Hi Steve,

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

On Wed, 20 Nov 2024 12:47:19 +0100
Jean-Michel Hautbois [off-list ref] wrote:

quoted

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