Re: Memory modified after free: Most recently used by acpitask

On Wed, 2004-11-24 at 16:49, Nate Lawson wrote:
> Gavin Atkinson wrote:
> > Hi,
> > 
> > Just got a panic on a 6-CURRENT (Thu Nov 18 16:36:35 GMT 2004) machine,
> > while copying a large amount of data around.
> > 
> > Seems to be an ACPI related reuse-after-free.  As far as I can tell, 20
> > bytes into the acpi_task structure is (int)ta_flags within the embedded
> > struct task, but I can't see use of this field in the ACPI code so ACPI
> > may be a red herring.
> > 
> > 
> > # cp -Rp /usr/* /var/usr
> > [about 10 minutes later]
> > Memory modified after free 0xc44a8420(28) val=0 _at_ 0xc44a8434
> > panic: Most recently used by acpitask
> 
> Unfortunately, the panic message doesn't tell you who modified it since 
> someone with a stray pointer (say, who allocated/freed it before acpi) 
> could overwrite it and it was only detected on the next malloc.  The way 
> I've found these is to boot -d (into ddb) and type "watch 0xc44a8420". 
> Then hit "c" to continue the boot.  Dump a "tr" any time the watchpoint 
> triggers and look for suspicious callers.

Sadly, I suspect it's not going to be that easy.  I've just had another
panic, same trigger and symptoms but different memory address.

Memory modified after free 0xc50441c0(28) val=0 _at_ 0xc50441d4
panic: Most recently used by acpitask

cpuid = 0
KDB: enter: panic
[thread 100111]
Stopped at      kdb_enter+0x2c: leave

I'll try taking the box to top-of-tree current in case it has already
been fixed - however that will probably have to wait until tomorrow now
as this machine cannot reboot without physical help.  Surely it seems
like quite a coincidence that both times it was 20 bytes into memory
once owned by acpitask, though?

Gavin