Re: [PATCH 6/7] mke2fs: check for pre-existing file system

["Lukáš Czerner" <lczerner@redhat.com>]

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On Wed, 30 Apr 2014, Lukáš Czerner wrote:

> Date: Wed, 30 Apr 2014 13:50:13 +0200 (CEST)
> From: Lukáš Czerner <lczerner@redhat.com>
> To: Theodore Ts'o <tytso@mit.edu>
> Cc: Ext4 Developers List <linux-ext4@vger.kernel.org>
> Subject: Re: [PATCH 6/7] mke2fs: check for pre-existing file system
> 
> On Sat, 26 Apr 2014, Theodore Ts'o wrote:
> 
> > Date: Sat, 26 Apr 2014 20:00:33 -0400
> > From: Theodore Ts'o <tytso@mit.edu>
> > To: Ext4 Developers List <linux-ext4@vger.kernel.org>
> > Cc: Theodore Ts'o <tytso@mit.edu>
> > Subject: [PATCH 6/7] mke2fs: check for pre-existing file system
> > 
> > Warn the system administrator if there is an existing file system on
> > the block device, and give the administrator an opportunity to abort
> > the mkfs operation.
> > 
> > Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
> > ---
> >  misc/mke2fs.c |  4 +++-
> >  misc/util.c   | 29 +++++++++++++++++++++++++++++
> >  misc/util.h   |  1 +
> >  3 files changed, 33 insertions(+), 1 deletion(-)
> > 
> > diff --git a/misc/mke2fs.c b/misc/mke2fs.c
> > index 799132a..3694ce5 100644
> > --- a/misc/mke2fs.c
> > +++ b/misc/mke2fs.c
> > @@ -1753,7 +1753,9 @@ profile_error:
> >  	profile_get_integer(profile, "options", "proceed_delay", 0, 5,
> >  			    &proceed_delay);
> >  
> > -	if (!check_plausibility(device_name, CREATE_FILE,
> > +	/* The isatty() test is so we don't break existing scripts */
> > +	if (!check_plausibility(device_name, CREATE_FILE |
> > +				(isatty(0) ? CHECK_FS_EXIST : 0),
> >  				&is_device) && !force)
> >  		proceed_question(proceed_delay);
> >  
> > diff --git a/misc/util.c b/misc/util.c
> > index afb0058..be16ebe 100644
> > --- a/misc/util.c
> > +++ b/misc/util.c
> > @@ -113,6 +113,9 @@ int check_plausibility(const char *device, int flags, int *ret_is_dev)
> >  	int fd, is_dev = 0;
> >  	ext2fs_struct_stat s;
> >  	int fl = O_RDONLY;
> > +	blkid_cache cache = NULL;
> > +	char *fs_type = NULL;
> > +	char *fs_label = NULL;
> >  
> >  	if (flags & CREATE_FILE)
> >  		fl |= O_CREAT;
> > @@ -148,6 +151,32 @@ int check_plausibility(const char *device, int flags, int *ret_is_dev)
> >  		return 0;
> >  	}
> >  
> > +	if ((flags & CHECK_FS_EXIST) && blkid_get_cache(&cache, NULL) >= 0) {
> > +		fs_type = blkid_get_tag_value(cache, "TYPE", device);
> > +		if (fs_type)
> > +			fs_label = blkid_get_tag_value(cache, "LABEL", device);
> > +		blkid_put_cache(cache);
> > +	}
> > +
> > +	if (fs_type) {
> > +		if (fs_label)
> > +			printf(_("%s contains a %s file system "
> > +				 "labelled '%s'\n"), device, fs_type, fs_label);
> > +		else
> > +			printf(_("%s contains a %s file system\n"), device,
> > +			       fs_type);
> > +		free(fs_type);
> > +		free(fs_label);
> > +		return 0;
> > +	}
> > +
> > +	/*
> > +	 * We should eventually replace this with a test for the
> > +	 * presence of a partition table.  Unfortunately the blkid
> > +	 * library doesn't test for partition tabels, and checking for
> > +	 * valid GPT and MBR and possibly others isn't quite trivial.
> > +	 */
> 
> That is not true. libblkid definitely can scan for partition or any
> other signature for that matter (lvm, mdraid, ...) and we should
> definitely utilize that.

Take a look at how xfs is doing this:

	pr = blkid_new_probe_from_filename(device);
	if (!pr)
		goto out;

	ret = blkid_probe_enable_partitions(pr, 1);
	if (ret < 0)
		goto out;

	ret = blkid_do_fullprobe(pr);
	if (ret < 0)
		goto out;

	/*
	 * Blkid returns 1 for nothing found and 0 when it finds a signature,
	 * but we want the exact opposite, so reverse the return value here.
	 *
	 * In addition print some useful diagnostics about what actually is
	 * on the device.
	 */
	if (ret) {
		ret = 0;
		goto out;
	}

	if (!blkid_probe_lookup_value(pr, "TYPE", &type, NULL)) {
		fprintf(stderr,
			_("%s: %s appears to contain an existing "
			"filesystem (%s).\n"), progname, device, type);
	} else if (!blkid_probe_lookup_value(pr, "PTTYPE", &type, NULL)) {
		fprintf(stderr,
			_("%s: %s appears to contain a partition "
			"table (%s).\n"), progname, device, type);
	} else {
		fprintf(stderr,
			_("%s: %s appears to contain something weird "
			"according to blkid\n"), progname, device);
	}
	ret = 1;


we should probably also add blkid_probe_enable_superblocks(pr, 1)
even though it is enabled by default but just to better readability.

I think that we probably want to do something similar to this.

Thanks!
-Lukas


> 
> It is true that "our" blkid library can not do that, but we should
> actually stop using that and possibly remove it from e2fsprogs since
> it's really out-of-date comparing to upstream libblkid from
> util-linux.
> 
> I have some patches to do that, I can revive and resent it.
> 
> Thanks!
> -Lukas
> 
> > +
> >  #ifdef HAVE_LINUX_MAJOR_H
> >  #ifndef MAJOR
> >  #define MAJOR(dev)	((dev)>>8)
> > diff --git a/misc/util.h b/misc/util.h
> > index 9de3fbf..745568e 100644
> > --- a/misc/util.h
> > +++ b/misc/util.h
> > @@ -20,6 +20,7 @@ extern char	*journal_location_string;
> >   */
> >  #define CHECK_BLOCK_DEV	0x0001
> >  #define CREATE_FILE	0x0002
> > +#define CHECK_FS_EXIST	0x0004
> >  
> >  #ifndef HAVE_STRCASECMP
> >  extern int strcasecmp (char *s1, char *s2);
> > 
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