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Re: [libvirt] [PATCH 2/4] Avoid create/unlink with block devs used for QEMU save



On Wed, Apr 21, 2010 at 02:49:42PM -0600, Eric Blake wrote:
> On 04/21/2010 10:56 AM, Daniel P. Berrange wrote:
> > It is possible to use block devices with domain save/restore. Upon
> > failure QEMU unlinks the path being saved to. This isn't good when
> > it is a block device !
> > 
> > +    struct stat sb;
> > +    int is_bdev = 0;
> 
> Should this be bool instead of int?
> 
> >  
> > +    /* path might be a pre-existing block dev, in which case
> > +     * we need to skip the create step, and also avoid unlink
> > +     * in the failure case */
> > +    if (stat(path, &sb) < 0) {
> > +        if (errno != ENOENT) {
> > +            virReportSystemError(errno, _("unable to access %s"), path);
> > +            goto endjob;
> > +        } else {
> > +            is_bdev = 0;
> > +        }
> > +    } else {
> > +        is_bdev = S_ISBLK(sb.st_mode);
> 
> although if you use bool, you'd have to explicitly compare against 0
> here, based on the gnulib restraints on stdbool.h.  Also, what about
> other non-regular files?  A directory will fail on the open, but what
> about named fifos (on the other hand, is anyone crazy enough to try a
> non-seekable file as the file to open?).  So I'm wondering if it's
> better to check for S_ISREG, changing is_bdev to is_reg, and changing
> the cleanup logic to only attempt unlink() if is_reg, rather than
> skipping if is_bdev.
> 
> > +    }
> > +
> > +
> >      /* Setup hook data needed by virFileOperation hook function */
> >      hdata.dom = dom;
> >      hdata.path = path;
> > @@ -4849,7 +4866,8 @@ static int qemudDomainSaveFlag(virDomainPtr dom, const char *path,
> >      /* Write header to file, followed by XML */
> >  
> >      /* First try creating the file as root */
> > -    if ((rc = virFileOperation(path, O_CREAT|O_TRUNC|O_WRONLY,
> > +    if (!is_bdev &&
> > +        (rc = virFileOperation(path, O_CREAT|O_TRUNC|O_WRONLY,
> 
> This is racy.  You stat() prior to open()ing.  Wouldn't it be better to
> open(), then fstat()?

That would require significant changes to the virFileOperation code
which is not something I think is worth the effort here. 

Daniel
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