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

From: Steven Rostedt
Date: Tue Dec 17 2024 - 17:53:50 EST


On Tue, 17 Dec 2024 14:24:08 -0800
Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> 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