lvreduce: check for file system that cannot be reduced

Add a new command line option --detectfs y|n, along with
the capability to detect if the LV contains a shrinkable
file system (using libblkid).

The new fs detection is performed by default to avoid
acccidental data loss by users who reduce an LV without
knowing that the file system cannot be reduced, or without
including the --resizefs option to reduce the reducible fs.
The new errors reported for these cases are:

$ lvreduce -L64M --detectfs y vg/lv
  Detected file system type xfs that cannot be reduced (from --detectfs).

$ lvreduce -L64M --detectfs y vg/lv
  Detected file system type ext4 that can be reduced (see --resizefs).

When detectfs is enabled, the lvreduce command will activate
the LV to check the fs type.

Using --detectfs n results in the same historical (and
somewhat dangerous) lvreduce behavior.

lib/device/dev-type.c

@@ -725,6 +725,39 @@ int get_fs_block_size(struct device *dev, uint32_t *fs_block_size)
 }
 #endif
 
+#ifdef BLKID_WIPING_SUPPORT
+int get_fs_can_reduce(const char *pathname, int *fs_can_reduce, char *fs_buf, int fs_buf_len)
+{
+	char *fs_str = NULL;
+
+	if ((fs_str = blkid_get_tag_value(NULL, "TYPE", pathname))) {
+		if (fs_buf && fs_buf_len)
+			strncpy(fs_buf, fs_str, fs_buf_len-1);
+
+		log_debug("Found blkid filesystem TYPE %s on %s", fs_str, pathname);
+
+		if (fs_str && !strcmp(fs_str, "ext4"))
+			*fs_can_reduce = 1;
+		else
+			*fs_can_reduce = 0;
+
+		free(fs_str);
+		return 1;
+	} else {
+		log_debug("No blkid filesystem TYPE found for %s", pathname);;
+		*fs_can_reduce = 0;
+		return 0;
+	}
+}
+#else
+int get_fs_can_reduce(const char *pathname, int *fs_can_reduce, char *fs_buf, int fs_buf_len)
+{
+	log_warn("Disabled blkid for fs type check.");
+	*fs_can_reduce = 0;
+	return 0;
+}
+#endif
+
 #ifdef BLKID_WIPING_SUPPORT
 
 static inline int _type_in_flag_list(const char *type, uint32_t flag_list)

lib/device/dev-type.h

@@ -102,4 +102,6 @@ int dev_is_lv(struct device *dev);
 
 int get_fs_block_size(struct device *dev, uint32_t *fs_block_size);
 
+int get_fs_can_reduce(const char *pathname, int *fs_can_reduce, char *fs_buf, int fs_buf_len);
+
 #endif

lib/metadata/lv_manip.c

@@ -6050,6 +6050,55 @@ int lv_resize(struct logical_volume *lv,
 	    (pvh != &vg->pvs))
 		log_print_unless_silent("Ignoring PVs on command line when reducing.");
 
+	if (lp->detectfs) {
+		char pathname[PATH_MAX];
+		char *dmname;
+		char fs_buf[16] = {0};
+		int fs_can_reduce = 0;
+
+		/* if lv is not active, activate it */
+		if (!lv_is_active(lv)) {
+			if (!activate_lv(cmd, lv)) {
+				log_error("Failed to activate %s for --detectfs.", display_lvname(lv));
+				return 0;
+			}
+			if (!sync_local_dev_names(cmd))
+				return_0;
+			activated = 1;
+		}
+
+		if (!(dmname = dm_build_dm_name(cmd->mem, vg->name, lv->name, NULL)))
+			return_0;
+
+		if (dm_snprintf(pathname, sizeof(pathname), "%s/%s", dm_dir(), dmname) < 0)
+			return_0;
+
+		/* use blkid to check fs type, we know which types can reduce */
+		if (!get_fs_can_reduce(pathname, &fs_can_reduce, fs_buf, sizeof(fs_buf))) {
+			log_error("Failed to find file system type for --detectfs.");
+			goto bad;
+		}
+
+		if (!fs_can_reduce) {
+			log_error("Detected file system type %s that cannot be reduced (from --detectfs).",
+				  fs_buf[0] ? fs_buf : "unknown");
+			goto bad;
+		}
+
+		/*
+		 * We've checked that the fs can reduce, but if the user has
+		 * not set --resizefs to actually do the fs reduction, then
+		 * report an error since it would unnecessarily cause data
+		 * loss.  i.e. require the user to add either --detectfs n or
+		 * --resizefs.
+		 */
+		if (!lp->resizefs) {
+			log_error("Detected file system type %s that can be reduced (see --resizefs).",
+				  fs_buf[0] ? fs_buf : "unknown");
+			goto bad;
+		}
+	}
+
 	/* Request confirmation before operations that are often mistakes. */
 	/* aux_lv never resize fs */
 	if ((lp->resizefs || (lp->resize == LV_REDUCE)) &&

