Re: [Qemu-devel] How are the temporary files (-snapshot) created on Linux?

[Jamie Lokier <jamie@shareable.org>]

Steve Fosdick wrote:
> On 12/09/08 11:18:50, Yann E. MORIN wrote:
> > Hello!
> > 
> > On Friday 12 September 2008 08:04:02 EQX wrote:
> > > The code says the -snapshot temporary files are created here:  
> > > /tmp/vl.*, but they are never visible for users. Using lsof,
> > > they have a state of 'deleted'. How does this work exactly?
> > > What type of file is this?
> > 
> > It's done via some incantation of open(2) followed by unlink(2),
> > something
> > like:
> > 
> >   int fd;
> >   fd = open( "/tmp/vl.xxx", O_CREAT|... );
> >   unlink( "/tmp/vl.xxx" );
> >   /* Use the file somehow */
> >   close( fd );
> > 
> > Regards,
> > Yann E. MORIN.
> 
> There is nothing special about the files concerned.  The key to this is 
> that, unlike some other operating systems, Linux (like Unix) allows an 
> open file to be deleted and has a well defined way to deal with that 
> happenning.
> 
> When an open file is deleted only the file name is actually deleted.  
> The data in the file (and any new data written to the file) are kept 
> until the last process to have the file open closes the file whereupon 
> the second half of the delete happens, i.e. deallocating the disk 
> storage and returning it to the free space.
> 
> This mechanism is exploited by a process that opens a file and 
> immediately deletes it like the example above for two reasons:
> 
> 1. It provides a way to guarantee that the file not exist after the 
> process concerned has finished even if it finishes abnormally or gets 
> killed for some reason.
> 
> 2. As a security measure.  Once the file name has been deleted there is 
> no way for any other process to stumble upon the file and open it.
> 
> The second point is negated somewhat by the /proc filesystem.

Also both points are negated for a brief time window by the fact you
have to have the file exist momentarily - there's no O_UNLINK flag to open.

-- Jamie