s390/extmem: return correct segment type in __segment_load()

Commit f05f62d0 ("s390/vmem: get rid of memory segment list")
reshuffled the call to vmem_add_mapping() in __segment_load(), which now
overwrites rc after it was set to contain the segment type code.

As result, __segment_load() will now always return 0 on success, which
corresponds to the segment type code SEG_TYPE_SW, i.e. a writeable
segment. This results in a kernel crash when loading a read-only segment
as dcssblk block device, and trying to write to it.

Instead of reshuffling code again, make sure to return the segment type
on success, and also describe this rather delicate and unexpected logic
in the function comment. Also initialize new segtype variable with
invalid value, to prevent possible future confusion.

Fixes: f05f62d0 ("s390/vmem: get rid of memory segment list")
Cc: <stable@vger.kernel.org> # 5.9+
Signed-off-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/mm/extmem.c

@@ -289,15 +289,17 @@ segment_overlaps_others (struct dcss_segment *seg)
 
 /*
  * real segment loading function, called from segment_load
+ * Must return either an error code < 0, or the segment type code >= 0
  */
 static int
 __segment_load (char *name, int do_nonshared, unsigned long *addr, unsigned long *end)
 {
 	unsigned long start_addr, end_addr, dummy;
 	struct dcss_segment *seg;
-	int rc, diag_cc;
+	int rc, diag_cc, segtype;
 
 	start_addr = end_addr = 0;
+	segtype = -1;
 	seg = kmalloc(sizeof(*seg), GFP_KERNEL | GFP_DMA);
 	if (seg == NULL) {
 		rc = -ENOMEM;
@@ -326,9 +328,9 @@ __segment_load (char *name, int do_nonshared, unsigned long *addr, unsigned long
 	seg->res_name[8] = '\0';
 	strlcat(seg->res_name, " (DCSS)", sizeof(seg->res_name));
 	seg->res->name = seg->res_name;
-	rc = seg->vm_segtype;
-	if (rc == SEG_TYPE_SC ||
-	    ((rc == SEG_TYPE_SR || rc == SEG_TYPE_ER) && !do_nonshared))
+	segtype = seg->vm_segtype;
+	if (segtype == SEG_TYPE_SC ||
+	    ((segtype == SEG_TYPE_SR || segtype == SEG_TYPE_ER) && !do_nonshared))
 		seg->res->flags |= IORESOURCE_READONLY;
 
 	/* Check for overlapping resources before adding the mapping. */
@@ -386,7 +388,7 @@ __segment_load (char *name, int do_nonshared, unsigned long *addr, unsigned long
  out_free:
 	kfree(seg);
  out:
-	return rc;
+	return rc < 0 ? rc : segtype;
 }
 
 /*