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



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()?

-- 
Eric Blake   eblake redhat com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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