Re: Question on debugging use-after-free memory issues.

[Jiri Kosina]

On Mon, 27 Jun 2011, Ben Greear wrote:

> I have a case where deleted memory is being passed into an RPC callback.  
> I enabled SLUB memory poisoning and verified that the data pointed to 
> has 0x6b...6b value.
> 
> Unfortunately, the rpc code is a giant maze of callbacks and I'm having 
> a difficult time figuring out where this data could be erroneously 
> deleted at.
> 
> So first question:
> 
> Given a pointer to memory, and with SLUB memory debuging on (and/or 
> other debugging options if applicable), is there a way to get any info 
> about where the memory was last deleted?
> 
> Second:  Any other suggestions for how to go about debugging this?
> 
> I hit this problem under load after multiple hours, so just adding 
> printks in random places may not be feasible...

First, this is not really a proper list for such questions. I'd propose 
kernel newbies community next time.

Anyway, I'd propose to start with kmemcheck (see 
Documentation/kmemcheck.txt). It could pin-point the problemtic spot 
immediately (or it might not).

-- 
Jiri Kosina
SUSE Labs

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