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

To: Eric Sandeen <sandeen@xxxxxxxxxx>
Subject: Re: [PATCH] mkfs: don't try to detect filesystems on regular files via blkid
From: Jim Meyering <jim@xxxxxxxxxxxx>
Date: Thu, 04 Feb 2010 17:46:23 +0100
Cc: xfs mailing list <xfs@xxxxxxxxxxx>
In-reply-to: <4B6AF6FC.6030101@xxxxxxxxxx> (Eric Sandeen's message of "Thu, 04 Feb 2010 10:34:04 -0600")
References: <4B6AF6FC.6030101@xxxxxxxxxx>
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@xxxxxxxxxx>
> Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx>
> ---
>
> 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.

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