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

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Message-ID: <2dc1cdfa-d33a-48b6-ab77-d04b06a3efe8@yoseli.org>
Date: Wed, 20 Nov 2024 16:59:55 +0100
From: Jean-Michel Hautbois <jeanmichel.hautbois@...eli.org>
To: Steven Rostedt <rostedt@...dmis.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

Hi Steve,

On 20/11/2024 16:31, Steven Rostedt wrote:
> 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?

Yes, I did, sorry I thought it was in the previous dump :-(.

kbuffer_load_subbuffer: kbuf 0x8001d520, ptr 0x8025e2a0, call read_8
read_8
swap_8
kbuffer_load_subbuffer: kbuf 0x8001d520, ptr 0x8025e2a8, read_long
read_long: call read_long at 0x60022ef4 with 0x8025e2a8
__read_long_4: call read_4 at 0x600230de with 0x8025e2a8
__read_4_sw with 0x8025e2a8
__read_4_sw: ptr=0x8025e2a8, value: 00001ff0
__read_4_sw: data=00001ff0
swap_4 for 00001ff0
__read_long_4: --> read_4 at 0x600230de: f01f0000
__read_4_sw with 0x8025e2a8
__read_4_sw: ptr=0x8025e2a8, value: 00001ff0
__read_4_sw: data=00001ff0
swap_4 for 00001ff0
read_long: --> read_long at 0x60022ef4: f01f0000
__read_long_4: call read_4 at 0x600230de with 0x8025e2a8
__read_4_sw with 0x8025e2a8
__read_4_sw: ptr=0x8025e2a8, value: 00001ff0
__read_4_sw: data=00001ff0
swap_4 for 00001ff0
__read_long_4: --> read_4 at 0x600230de: f01f0000
__read_4_sw with 0x8025e2a8
__read_4_sw: ptr=0x8025e2a8, value: 00001ff0
__read_4_sw: data=00001ff0
swap_4 for 00001ff0
kbuffer_load_subbuffer: --> read_long,  flags=f01f0000
kbuffer_load_subbuffer: --> size=1f0000
kbuffer_load_subbuffer: kbuf->data=0x8025e2ac, kbuf->size=1f0000
kbuffer_load_subbuffer: kbuf 0x8001d520, ptr 0x8044e2ac, read_long 
lost_events
read_long: call read_long at 0x60022ef4 with 0x8044e2ac
__read_long_4: call read_4 at 0x600230de with 0x8044e2ac
__read_4_sw with 0x8044e2ac
Segmentation fault

JM