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Re: [dm-devel] [PATCH 18/32] libmultipath: fix tur memory misuse



On Sun, Aug 05, 2018 at 03:43:18PM +0000, Bart Van Assche wrote:
> On Wed, 2018-08-01 at 15:57 -0500, Benjamin Marzinski wrote:
> > when tur_thread() was calling tur_check(), it was passing ct->message as
> > the copy argument, but copy_msg_to_tcc() was assuming that it was
> > getting a tur_checker_context pointer. This means it was treating
> > ct->message as ct. This is why the tur checker never printed checker
> > messages. Intead of simply changing the copy argument passed in, I just
> > removed all the copying code, since it is completely unnecessary. The
> > callers of tur_check() can just pass in a buffer that it is safe to
> > write to, and copy it later, if necessary.
> > [ ... ]
> > -#define TUR_MSG(fmt, args...)					\
> > -	do {							\
> > -		char msg[CHECKER_MSG_LEN];			\
> > -								\
> > -		snprintf(msg, sizeof(msg), fmt, ##args);	\
> > -		copy_message(cb_arg, msg);			\
> > -	} while (0)
> > -
> > [ ... ]
> > -static void copy_msg_to_tcc(void *ct_p, const char *msg)
> > -{
> > -	struct tur_checker_context *ct = ct_p;
> > -
> > -	pthread_mutex_lock(&ct->lock);
> > -	strlcpy(ct->message, msg, sizeof(ct->message));
> > -	pthread_mutex_unlock(&ct->lock);
> > -}
> 
> The above code was introduced because multiple threads can access ct->message
> concurrently. Does your patch reintroduce the race conditions on the ct->message
> buffer that were fixed by commit f63f4d115813 ("libmultipath/checkers/tur: Protect
> tur_checker_context.message changes")?

It shouldn't. Since the tur checker thread only has access to the
tur_checker_context, the only race we have to worry about here is on
access to ct->message. The main checker code is free to pass c->message
to tur_check(), since the tur checker thread doesn't have access to
that.  With this patch, tur_thread() passes tur_check() a local variable
and copies that variable to ct->message while holding the mutex.  the
main checker code copies ct->message to c->message while holding the
same mutex. The main checker code even clears ct->message before
starting the thread while holding the mutex.

If you see something I've missed, please let me know.

-Ben

> 
> Bart.
> 


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