racy updates of /etc/mtab

[Rob Landley]

On Thursday 14 September 2006 12:32 pm, Denis Vlasenko wrote:
> I think we should not modify /etc/mtab in place, as we do today,
> but create /etc/mtab.new, write new contents into it and
> then rename it. With exception: if /etc/mtab is a symlink,
> we probably shouldn't touch it (it is most likely link to /proc).
> 
> This approach will be race-free. Do you agree with such a plan?

Why will it be race-free?  If two updates happen at the same time, both read 
the old file to create their new files, both rename their new files back to 
the old name.  The second rename deletes the first, so an update gets 
dropped.

I'm trusting addmntent() and such to handle these details for us.  If the 
library isn't doing the appropriate locking for legacy mtab support, fix the 
library.

Rob
-- 
Never bet against the cheap plastic solution.