linux-kernel - Re: [PATCH RFC 0/2] Add basic tracing support for m68k

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Message-ID: <20241120103150.3442d658@gandalf.local.home>
Date: Wed, 20 Nov 2024 10:31:50 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Jean-Michel Hautbois <jeanmichel.hautbois@...eli.org>
Cc: linux-m68k@...ts.linux-m68k.org, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, Geert Uytterhoeven
 <geert@...ux-m68k.org>, Greg Ungerer <gerg@...ux-m68k.org>, Tomas Glozar
 <tglozar@...hat.com>
Subject: Re: [PATCH RFC 0/2] Add basic tracing support for m68k

On Wed, 20 Nov 2024 12:47:19 +0100
Jean-Michel Hautbois <jeanmichel.hautbois@...eli.org> wrote:

> Long story short: it fails at kbuffer_load_subbuffer() call in
> read_cpu_pages().
> 
> I added printf in the kbuffer helpers in libevent, and it finishes at:
> __read_long_4: call read_4 at 0x600230c2
> __read_4_sw: ptr=0x8044e2ac
> 
> static unsigned int __read_4_sw(void *ptr)
> {
> 	printf("%s: ptr=%p, value: %08x\n", __func__, ptr, *(unsigned int *)ptr);
> 	unsigned int data = *(unsigned int *)ptr;
> 	printf("%s: data=%08x\n", __func__, data);
> 
> 	return swap_4(data);
> }
> 
> As soon as ptr is dereferenced, the segfault appears.
> ptr should be ok though, as the address is valid afaik...

But you don't know what ptr it failed on, right?

If dereferencing a pointer will crash, the below line:

 	printf("%s: ptr=%p, value: %08x\n", __func__, ptr, *(unsigned int *)ptr);

Will crash before printing, because you are dereferencing ptr. Perhaps you
should change this to:

 	printf("%s: ptr=%p\n" value: %08x\n", __func__, ptr);
	printf("    value: %08x\n", *(unsigned int *)ptr);

And that way you will see what 'ptr' is before the crash. Or did you do
that already?

-- Steve


> 
> I must say that now I am stuck :-(.