[PATCH] blkdiscard: Refuse to proceed if signatures are found

[Lukas Czerner <lczerner@xxxxxxxxxx>]

Currently the blkdiscard has the ability to wipe out entere device in a
matter of seconds. This is fine as long as it's intentional, it is
potentially catastrophic if it's not.

With this commit blkdiscard will check for existing signatures on the
device and refuse to continue if any are found unless the operation is
forced with the -f option.

In an attempt to avoid breaking existing automation scripts the force is
only required when stdin refers to a terminal.

Signed-off-by: Lukas Czerner <lczerner@xxxxxxxxxx>
---
 sys-utils/Makemodule.am |  2 +-
 sys-utils/blkdiscard.c  | 54 +++++++++++++++++++++++++++++++++++++++--
 2 files changed, 53 insertions(+), 3 deletions(-)

diff --git a/sys-utils/Makemodule.am b/sys-utils/Makemodule.am
index 5855e1cc1..b5f6c1b1b 100644
--- a/sys-utils/Makemodule.am
+++ b/sys-utils/Makemodule.am
@@ -165,7 +165,7 @@ if BUILD_BLKDISCARD
 sbin_PROGRAMS += blkdiscard
 dist_man_MANS += sys-utils/blkdiscard.8
 blkdiscard_SOURCES = sys-utils/blkdiscard.c lib/monotonic.c
-blkdiscard_LDADD = $(LDADD) libcommon.la $(REALTIME_LIBS)
+blkdiscard_LDADD = $(LDADD) libblkid.la libcommon.la $(REALTIME_LIBS)
 endif
 
 if BUILD_BLKZONE
diff --git a/sys-utils/blkdiscard.c b/sys-utils/blkdiscard.c
index e83f69b11..f9eb6b7b0 100644
--- a/sys-utils/blkdiscard.c
+++ b/sys-utils/blkdiscard.c
@@ -37,6 +37,7 @@
 #include <sys/stat.h>
 #include <sys/time.h>
 #include <linux/fs.h>
+#include <blkid/blkid.h>
 
 #include "nls.h"
 #include "strutils.h"
@@ -106,11 +107,45 @@ static void __attribute__((__noreturn__)) usage(void)
 	exit(EXIT_SUCCESS);
 }
 
+/*
+ * Check existing signature on the open fd
+ * Returns	0  if no signature was found
+ * 		1  if a signature was found
+ * 		<0 on error
+ */
+static int device_empty(int fd, char *path)
+{
+	const char *type;
+	blkid_probe pr = NULL;
+	int ret = -1;
+
+	pr = blkid_new_probe();
+	if (!pr || blkid_probe_set_device(pr, fd, 0, 0))
+		return ret;
+
+	blkid_probe_enable_superblocks(pr, TRUE);
+	blkid_probe_enable_partitions(pr, TRUE);
+
+	ret = blkid_do_fullprobe(pr);
+	if (ret)
+		return ret;
+
+	if (!blkid_probe_lookup_value(pr, "TYPE", &type, NULL)) {
+		warnx("%s contains existing file system (%s).",path ,type);
+	} else if (!blkid_probe_lookup_value(pr, "PTTYPE", &type, NULL)) {
+		warnx("%s contains existing partition (%s).",path ,type);
+	} else {
+		warnx("%s contains existing signature.", path);
+	}
+
+	blkid_free_probe(pr);
+	return ret;
+}
 
 int main(int argc, char **argv)
 {
 	char *path;
-	int c, fd, verbose = 0, secsize, force = 0;
+	int c, fd, ret, verbose = 0, secsize, force = 0;
 	uint64_t end, blksize, step, range[2], stats[2];
 	struct stat sb;
 	struct timeval now, last;
@@ -184,7 +219,7 @@ int main(int argc, char **argv)
 		errtryhelp(EXIT_FAILURE);
 	}
 
-	fd = open(path, O_WRONLY | (force ? 0 : O_EXCL));
+	fd = open(path, O_RDWR | (force ? 0 : O_EXCL));
 	if (fd < 0)
 		err(EXIT_FAILURE, _("cannot open %s"), path);
 
@@ -217,6 +252,21 @@ int main(int argc, char **argv)
 		errx(EXIT_FAILURE, _("%s: length %" PRIu64 " is not aligned "
 			 "to sector size %i"), path, range[1], secsize);
 
+	 /* Check for existing signatures on the device */
+	if ((ret = device_empty(fd, path)) == 0) {
+		/*
+		 * Only require force in interactive mode to avoid
+		 * breaking existing scripts
+		 */
+		if (!force && isatty(STDIN_FILENO)) {
+			errx(EXIT_FAILURE,
+			     _("This is destructive operation, data will " \
+			       "be lost! Use the -f option to override."));
+		}
+		warnx(_("Operation forced, data will be lost!"));
+	} else if (ret < 0)
+		err(EXIT_FAILURE, _("failed to probe the device"));
+
 	stats[0] = range[0], stats[1] = 0;
 	gettime_monotonic(&last);
 
-- 
2.21.3