Re: [PATCH] [0/11] SYSCTL: Use RCU to avoid races with string sysctls

[Andi Kleen <andi@firstfloor.org>]

On Sun, Dec 20, 2009 at 05:59:59PM -0800, Eric W. Biederman wrote:
> Andi Kleen <andi@firstfloor.org> writes:
> 
> > With BKL-less sysctls most of the writable string sysctls are racy. There
> > is no locking on the reader side, so a reader could see an inconsistent
> > string or worse miss the terminating null and walk of beyond it.
> 
> The walk will only extend up to the maximum length of the string.
> So the worst case really is inconsistent data.

It could still miss the 0 byte and walk out, can't it?

> This is an unfortunate corner case.  This is not a regression as this
> has been the way things have worked for years.  So probably 2.6.34
>  material.

The one that's a clear regression is the core pattern one, that 
was protected before by the BKL. A lot of others were always
broken yes.

> 
> > This patch kit adds a new "rcu string" variant to avoid these 
> > problems and convers the racy users. One the writer side the strings are 
> > always copied to new memory and the readers use rcu_read_lock()
> > to get a stable view. For readers who access the string over
> > sleeps the reader copies the string. 
> 
> I will have to look more after the holidays.  This rcu_string looks like
> it introduces allocations on paths that did not use them before, which
> has me wondering a bit.

On the reader side about all of them allocated before, e.g. for
call_usermodehelper.

If the strings were made a bit smaller this could be also 
put on the stack, but I didn't dare for 256 bytes. 

-Andi

-- 
ak@linux.intel.com -- Speaking for myself only.