kpartx: dasd: avoid negative partition size

Make sure that size - sp[0].start does not result in a negative value.
Found withe the help of Claude Sonnet 4.6.

Signed-off-by: Martin Wilck <mwilck@suse.com>
Reviewed-by: Benjamin Marzinski <bmarzins@redhat.com>

Author: mwilck

kpartx/dasd.c

@@ -71,7 +71,7 @@ int read_dasd_pt(int fd, __attribute__((unused)) struct slice all,
 	int retval = -1;
 	int blocksize;
 	uint64_t disksize;
-	uint64_t offset, size, fmt_size;
+	uint64_t offset, size, fmt_size, start;
 	dasd_information_t info;
 	struct hd_geometry geo;
 	char type[5] = {0,};
@@ -214,8 +214,11 @@ int read_dasd_pt(int fd, __attribute__((unused)) struct slice all,
 			offset = info.label_block + 1;
 			size   = sectors512(label[8], blocksize);
 		}
-		sp[0].start = sectors512(offset, blocksize);
-		sp[0].size  = size - sp[0].start;
+		start = sectors512(offset, blocksize);
+		if (start >= size)
+			goto out;
+		sp[0].start = start;
+		sp[0].size = size - start;
 		retval = 1;
 	} else if ((strncmp(type, "VOL1", 4) == 0) &&
 		(!info.FBA_layout) && (!memcmp(info.type, "ECKD",4))) {
@@ -288,8 +291,11 @@ int read_dasd_pt(int fd, __attribute__((unused)) struct slice all,
 		} else
 			size = disksize;
 
-		sp[0].start = sectors512(info.label_block + 1, blocksize);
-		sp[0].size  = size - sp[0].start;
+		start = sectors512(info.label_block + 1, blocksize);
+		if (start >= size)
+			goto out;
+		sp[0].start = start;
+		sp[0].size = size - start;
 		retval = 1;
 	}