[PATCH] mkfs: don't try to detect filesystems on regular files via blkid

[Jim Meyering]

Eric Sandeen wrote:
> https://bugzilla.redhat.com/show_bug.cgi?id=561870
>
> # dd if=/dev/zero of=k bs=1MB count=2 seek=20; mkfs.xfs k
> # mkfs.xfs: probe of k failed, cannot detect existing filesystem.
> # mkfs.xfs: Use the -f option to force overwrite.
>
> blkid fails to do a probe of a regular file.
>
> I wish blkid would cope with this, but for now it might
> be better to just turn it off.
>
> Reported-by: Jim Meyering <meyering at redhat.com>
> Signed-off-by: Eric Sandeen <sandeen at redhat.com>
> ---
>
> diff --git a/mkfs/xfs_mkfs.c b/mkfs/xfs_mkfs.c
> index 9baf116..de87647 100644
> --- a/mkfs/xfs_mkfs.c
> +++ b/mkfs/xfs_mkfs.c
> @@ -300,10 +300,15 @@ check_overwrite(
>  	int		fd;
>  	long long	size;
>  	int		bsz;
> +	struct stat	statbuf;
>
>  	if (!device || !*device)
>  		return 0;
>
> +	/* blkid can't get info from a regular file */
> +	if (!stat(device, &statbuf) && S_ISREG(statbuf.st_mode))
> +		return 0;
> +
>  	ret = -1; /* will reset on success of all setup calls */
>
>  	fd = open(device, O_RDONLY);

Hi Eric,

Did you consider calling fstat after opening,
rather than "stat" before?

        /* blkid can't get info from a regular file */
        if (!fstat(fd, &statbuf) && S_ISREG(statbuf.st_mode)) {
                close(fd);
                return 0;
        }

That's slightly more efficient, and not prone to confusion
in the unlikely event that "device" changes inode between
the stat and the open calls.