32 buf[];	offset:24;	size:0;	signed:0;

print fmt: "%ps: %s", (void *)REC->ip, REC->fmt

In this case, the "print fmt" is ignored.

Where the buf value holds the binary storage from vbin_printf() and written
in trace_vbprintk(). Yeah, it looks like it does depend on the arguments
being word aligned.

-- Steve