lib/metadata/metadata-exported.h

@@ -687,6 +687,7 @@ struct lvresize_params {
 	int nosync;
 	int nofsck;
 	int resizefs;
+	int detectfs;
 
 	unsigned mirrors;
 	uint32_t stripes;

tools/args.h

@@ -155,6 +155,10 @@ arg(cachesize_ARG, '\0', "cachesize", sizemb_VAL, 0, 0,
 arg(check_ARG, '\0', "check", 0, 0, 0,
     "Check the content of the devices file.\n")
 
+arg(detectfs_ARG, '\0', "detectfs", bool_VAL, 0, 0,
+    "Check the LV for file system information. This activates the LV if\n"
+    "needed so that the LV can be read to look for fs superblocks.\n")
+
 arg(commandprofile_ARG, '\0', "commandprofile", string_VAL, 0, 0,
     "The command profile to use for command configuration.\n"
     "See \\fBlvm.conf\\fP(5) for more information about profiles.\n")

tools/command-lines.in

@@ -1449,7 +1449,7 @@ DESC: Remove the devices file entry for the given PVID.
 
 lvreduce --size NSizeMB LV
 OO: --autobackup Bool, --force, --nofsck, --noudevsync,
---reportformat ReportFmt, --resizefs
+--reportformat ReportFmt, --resizefs, --detectfs Bool
 ID: lvreduce_general
 
 ---
@@ -1479,7 +1479,7 @@ lvresize --size SSizeMB LV
 OO: --alloc Alloc, --autobackup Bool, --force,
 --nofsck, --nosync, --noudevsync, --reportformat ReportFmt, --resizefs,
 --stripes Number, --stripesize SizeKB, --poolmetadatasize PSizeMB,
---type SegType
+--type SegType, --detectfs Bool
 OP: PV ...
 ID: lvresize_by_size
 DESC: Resize an LV by a specified size.
@@ -1488,7 +1488,7 @@ lvresize LV PV ...
 OO: --alloc Alloc, --autobackup Bool, --force,
 --nofsck, --nosync, --noudevsync,
 --reportformat ReportFmt, --resizefs, --stripes Number, --stripesize SizeKB,
---type SegType
+--type SegType, --detectfs Bool
 ID: lvresize_by_pv
 DESC: Resize an LV by specified PV extents.
 

tools/lvresize.c

@@ -39,6 +39,13 @@ static int _lvresize_params(struct cmd_context *cmd, int argc, char **argv,
 	else
 		lp->resize = LV_ANY;
 
+	if (lp->resize == LV_REDUCE) {
+		if (arg_is_set(cmd, detectfs_ARG))
+			lp->detectfs = arg_int_value(cmd, detectfs_ARG, 1);
+		else
+			lp->detectfs = 1;
+	}
+
 	lp->sign = lp->poolmetadata_sign = SIGN_NONE;
 
 	if ((lp->use_policies = arg_is_set(cmd, usepolicies_ARG))) {