ufs: fix ufs_read_cylinder() failure handling

1) ufs_load_cylinder() should return NULL on ufs_read_cylinder() failures.
ufs_error() is not enough.  As it is, IO failure on attempt to read a part
of cylinder group metadata is likely to end up with an oops.

2) we drop the wrong buffer heads when undoing sb_bread() on IO failure
halfway through the read - we need to brelse() what we've got from
sb_bread(), TYVM...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Author: Al Viro

fs/ufs/cylinder.c

@@ -26,7 +26,7 @@
  * Read cylinder group into cache. The memory space for ufs_cg_private_info
  * structure is already allocated during ufs_read_super.
  */
-static void ufs_read_cylinder (struct super_block * sb,
+static bool ufs_read_cylinder(struct super_block *sb,
 	unsigned cgno, unsigned bitmap_nr)
 {
 	struct ufs_sb_info * sbi = UFS_SB(sb);
@@ -46,9 +46,11 @@ static void ufs_read_cylinder (struct super_block * sb,
 	 * We have already the first fragment of cylinder group block in buffer
 	 */
 	UCPI_UBH(ucpi)->bh[0] = sbi->s_ucg[cgno];
-	for (i = 1; i < UCPI_UBH(ucpi)->count; i++)
-		if (!(UCPI_UBH(ucpi)->bh[i] = sb_bread(sb, UCPI_UBH(ucpi)->fragment + i)))
+	for (i = 1; i < UCPI_UBH(ucpi)->count; i++) {
+		UCPI_UBH(ucpi)->bh[i] = sb_bread(sb, UCPI_UBH(ucpi)->fragment + i);
+		if (!UCPI_UBH(ucpi)->bh[i])
 			goto failed;
+	}
 	sbi->s_cgno[bitmap_nr] = cgno;
 
 	ucpi->c_cgx	= fs32_to_cpu(sb, ucg->cg_cgx);
@@ -67,13 +69,14 @@ static void ufs_read_cylinder (struct super_block * sb,
 	ucpi->c_clusteroff = fs32_to_cpu(sb, ucg->cg_u.cg_44.cg_clusteroff);
 	ucpi->c_nclusterblks = fs32_to_cpu(sb, ucg->cg_u.cg_44.cg_nclusterblks);
 	UFSD("EXIT\n");
-	return;	
+	return true;
 
 failed:
 	for (j = 1; j < i; j++)
-		brelse (sbi->s_ucg[j]);
+		brelse(UCPI_UBH(ucpi)->bh[j]);
 	sbi->s_cgno[bitmap_nr] = UFS_CGNO_EMPTY;
 	ufs_error (sb, "ufs_read_cylinder", "can't read cylinder group block %u", cgno);
+	return false;
 }
 
 /*
@@ -156,15 +159,14 @@ struct ufs_cg_private_info * ufs_load_cylinder (
 				UFSD("EXIT (FAILED)\n");
 				return NULL;
 			}
-			else {
-				UFSD("EXIT\n");
-				return sbi->s_ucpi[cgno];
-			}
 		} else {
-			ufs_read_cylinder (sb, cgno, cgno);
-			UFSD("EXIT\n");
-			return sbi->s_ucpi[cgno];
+			if (unlikely(!ufs_read_cylinder (sb, cgno, cgno))) {
+				UFSD("EXIT (FAILED)\n");
+				return NULL;
+			}
 		}
+		UFSD("EXIT\n");
+		return sbi->s_ucpi[cgno];
 	}
 	/*
 	 * Cylinder group number cg is in cache but it was not last used, 
@@ -195,7 +197,10 @@ struct ufs_cg_private_info * ufs_load_cylinder (
 			sbi->s_ucpi[j] = sbi->s_ucpi[j-1];
 		}
 		sbi->s_ucpi[0] = ucpi;
-		ufs_read_cylinder (sb, cgno, 0);
+		if (unlikely(!ufs_read_cylinder (sb, cgno, 0))) {
+			UFSD("EXIT (FAILED)\n");
+			return NULL;
+		}
 	}
 	UFSD("EXIT\n");
 	return sbi->s_ucpi[0];