e2fsck: check for zero length extent

If an extent has e_len set to zero, the kernel will oops with a
BUG_ON.  Unfortunately, e2fsck wasn't catching this case.  The kernel
needs to be fixed to notice this case and call ext4_error() instead of
failing an assertion check, but e2fsck should catch this case and
repair it (by deleting the errant extent).

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

e2fsck/pass1.c

@@ -1778,6 +1778,8 @@ static void scan_extent_node(e2fsck_t ctx, struct problem_context *pctx,
 			problem = PR_1_EXTENT_BAD_START_BLK;
 		else if (extent.e_lblk < start_block)
 			problem = PR_1_OUT_OF_ORDER_EXTENTS;
+		else if (extent.e_len == 0)
+			problem = PR_1_EXTENT_LENGTH_ZERO;
 		else if (is_leaf &&
 			 (extent.e_pblk + extent.e_len) >
 			 ext2fs_blocks_count(ctx->fs->super))

e2fsck/problem.c

@@ -936,6 +936,11 @@ static struct e2fsck_problem problem_table[] = {
 	  N_("The bad @b @i looks @n.  "),
 	  PROMPT_CLEAR, 0 },
 
+	/* Extent has zero length */
+	{ PR_1_EXTENT_LENGTH_ZERO,
+	  N_("@i %i has zero length extent\n\t(@n logical @b %c, physical @b %b)\n"),
+	  PROMPT_CLEAR, 0 },
+
 	/* Pass 1b errors */
 
 	/* Pass 1B: Rescan for duplicate/bad blocks */

e2fsck/problem.h

@@ -550,6 +550,9 @@ struct problem_context {
 /* Invalid bad inode */
 #define PR_1_INVALID_BAD_INODE		0x010065
 
+/* Extent has zero length */
+#define PR_1_EXTENT_LENGTH_ZERO		0x010066
+
 /*
  * Pass 1b errors
  */

tests/f_ext_zero_len/expect.1

@@ -0,0 +1,13 @@
+Pass 1: Checking inodes, blocks, and sizes
+Inode 12 has zero length extent
+	(invalid logical block 0, physical block 37)
+Clear? yes
+
+Pass 2: Checking directory structure
+Pass 3: Checking directory connectivity
+Pass 4: Checking reference counts
+Pass 5: Checking group summary information
+
+test_filesys: ***** FILE SYSTEM WAS MODIFIED *****
+test_filesys: 12/16 files (0.0% non-contiguous), 21/100 blocks
+Exit status is 1

tests/f_ext_zero_len/expect.2

@@ -0,0 +1,7 @@
+Pass 1: Checking inodes, blocks, and sizes
+Pass 2: Checking directory structure
+Pass 3: Checking directory connectivity
+Pass 4: Checking reference counts
+Pass 5: Checking group summary information
+test_filesys: 12/16 files (0.0% non-contiguous), 21/100 blocks
+Exit status is 0

tests/f_ext_zero_len/name

@@ -0,0 +1 @@
+extent with zero length